On the nature of the galactic early-B hypergiants
Key Words.:stars:evolution - stars:early type - stars:supergiant
Aims:Despite their importance to a number of astrophysical fields, the lifecycles of very massive stars are still poorly defined. In order to address this shortcoming, we present a detailed quantitative study of the physical properties of four early-B hypergiants (BHGs) of spectral type B1-4 Ia; Cyg OB2 #12, Sco, HD 190603 and BP Cru. These are combined with an analysis of their long-term spectroscopic and photometric behaviour in order to determine their evolutionary status.
Methods:Quantitative analysis of UV–radio photometric and spectroscopic datasets was undertaken with a non-LTE model atmosphere code in order to derive physical parameters for comparison with apparently closely related objects, such as B supergiants (BSGs) and luminous blue variables (LBVs), and theoretical evolutionary predictions.
Results:The long-term photospheric and spectroscopic datasets compiled for the early-B HGs revealed that they are remarkably stable over long periods (40 yrs), with the possible exception of Sco prior to the 20 century; in contrast to the typical excursions that characterise LBVs. Quantitative analysis of Sco, HD 190603 and BP Cru yielded physical properties intermediate between BSGs and LBVs; we therefore suggest that BHGs are the immediate descendants and progenitors (respectively) of such stars, for initial masses in the range . Comparison of the properties of Sco with the stellar population of its host cluster/association NGC 6231/Sco OB1 provides further support for such an evolutionary scenario. In contrast, while the wind properties of Cyg OB2 #12 are consistent with this hypothesis, the combination of extreme luminosity and spectroscopic mass () and comparatively low temperature means it cannot be accommodated in such a scheme. Likewise, despite its co-location with several LBVs above the Humphreys-Davidson (HD) limit, the lack of long term variability and its unevolved chemistry apparently excludes such an identification. Since such massive stars are not expected to evolve to such cool temperatures, instead traversing an O4-6IaO4-6IaWN7-9ha pathway, the properties of Cyg OB2 #12 are therefore difficult to understand under current evolutionary paradigms. Finally, we note that as with AG Car in its cool phase, despite exceeding the HD limit, the properties of Cyg OB2 #12 imply that it lies below the Eddington limit - thus we conclude that the HD limit does not define a region of the HR diagram inherently inimical to the presence of massive stars.
Very massive stars are of considerable astrophysical interest given their role in driving galactic evolution via the copious production of ionising radiation and the deposition of chemically processed material and mechanical energy into the interstellar medium. They are thought to be the progenitors of Type Ibc and II supernovae (SNe) and, in low metallicity environments, gamma ray bursts (GRBs). The diverse nature of their core collapse SNe is also mirrored in the nature of their relativistic remnants which, for a given progenitor mass, may be either a neutron star or black hole depending on their pre-SNe (binary mediated?) mass loss history (e.g. Ritchie et al. magnetar ()). Moreover, with during particular phases of their post-main sequence evolution, they have the potential to probe the distance to - and chemical composition and star formation history of - external galaxies out to distances of to 30 Mpc with the current and next generation of ground based telescopes (Kudritzki kd ()).
Unfortunately, despite considerable theoretical efforts (e.g. Meynet & Maeder meynet (), Heger et al. heger ()) we currently lack a comprehensive theoretical framework to fully understand and exploit all aspects of these stars; a problem compounded by comparatively weak observational constraints on a number of key physical processes that drive massive stellar evolution, such as the influence of (metallicity dependent) mass loss, the role of rotational mixing and the effect of binary interactions. In this regard, a critical test of evolutionary theory is the accurate reproduction of the properties of stars within the short lived transitional phase between the main sequence (MS) and H-depleted Wolf-Rayets (WRs), which is populated by a diverse ‘zoo’ of disparate objects such as blue and yellow hypergiants (B/YHGs) and luminous blue variables (LBVs)/P-Cygni supergiants. For example, a crucial observational finding that must be replicated is the apparent dearth of cool evolved stars at high luminosities (the empirical Humphreys-Davidson (HD) limit; Humphreys & Davidson hdlimit ()); in this regard the YHG/RSG populations of M31 (Drout et al. drout ()) and the Galactic cluster Westerlund 1 (Clark et al. clark10 ()) already pose problems for current theoretical models.
Another manifestation of this uncertainty is an inability to incorporate the various members of the post-MS ‘zoo’ into a coherent evolutionary scheme as a function of initial stellar mass. Various authors (e.g. Langer et al. langer (), Crowther et al. pacevol (), Martins et al. martins07 (), martins08 ()) have proposed scenarios which have increasingly been informed by quantitative non-LTE model atmosphere analysis of different stellar populations, latterly located within coeval young massive Galactic clusters. In order to build on this approach we present an analysis of the properties of the BHGs of early B spectral type, which hitherto have escaped systematic study.
The Ia luminosity class was first applied to a group of four highly luminous () B ‘super-supergiants’ within the LMC by Keenan (keenanBHG ()), which were later described as ‘hypergiants’ by van Genderen et al. (vGBHG ()). These are further distinguished from normal BSGs by the presence of (P Cygni) emission in the Balmer series (cf. HD 190603; Lennon et al. lennon ()). Currently, to the best of our knowledge only 16 (candidate) BHGs have been identified within the Galaxy, of which eight are of early (B4) and eight of late (B5) spectral type; these are summarised in Table 1. Of the early BHGs, Kaper et al. (kaper06 ()) have already presented the results of a quantitative analysis of BP Cru (= Wray 977; the mass donor in the High Mass X-ray Binary GX301-2); in this paper we present the result of equivalent analyses of Cyg OB2 #12, Sco(=HD 152236) and HD 190603. Foreshadowing Sect. 4, while the latter two objects have previously been subject to similar studies (e.g. Crowther et al. pacBSG (), Searle et al. searle ()), we employ more extensive multiwavelength datasets that enable significantly more accurate determinations of physical parameters; for example the lack of wind contamination in the higher Balmer and Paschen lines sampled here permitting a more robust determination of the surface gravity. Insufficient data exist to undertake comparable modeling of the remaining early B hypergiants, although we are currently in the process of obtaining the requisite observations.
Of these stars, Cyg OB2 #12 is of particular interest as it has long been recognised as one of the intrinsically brightest, and potentially most luminous stars in the Galaxy (=Schulte 12 ; Sharpless sharpless (), Schulte schulte ()), lying well above the HD limit and so should provide a stringent test of current evolutionary theory. Moreover, BHGs have previously been associated with the LBV phenomenon by various authors (e.g. Clark et al. clark05 ()) and LBVs in turn have been implicated as both critical to the formation of WRs and as the immediate precursors of type II SNe (Smith & Conti sc08 ()). Finally, Cyg OB2 #12 and Sco are generally thought to be located within the Cyg OB2 and Sco OB1 associations respectively, which have both benefited from multiple (recent) studies111e.g. Cyg OB2: Massey & Thompson (massey ()), Knödlseder (knodlseder ()), Comerón et al. (comeron ()), Hanson (hanson ()) & Negueruela et al. (iggy ()) and Sco OB1: Reipurth (reipurth ()), Sana et al. (sana06 (),sana07 (), sana08 ()) and Raboud et al. (raboud ()). and hence in principal should help in the assessment of both the nature of their progenitors as well as their placement in a post-MS evolutionary sequence.
The paper is structured as follows. In Sect. 2 we briefly detail the new observations of the 3 BHGs analysed in this work (Cyg OB2 #12, Sco and HD 190603; we utilise the dataset and analysis of Kaper et al. (kaper06 ()) for BP Cru), while summarising the consolidated observational datasets in Sect. 3. This enables us to assess the degree of short and long-term variability demonstrated by the program stars as well as providing the spectra and optical–radio spectral energy distributions (SEDs) for the non-LTE model atmosphere quantitative analysis. The results of this are presented in Sect. 4, discussed in Sect. 5 and conclusions presented in Sect. 6. Finally, an extensive summary of the (historical) observational properties of Galactic BHGs is documented in Appendix A, the spectropolarimety of Cyg OB2 #12 and Sco discussed in Sect. 4 is presented in Appendix B and Appendix C contains an analysis of the properties of the host cluster (NGC 6231) and association (Sco OB1) of Sco.
|BP Cru||B1 Ia||12 26 37.56||62 46 13.2||10.68||-|
|HD 169454||B1 Ia||18 25 15.19||13 58 42.3||6.65||-|
|Sco||B1.5 Ia||16 53 49.73||42 21 43.3||4.78||Sco OB1|
|HD 190603||B1.5 Ia||20 04 36.17||+32 13 07.0||5.66||-|
|HD 80077||B2.5 Ia||09 15 54.79||49 58 24.6||7.57||Pismis 11|
|Cyg OB2 #12||B3-4 Ia||20 32 40.96||+41 14 29.3||11.47||Cyg OB2|
|Wd1-5||WNL/B Ia||16 46 02.97||45 50 19.5||17.49||Wd1|
|Wd1-13||WNL/B Ia||16 47 06.45||45 50 26.0||17.19||Wd1|
|Wd1-7||B5 Ia||16 46 03.62||45 50 14.2||15.57||Wd1|
|Wd1-33||B5 Ia||16 47 04.12||45 50 48.3||15.61||Wd1|
|HD 183143||B7 Iae||19 27 26.56||+18 17 45.2||6.92||-|
|HD 199478||B9 Iae||20 55 49.80||+47 25 03.6||5.73||-|
|HD 168625||B8 Ia||18 21 19.55||16 22 26.1||8.44||-|
|Wd1-42a||B9 Ia||16 47 03.25||45 50 52.1||-||Wd1|
|HD 168607||B9 Ia||18 21 14.89||16 22 31.8||-||-|
|HD 160529||B8-A9 Ia||17 41 59.03||33 30 13.7||-||-|
Note that no -band magnitude is available for Wd1-42a, while both HD 168607 and 160529 are known large amplitude photometric variables (Sect. A.6).
2 Observations and data reduction
In order to accomplish the goals of the paper we have compiled extensive datasets for Cyg OB2 #12, Sco and HD 190603, utilising both new and published observations. A full presentation of the data for these (and other) BHGs may be found in Appendix A, while we briefly describe the new observations undertaken and data reduction employed below. In appendix B we also present the results of analysis of previously unpublished spectropolarimetric observations of Cyg OB2 #12 and Sco; while these are not directly employed in the quantitative modeling of these stars they serve as a useful check on the geometry of the cirumstellar environment of these stars.
2.1 Cyg OB2 #12
New high resolution and S/N spectra of Cyg OB2 #12 were obtained through the William Heschel Telescope (WHT) service programme on 2008 July 28, employing the ISIS double-beam spectrograph equipped with the R1200B and R1200R gratings; a summary of the resultant wavelength ranges is given in Table 2. These observations were supplemented with a further five epochs of previous unpublished archival observations dating from 1992-2007; these too are summarised in Table 2. All spectra were reduced in a consistent manner using the Starlink packages Figaro and Kappa, with selected regions presented in Figs. 1-2 and A.1. Continuum normalisation was accomplished via spline fitting and division, yielding uncertainties of a few percent at most. Likewise, the RMS on the fits to the arc spectra are of the order of 0.03 pixels, resulting in a negligible error for wavelength callibration.
Unfortunately the distance and reddening to Cyg OB2 #12 preclude UV observations, so the 4000–9000Å spectrum obtained in 2008 formed the primary dataset employed in this study. This was supplemented with the 1m spectrum of Conti & Howarth (conti ()), which encompasses the important He i 1.083m transition, as well as the flux callibrated ISO-SWS spectrum presented by Whittet et al. (whittet ()) and an archival Spitzer Space Telescope IRS spectrum (Houck et al. houck ()). An optical – radio spectral energy distribution (SED) was constructed from continuum flux measurements from the literature (Tables A.1–A.3). While these observations were not contemporaneous, as demonstrated in Sect. A.1 there is no evidence for substantial variability/evolution over the timeframe spanned by these observations. Hence we are confident that such an approach is well justified and this is supported a posteriori by the excellent fits to the combined dataset.
A total of 18 new spectra have been utilised for the analysis of the long-term behaviour of Sco. Unpublished ESO 2.2m/FEROS spectra from 1998-9 (Program ID 063.H-0080; PI:Kaufer) and 2005 (Program ID 075.D-0103(A); PI:Dufton) were kindly provided by Otmar Stahl (2010; priv. comm.) and Phillip Dufton (2010; priv. comm.) respectively. Data reduction was accomplished via the custom pipeline described in Stahl et al. (feros ()). Continuum normalisation was accomplished by first dividing the spectra by the instrumental response curve, with a subsequent division by a spline function; as before, the errors in this procedure are of the order of a few percent. The spectrum from 2006 February 17 was obtained by us with the ESO NTT/EMMI; standard reduction procedures were employed and are discussed in Negueruela et al. (in prep.). Finally new blue and red end spectra from 2009 March 06 were obtained from the ESO archive (Program ID 082.C-0566(A); PI:Beletsky). These were obtained with the VLT/UVES with cross disperser gratings CD1, 2 and 3 and were reduced using the standard ESO pipeline. A full summary of these new observations is provided in Table 2 and selected spectra are presented in Figs. 1-2 and A.1. The full dataset used for modeling also includes archival IUE and ISO spectra, as well as published photometry summarised in Tables A1-3.
2.3 Hd 190603
Finally, we make use of solely archival data for the quantitative analysis of HD190603. As with the other objects, the sources of data used in constructing the SED are given in Tables A1-3. Spectroscopic data were taken from Lennon et al. (3950-5000Å+ H; lennon ()), Rivinius et al. (4050-6800Å; rivinius ()), Andrillat et al. (8390-8770Å) as well as one from the IACOB database (3710-6800Å; Sergio Simón-Díaz, priv. comm. 2011).
|1992 July 17||INT||4036-4836||4000|
|1995 July 17||WHT||6172-6980||4000|
|1998 Aug. 09||WHT||3940-5737||8000|
|2000 July 17||INT||3940-5737||8000|
|2007 August 22||WHT||7600-8900||10000|
|2008 July 22||WHT||4000-4700||4500|
|1998 October 7||ESO 2.2-m||4000-8950||48000|
|1999 July–Aug.||ESO 2.2-m||4000-8950||48000|
|2005 April 24||ESO 2.2m||3800-6800||48000|
|2006 February 17||NTT||3933-7985||9840|
|2009 March 06||VLT||3060-5600|
Note this wavelength coverage was obtained via 5 overlapping observations. Approximate usable wavelengths. 14 spectra obtained between July 17 to August 4.
3 The observational properties of BHGs
BHGs form a spectroscopically homogeneous class of objects that are distinguished from BSGs by the presence of Balmer emission lines. However, they have been implicated in the LBV phenomenon (Sect. 1) and so it is of interest to compare their 0.4-4.1m spectra with those of bona fide LBVs in both hot and cool phases. Suitable comparison spectra across this wavelength range are provided by, amongst others, Stahl et al. (stahl ()), Hillier et al. (316 ()), Groh et al. (groh ()), Clark et al. (clark03 (), clark09 (), liege ()) and Lenorzer et al. (lenorzer ()). Significant differences are evident; specifically the weakness of the H i and He i lines in the BHG spectra, as well as the absence of emission in low excitation metallic lines such as Fe ii and iii that characterise the spectra of LBVs.
A second observational characteristic of some LBVs is the presence of circumstellar dust – indeed the presence of a detached circumstellar nebula has been advanced to support the LBV nature of the late-B hypergiant HD 168625. Of the early BHGs in question, only BP Cru is known to show evidence for warm dust (e.g. Fig. 2; Moon et al. moon ()), although with , considerably less than is associated with either LBVs or supergiant B[e] stars (Egan et al. egan2 (), Clark et al. clark03 (), Kastner et al. kastner ()).
However, the defining characteristic of LBVs is their dramatic spectroscopic and photometric variability over timescales of months–years. Motivated by reports of spectral variability amongst both early- and late-B HGs we undertook an exhaustive literature search for these objects, which we present in Appendix A, and briefly discuss here. As highlighted by previous authors, there is strong observational evidence for a physical association between LBVs and low-luminosity, late-B HGs such as HD 160529 and HD 168607; however, the link is far from proven for earlier, more luminous BHGs. Owing to their luminosities extensive spectroscopic and photometric datasets exist for such stars, with the majority extending back to the mid-20th century, for Cyg OB2 #12 and HD 190603 to the turn of last century and (sparse) photometric data for Sco potentially many centuries before that (Appendix A)222Unfortunately, in many cases early observations are presented in the literature without precise observational dates. Nevertheless, the similarity of these data to more recent observations supports the lack of long term variability in such cases.
Cyg OB2 #12 has long been suspected of being spectroscopically variable, being variously classified as B3-8 Ia in the literature (e.g. Table A.4 and refs. therein). However, upon close examination of the spectra and classification criteria employed, we conclude that long-term evolution of the stellar temperature between 1954-2008 is probably absent (Appendix A.1.2) and that the reports of such behaviour result from comparison of low S/N and resolution spectra and, crucially, the sensitivity of the commonly employed He i 4471/Mg ii 4481 criterion to the properties of the photosphere/wind transition zone as well as photospheric temperature (Sect. 4.1). This conclusion is bolstered by the lack of secular photospheric variability dating back to the 1890s, although low level, apparently aperiodic variability does appear present (e.g. Gottlieb & Liller gottlieb ()).
Based on the datasets discussed in Appendix A, similar conclusions may be drawn for the remaining early- (BP Cru, HD 80077, HD 190603, HD 168454, Wd1-5 and 13) and mid-B HGs (Wd1-7 and 33), with the potential exception of Sco. Spectroscopically, it is the best sampled of all the BHGs and shows no evidence for evolution between 1891-2009 (Table A.5). Likewise, it appears to have remained photometrically stable since at least 1949 (Table A.1), although isolated historical photometric observations dating back many centuries are available in the literature, from which Sterken et al. (sterken ()) infer possible LBV like variability in the 18-19th century.
However, while long-term variability appears rare or absent amongst the early-B HGs, rapid (day to day) line profile variability (LPV) in both wind dominated emission and photospheric absorption lines appears ubiquitous, persistent and well documented (e.g. Fig. 1 and Appendix A and references therein). These behaviours are thought to arise from time variable wind structure and photospheric pulsations respectively, with the former also potentially giving rise to variable radio continuum emission (e.g. González & Cantó gonzalez ()), although changes in the the degree of ionization of the wind are another possible cause of this phenomenon.
Likewise, rapid, low amplitude photometric variability is present amongst all the BHGs (e.g. Fig. 3 and Appendix A). Such behaviour has long been recognised as being characteristic of luminous stars of all spectral types (the ‘ Cygni’ variables; e.g. Burki burki78 (), van Leeuwen et al. vanL98 (), Clark et al. clark10 ()) and again is typically attributed to photospheric pulsations.
We conclude that the temporal behaviour of the early-B HGs appears entirely typical of the wider population of luminous, early-spectral-type non-LBV stars, with the potential exception of Sco in the 18-19th centuries and a possible sudden m0.4 mag ‘glitch’ in the lightcurve of Cyg OB2 #12 in the mid-1940s (Gottlieb & Liller gottlieb ()).
4 Quantitative modeling
|Species||# full levels||# super levels||# of transitions|
Not used in Cyg OB2 #12.
Cyg OB #12
|Cyg OB #12||B3-4||12.5||38||50||0.02||10||2.7||0.21||0.55|
Note that corresponds to , the H/He ratio is given by number and in terms of . Abundances are relative to solar values from Anders & Grevesse (anders ()) and have uncertainties of, typically, 0.2dex; if we use the values from Asplund et al. (asplund ()) as a reference the derived ratios need to be scaled by 1.49, 1.86 and 1.86 for C, N and O respectively. The results presented here assume a distance of 1.75 kpc for Cyg OB2 #12 (). Apart from the assumed distance of 1.64 kpc for Sco (, Sana et al 2006) we also provide corresponding stellar properties for the other distance estimate (1.99 kpc, ) assumed in previous spectroscopic studies. For HD190603 we display the stellar properties assuming a distance of 1.57 kpc () as well as those which would result if the object had the same as Sco. Finally a distance of 3.04kpc (Kaper et al 2006) is assumed for BP Cru
In order to more fully address the physical properties of Cyg OB2 #12, Sco and HD 190603 we have employed the non-LTE model atmosphere code CMFGEN (Hillier & Miller hil98 (), hil99 ()). This solves the radiative-transfer equation for a spherically symmetric wind in the co-moving frame under the constraints of radiative and statistical equilibrium. Since the code does not solve for the wind structure from first (physical) principles, a velocity structure must be chosen; we adopted a standard -type law333The -type law is given by , where and and are the radius and velocity at the connection point between the -law and the hydrostatic structure.. Given the relatively high wind densities of BHGs, the role of the hydrostatic structure and the transition region between photosphere and wind becomes crucial to interpret the spectra (e.g. Sect. 4.1 and Fig. 8). Because of this, we used a hydrostatic density structure at depth (the ‘pseudo-photosphere’) matched to a -law wind, requiring the density to be continuous at the transition. The location of the transition is fixed by the adopted base velocity of the wind, (a free parameter in the fitting), which specifies the density through the equation of mass continuity. This approach is similar to that of, e.g., Santolaya-Rey et al. (sph97 ()), except that in our implementation the flux-weighted mean opacity is used in preference to the Rosseland mean opacity Several comparisons using ‘exact’ photospheric structures from TLUSTY (Hubeny & Lanz hub95 ()) showed excellent agreement with our method.444The latest version of CMFGEN, which allows the user to compute the exact hydrostatic structure, showed full consistency with our method. At this stage, we prefer our approach as it allows for more flexibility to study the crucial transition region..
The main atomic processes and data sets are discussed in Hillier & Miller (hil98 (), see Dessart & Hillier dess10 () for an updated description of the atomic data). The list of model atoms utilized in our calculations is provided in Table 3, including for each ion the number of super and full levels and number of bound-bound transitions. The CMFGEN model is then prescribed by the stellar radius, , the stellar luminosity, , the mass-loss rate, , the velocity field, (defined by , and the terminal velocity, ), the volume filling factor, f, characterizing the clumping of the stellar wind, and elemental abundances. The interstellar reddening parameters and are also obtained by fitting the model SEDs to the available photometric data. Finally, the equatorial rotation velocity, , and the atmospheric macroturbulent velocity, , were estimated in Fourier space (Simón-Díaz & Herrero simondiaz ()) from a selected sample of photospheric lines.
Given the number of free parameters required to specify a fit it is not possible to survey the full parameter space systematically in order to establish robust error estimates; hence errors quoted in this paper represent the range of values for which an acceptable fit to the data may be obtained. Nevertheless, we discuss error estimates for key parameters for each of the stars analysed below. The validity of this technique has been demonstrated by calibration to stars of similar temperature and luminosity for which UV, optical and near-IR data were available (Najarro et al. paco99 (), Najarro paco01 ()).
A comparison of the predicted SEDs to our data for the three stars is presented in Fig. 2, while selected regions of the synthetic spectra are overplotted on the observed 4090-11050Å spectrum of CygOB2 #12 in Fig. 4, the UV–near IR (1200–22100Å) spectrum of Sco in Figs. 5 & 6 and the optical (4000–7000Å) spectrum of HD 190603 in Fig. 7. The optical data shown in Fig. 6 correspond to the 2006 ESO EMMI/NTT run. Fits to the earlier FEROS spectra supplied by Otmar Stahl - which provide a more extended wavelength coverage - are shown in the Appendix (electronic version only) in Figs.18 and 19. The latter were used to obtain the final stellar properties as they encompass the high Balmer and Paschen lines which constrain the surface gravity. A summary of the results of the analyses for the 3 stars is presented in Table 4, along with the parameters of the B1 Ia star BP Cru, obtained via an identical methodology by Kaper et al. (kaper06 ()).
4.1 Cyg OB2 #12
For the purposes of modeling Cyg OB2 #12 we employed the dataset described in Sect. 3 and Appendix A. We have also assumed membership of CygOB2 and hence a distance, d1.75 kpc (e.g. Negueruela et al. iggy () and refs. therein); we return to this issue below.
Temperature, gravity and Luminosity
The available optical and NIR spectra allowed us to make use of several ionization equilibria to estimate the effective temperature, and thus constrain the ionization structure of Cyg OB2 #12. Hence, we were able to utilize simultaneously Si iii/Si ii, O ii/O i, N ii/N i and Fe iii/Fe ii line ratios. The S iii/S ii equilibrium was only used for a consistency check, as the weak diagnostic S iii 4253Å line suffers from a poorer S/N ratio in the blue spectral region due to reddening. We refer the reader to Sec A.1.2 for a detailed description of the available diagnostic lines.
When analyzing the ionization equilibria, we found that while lines belonging to higher ionization stages react sensitively to effective temperature and to lesser extent to gravity, those corresponding to the lower ionization stage display a high sensitivity not only to temperature and gravity, but also the location of the transition region between photosphere and wind, characterised by (especially the Si ii, Fe ii and O i lines). This sensitivity is clearly illustrated in Fig. 8 (right panel), where the choice of significantly affects the resulting strength of the Fe ii 5170Å line, leaving the Fe iii lines basically unaffected. Thus, if a lower transition velocity is chosen, a lower effective temperature is required to match the Fe iii/Fe ii ratio. As we will show later, this effect is responsible for our somewhat lower values when compared to other studies (see Sect.4.2). Interestingly, Fig. 8 (left panel) also shows how the ratio of the diagnostic lines He i 4471 and Mg ii 4481Å, which are used for spectral typing, also shows a moderate dependence on the transition region. Thus, a model with will yield equally strong He i 4471 and Mg ii 4481Å, while if the transition takes place at , He i 4471Å clearly becomes stronger than Mg ii 4481Å.
We note that this moderate dependence on the transition region becomes important in BHGs, due to the presence of a strong, dense wind and should be negligible in B supergiants, where the absorption lines will form in deeper photospheric layers that are largely unaffected by the photosphere-wind transition region. Therefore, despite the large number of diagnostics, our final estimate of the effective temperature, (kK, see Table 4) is subject to a moderate uncertainty. We find +800K and -500K as upper and lower error bounds.
Once the effective temperature was obtained, and assuming a distance of d1.75 kpc, we proceeded to fit the observed SED of Cyg OB2 #12 from the optical through radio (see Fig.2) and hence derived the reddening, stellar radius and, therefore, the stellar luminosity. We found and a reddening parameter , corresponding to . This value shows excellent agreement with the found by Torres et al. torres (), although they obtained a higher total to selective extinction parameter ()
From these values we obtained a stellar radius of 246 and a final luminosity of . The temperature and resultant luminosity are broadly comparable to previous qualitative studies (e.g. Massey & Thompson massey (), Hanson hanson ()), confirming that it is an extraordinarily luminous, but comparatively cool BHG.
The Paschen lines in the I Band provide the best constraints for the surface gravity, especially the run of the line overlap among the higher members (Fig. 4). Compared to the classical optical diagnostic lines - H and H - the higher Paschen series lines are significantly less affected by the stellar wind (and consequently clumping) and the wind/photosphere transition region. Nevertheless, the influence of the latter once again translates into a larger uncertainty in the (lower) error. Thus, we find log, corresponding to a spectroscopic mass of .
Wind properties and clumping
Unfortunately, the moderate reddening affecting Cyg OB2 #12 prevents us from securing UV observations from which one might derive firm estimates. Furthermore, as with Sco and HD 190603, Cyg OB2 #12’s stellar wind is not sufficiently dense that alternative diagnostic lines such as H or He i 10830Å can reach their full potentiallity and unambiguously yield the wind terminal velocity.
Highlighting this difficulty, Souza & Lutz (souza ()) suggested a terminal velocity of 1400kms, based on the presence of a blueshifted absorption feature in the H profile; a value which was subsequently used to obtain the mass-loss rate from radio (Abbott et al. abbott ()) and IR measurements (Leitherer et al. leitherer ()). Subsequently, Klochkova & Chentsov (klochkova ()) revised the estimate of the wind’s terminal velocity significantly downwards. Based on higher resolution H observations they identified strong electron scattering wings extending up to 1000kms, as well as blueshifted absorption up to kms which they attributed to the wind’s true . However our models indicate that this blueshifted feature does not directly reflect but rather results from the run of the density and ionization structures within the wind which shapes H.
This is shown in Fig. 9, where models with ranging from 400 to 1000kmsyield an absorption feature around 100-200kms to the blue of H in the resultant synthetic spectra. Moreover, values of significantly below 400kms appear too low to reproduce the high velocity line emission while, depending on the normalization errors of the H profile, values between 400 and 1000kms are potentially consistent. We also found that the blue absorption component of the He i 10830Å line is likewise not able to distinguish between km and km. We note that if the wind density of Cyg OB2 #12 was a factor of two higher, the blue absorption component would develop up to the corresponding value of .
Nevertheless, an upper limit on may be obtained by means of the Br/He i 4.048m complex. This is apparent in Fig. 9, where we see that ISO observations of this feature clearly resolve both components. Our models indicate that if is above 500kms, both components are blended, while strong emission bluewards of 400kmsis present, which is not seen in the data. On their own, these observations lack sufficient S/N and spectral resolution to accurately constrain , but in conjunction with H they jointly provide stronger constraints. Following this approach, we adopt kms for the remainder of the paper, while recognising that values between 300 and 1000kms cannot formally be discarded at present. Regarding this, we suggest that ground based high resolution spectra of Br and with sufficient S/N to trace the line wings may constitute the best observational constraint available to estimate .
Likewise, we make use of the shape of H and the Br/He i 4.048m complex to estimate , the parameter characterizing the velocity law. We obtain indicating a relatively flat velocity field. Values of below 2 or above 4.5 can be ruled out from the line fits.
We are, however, unable to fully reproduce the blue shoulder of the H line, noting that this is the case irrespective of the terminal wind velocity adopted in the modeling. In this regard we highlight comparable discrepancies between the synthetic and observed H profiles for Sco and HD 190603 (cf. an inability to reproduce the P Cygni absorption features in these stars; Figs. 6 and 7) while in many cases the model fits to BSGs presented by both Crowther et al. (pacBSG ()) and Searle et al. (searle ()) also suffer similarly.
Regarding Cyg OB2 #12, the lack of changes in polarisation through the line (Appendix B) strongly argues against any large scale wind asymmetries that might have been supposed to explain this feature. Hence we feel confident in the application of CMFGEN - which adopts spherical symmetry - to this and other stars in this study, while noting that this discrepancy might indicate a shortfall in the physics employed. Subject to this - and the preceding - caveats, we emphasise that the presence of this disagreement between model and observations does not affect our ability to determine the terminal velocity of the wind following the methodology described above555For completeness we also explored the possibility that the discrepancy between observations and model could be due to the presence of (spatially unresolved) blueshifted nebular emission. Fitting an additional simple gaussian profile to the blue emission ‘shoulder’ in the H and lines, scaled assuming typical nebular line intensity ratios (H/2.9 and H/2.2), resulted in significantly improved line fits. However, we caution that the adoption of such a solution appears premature at present as no other expected nebular emission lines such as [N ii], [S ii] or [O i] are present in the spectrum of Cyg OB2 #12; further adaptive optics or coronographic observations to search for compact nebular emission would be of value to determine if such an approach is physically well motivated..
Finally, we were able to derive a mass-loss rate of yr (Table 4). Of course this value is bound to the adopted and clumping law. Thus, if values of 300, 700 and 1000kmswere adopted, fits of similar quality would be obtained for , 4.0 and yr respectively.
The main observational constraints which set the run of the clumping law are the H and Br emission components and the IR and submillimeter + radio continuum. An onset of the clumping at relatively high velocity ( kms) is required to avoid strong emission in the above lines and too great an excess in the IR continuum. On the other hand, we find that a final clumping value of 0.04 is required to reproduce the submillimeter and radio continuum (see Fig.2).
Our derived mass loss rate presents one of the major differences with respect to previous works. The significantly lower clumped mass loss rate of yr corresponds to an unclumped (/) rate of yr; a factor of 3 lower than that reported by Leitherer et al. (leitherer ()), due to the considerably smaller wind terminal velocity () adopted (400 versus 1400kms). The effects of the high degree of wind clumping found for Cyg OB2 #12 mirror recent findings for other massive (evolved) stars (e.g. Najarro et al. paco09 (), Groh et al. agcar (), hrcar ()).
Nevertheless, such a clumping corrected value is in excess of that found for the less luminous and massive B supergiants studied by Crowther et al. (pacBSG ()) and Searle et al. (searle ()). For wind velocities kms the clumping corrected mass loss rate approaches those of known LBVs (Fig. 11; Sect. 5), although the terminal velocity is comparable to BSGs of similar temperature. Adopting kms leads to a mass loss rate comparable to those of the LBVs, but such a wind velocity is significantly greater than those determined for both LBVs and BSGs of equivalent spectral type. With respect to this, the lack of the low excitation metallic emission lines that characterise cool phase LBV spectra (Sect. 3) is a result of the extremely large radius found for Cyg OB2 #12.
Fortunately, unlike stars such as HDE 316285 (Hillier et al. 316 ()), Cyg OB2 #12 does not suffer from a /He-abundance degeneracy. Since, as previously described, can be accurately determined from e.g. the Si iii/Si ii or O ii/O i equilibria, we may therefore reasonably constrain the He/H ratio. In Fig. 10 we overplot synthetic spectra constructed with a wide range of He/H abundances on selected He i transitions; note that as expected the best constraints are provided by the weaker, non saturated, transtions. This reveals one of the fundamental results of our study - that Cyg OB2 #12 demonstrates a solar H/He=10 ratio (by number). Unlike the rest of BHGs analysed in this paper (see Table 4), which clearly demonstrate surface helium enrichment, the He i lines of Cyg OB2 #12 are best reproduced assuming no helium enrichment at all (noting that previous studies of BSGs adopted H/He=5 for all stars considered; Crowther et al. pacBSG (), Searle et al. searle ()). Furthermore, even better fits to some of the strong He i lines in the - (He i 6678Å), - (He i 8581 and 8845Å) and -bands (He i 10830Å) are obtained if we assume He to be underabundant. Moreover, we find that H/He8.0 appears to provide a robust lower limit to the abundance ratio, with lower values resulting in unacceptably poor fits to the data (Fig. 10).
We note that given the current stellar temperature and luminosity, the lack of He enrichment at the stellar surfaces is at odds with the predictions from evolutionary models and challenges present theory of stellar evolution of very massive stars. We discuss this somewhat unexpected finding, and the implications for the evolutionary state of Cyg OB2 #12 in Sect. 5. From Table 4 we see indications of CNO processing (N enhancement and CO depletion) in Cyg OB2 #12, though to a lesser extent than for Sco and HD190603. This result is consistent with the derived H/He ratios.
4.1.1 The unexpected luminosity of Cyg OB2 #12
A critical finding of this analysis is the rather extreme luminosity of Cyg OB2 #12 (e.g. Fig. 12 and Sect. 5); are we overestimating this? An obvious explanation is that a chance alignment with its ‘host’ association leads us to overestimate its distance. If this were the case, the large degree of reddening demonstrated by Cyg OB2 #12 would either be due to a (narrow) sightline of anomalously high interstellar extinction towards it and only it - but which nonetheless yielded Diffuse Interstellar Bands (DIBs) comparable to those found for other bona fide association members (e.g. Hanson hanson ()) - or would be circumstellar in origin (e.g. Massey & Thompson massey ()), thus permitting a more normal run of interstellar reddening for a lower distance. However, our model fits to the expanded SED show no evidence for an IR excess at wavelengths shorter than 30m; suggesting that any circumstellar component would have to be significantly cooler than observed around known LBVs, where pronouned emission from warm dust is detected at 25m (e.g. Egan et al. egan2 (), Clark et al. clark03 ()).
A second alternative is that it is a multiple system. Given the recent suggestion of a high binary fraction within CygOB2 (Kiminki et al. kiminki2 ()) and the hard, luminous X-ray emission from Cyg OB2 #12 (Albecete Columbo et al. albacete (), Rauw et al. rauw ()) this would appear to be physically well motivated. To reduce the luminosity of Cyg OB2 #12 such that it was marginally consistent with current theoretical predictions (Sect. 5) would require a companion(s) of equal luminosity. However clear spectroscopic signatures of binarity in terms of (i) radial velocity shifts, (ii) the presence of double lines and/or (iii) the dilution of spectral features appear absent666In particular we note that no (anti-phased) radial velocity shifts indicative of binarity are present in the additional emission components of the H, and lines.. Moreover, the excellent model fit to the observed spectral energy distribution also excludes a companion of comparable luminosity but very different temperature.
While not fatal to a binary scenario, these observational constraints would require any putative binary companion of comparable luminosity to have an identical spectral type - to avoid its spectral signature being visible - and for the binary to either be very wide or seen face on - to avoid RV shifts in the spectrum. However, the temperature of the hard X-ray component (keV) implies, via the strong shock condition, that the companion would have to have a wind velocity of 1300kms - inconsistent with the emission arising in the wind collision zone produced by such a ‘twin’ and necessitating the presence of a third, unseen star to yield the high energy emission. While we cannot exclude such a ‘finely tuned’ hierarchical system, we find no compelling evidence for one either.
Lastly, one might suppose that Cyg OB2 #12 were currently undergoing a long-term‘LBV eruption’ resulting in an increased luminosity. However, while the physical properties of such events are ill constrained we find the current physical properties to be distinct from those of quiescent LBVs (e.g. Sect. 5), while mass loss rates of yr are typically inferred for such stars in outburst (Clark et al. clark09 ()). Therefore, while we may not formally exclude any of these possibilities, we find no compelling observational evidence to support them either and hence consider it likely that the primary BHG within Cyg OB2 #12 is indeed unexpectedly luminous in comparison to theoretical predictions. Moreover, even if its luminosity were reduced via one of these scenarios such that it was consistent with current evolutionary tracks, we note that the revised mass loss rate () and (unchanged) chemistry would still remain discrepant with respect to field BSGs and BHGs.
4.2 Sco and HD 190603
For the analysis of Sco we adopted a distance kpc, based upon the revised estimate for NGC6231/Sco OB1 (Appendix C); for convenience we also show results for the slightly larger distance of kpc adopted in previous works to enable a direct comparison(Crowther et al. pacBSG ()). Unfortunately, HD 190603 is not associated with a host cluster making the distance significantly more uncertain. For consistency to prior studies we adopt an identical distance ( kpc; Crowther et al. pacBSG ()), while also presenting results under the assumption that it has an identical intrinsic visual magnitude to Sco.
As in the case of Cyg OB2 #12, several ionization equilibria could be used to determine the stellar temperature. Interestingly we could use three ionization stages of silicon (Si iv/Si iii/Si ii) for both objects. In the case of HD190603, our hottest object, the Fe ii and N i lines are not detected so we used them in our models as indicators of a lower limit for . On the other hand, He ii 4686Å was weakly detected on HD190603 allowing to use as well the He ii/He i ionization criterium.
Since UV spectra are available for both objects, the terminal velocity, , could be accurately determined for both objects (e.g. Fig. 5). Given that we are fitting non-simultaneous observations from the UV to the IR we might expect to find some transitions in this wavelength range which are not well fitted by our synthetic spectra. This appears to be the case for C iv 1548-1551Å, which is significantly underestimated by our models. Moreover these, as well as the Si iv 1394-1403Å UV lines, are very sensitive to X-rays in this parameter domain; thus they should be regarded with caution.
The main stellar properties of both objects are presented in Table 4. Uncertainties in stellar parameters are similar to those derived for Cyg OB2 #12, except for the effective temperature, which is slightly better constrained K and the terminal velocity with kms. Errors on the H/He ratio are of the order of 20%. Interstellar reddening was also determined; for Sco we found and and for HD 190603 and .
We found both stars to be less extreme than Cyg OB2 #12 in terms of luminosity, although Sco is still significantly in excess of the range of luminosities spanned by Galactic O9-B5 Ia/b supergiants (Searle et al. searle (), Crowther et al. pacBSG ()), even with the downwards revision of the distance. HD 190603 lies in the upper reaches of this distribution, but we caution that this could be subject to revision given the uncertainties in its distance estimate. As expected from their earlier spectral types, they are hotter than Cyg OB2 #12 , although the temperatures we find are slightly lower (1kK) than previous estimates for these objects. These lower temperature estimates are due in part to the effect of the wind transition region (see Sect. 4.1) and to the use of all available ionization equilibria criteria. Following from these results, spectroscopic mass estimates are significantly lower than that of Cyg OB2 #12 for both stars.
Terminal wind velocities are similar to Cyg OB2 #12, but lie at the lower range of values found for BSGs of comparable spectral type; in contrast the clumping corrected mass loss rate for HD 190603 lies at the upper range of those found for B1-2 Ia/b supergiants while that of Sco is considerably in excess of this range (Fig. 11). These compare favourably with previous estimates for these stars (Searle et al. searle (), Crowther et al. pacBSG (), Markova & Puls markova ()); although as with Cyg OB2 #12, the high degree of clumping required to fit the spectra provide a significant downwards revision of absolute mass loss rates between this and prior studies. Values of log are also lower than previous estimates of both stars, due to the greater accuracy afforded us by the inclusion of the higher Balmer and Paschen lines in the spectroscopic datasets.
Finally, in contrast to Cyg OB2 #12, but in line with estimates for field Galactic BSGs, we find a moderate H-depletion (or He-enrichment; H/He5 by number) for both stars as well as evidence for stronger CNO processing (higher N).
While the galactic BHG population spans over an order of magnitude in luminosity (Fig. 12), when combined with previous modeling results for BP Cru (Table 4; Kaper et al. kaper06 ()), our analyses show the early B1-4 Ia hypergiants to be, as expected, rather luminous evolved stars, with HD 80077 and CygOB2 #12 appearing to lie above the empirical HD limit (Fig. 12; Marco & Negueruela marco ()). In comparison to Galactic BSGs of the same spectral subtypes they are overluminous and support significantly higher mass loss rates, with wind terminal velocities at the lower end of the range populated by the BSGs.
5.1 The evolutionary status of BHGs
Subject to these observational constraints how may single
(early) BHGs be understood in evolutionary terms? Several authors
have presented evolutionary sequences for massive ()
stars, including Langer et al. (langer ()), Crowther et
al. (pacevol ()), Meynet & Maeder (meynet ()) and Martins et al. (martins07 (),
martins08 ()). After the initial O-type main-sequence
phase, the recent empirical evolutionary scheme of the latter authors may be summarized as:
: O Ia OIaf WNL + abs WN7
: O/B Ia WN9-11h LBV WN8 WN/C
While not explicit in the above scheme, the properties of LBVs such as AG Car (Groh et al. groh11 ()) and
the Pistol star (Najarro et al. paco09 ()) strongly imply that even very massive
stars may experience an LBV phase. Moreover, it is likely that a further
subdivision will occur for to accommodate
the high luminosity (log) cool hypergiants
present in clusters such as Westerlund 1 (Wd 1; Clark et
al. me ()):
: O OB Ia cool Ia WN
noting that the evolution of spectral classification through the red loop and the final state reached prior to SN is particularly uncertain.
Given that the physical properties of the BHGs appear more extreme than the field BSGs and that they appear co-located with the LBVs on the HR diagram (Fig. 12) a close physical association is suggested and we propose that the early BHGs - with the notable exception of CygOB2 #12 - are the immediate evolutionary progenitors of LBVs for stars of . Thus, they would form lower mass analogues of the O4-6 Iaf stars in the Arches, which Najarro et al. (paco04 ()) and Martins et al. (martins08 ()) demonstrate are intermediate between the O4-6 Ia and WN7-9ha stars both in terms of the degree of (increasing) chemical evolution and also wind properties, with advancing evolutionary state leading to a reduction in terminal wind velocities coupled with an increase in mass loss rate.
The BHGs appear to follow an analogous evolutionary template, with wind properties approaching those of the LBVs (Fig. 11). Moreover, the BHGs also appear less chemically evolved than (candidate) LBVs777HD 316285 (H/He1.5), AG Car (H/He2.3), W243 (H/He5), P Cygni (H/He3.3), FMM362 (H/He2.8) and the Pistol Star (H/He1.5; Hillier et al. 316 (), Groh et al. agcar (), Ritchie et al. ritchie (), Najarro et al. paco01 (), paco09 (), respectively)., again suggesting that they occupy an earlier evolutionary phase. Such an hypothesis appears bolstered by the apparent lack of long-term(secular) variability demonstrated by the early-B HGs (Sect. 3), although we are aware that bona fide LBVs such as P Cygni may also encounter long periods of quiescence.
We emphasise that this evolutionary scheme is appropriate for single stars, but it also appears possible that early-B HGs may form via binary interaction and, through such a channel, potentially from lower mass stars. Wellstein & Langer (wellstein ()) present the results of simulations that show the properties of the known binary BP Cru are consistent with quasi-conservative mass transfer in a progenitor system. Likewise, the BHG/WNVL transitional object Wd1-13 is the primary of a 9.27 day period binary (Ritchie et al. magnetar ()). This configuration is too compact for the primary to have passed through a BSG phase, and hence presumably formed via an episode of binary mediated mass loss (Sect. A.5), with a likely progenitor mass of . In this respect we highlight that Wd1-13 appears spectroscopically distinct from the BHGs considered here, showing pronounced (variable) emission lines in the Paschen series (Fig. A.2 and Ritchie et al. magnetar ()).
Finally, is it possible to place the later (lower luminosity) BHGs into comparable sequences? The consistency of the population of B1-3 Ia and cooler B5-9 Ia stars within Wd 1 with theoretical evolutionary isochrones (Negueruela et al. iggy10 ()) suggests an extension of the paradigm that BHGs are the more physically extreme direct descendants of hotter O/BSGs to lower progenitor masses (). However, the placement of these BHGs in a pre- or post-RSG/LBV phase is currently uncertain in the absence of tailored abundance analyses - ESO/VLT observations currently underway will allow this to be directly addressed. Finally, following previous suggestions in the literature (Sect. A.5) - and again in the absence of quantitative analyses - it appears likely that lower luminosity (log) BHGs are post-RSG objects possibly encountering an LBV phase.
5.2 Constraints from the host stellar aggregate
Do other observations support such an evolutionary scheme? Following the above discussion, the stellar populations of both the Arches and Wd 1 are clearly consistent. However we would also predict the presence of early BHGs within the Quintuplet cluster, which is likely to be intermediate in age between these clusters (Figer et al. figer ()). While neither these authors nor Liermann et al. (liermann ()) report the presence of BHGs, we note that e.g. LHO96, 100 and 146 - which are classified as O6-8 If by Liermann et al. - have K band spectra identical to that of Sco (Fig. 6). Simply applying the bolometric correction derived from our analysis of Sco yields an estimated log for these stars; directly comparable to the Pistol Star and FMM362 (Najarro et al. paco09 ()) and hence consistent with a pre-LBV BHG identification. Further high S/N and resolution observations of these stars would permit quantitative testing of this assertion by virtue of an accurate determination of temperature, luminosity and wind properties.
Encouragingly, stars with a similar K band spectral morphology are also present in the Galactic Centre cluster, where they too are found to be of higher temperature and possess faster, lower mass loss rate winds than the more evolved WNL - and candidate LBV - stars present (Najarro et al. paco97 (), Martins et al. martins07 ()).
Importantly, are the properties of Sco and Cyg OB2 #12 and their host stellar aggregates consistent with this evolutionary hypothesis? Unfortunately, observations of Cyg OB2 and NGC6231/Sco OB1 raise the possibility of the stars in either association being non-coeval (Negueruela et al. iggy (); Appendix C). Nevertheless, the physical properties of Sco appear consistent with the population of (initially) rapidly rotating, 5 Myr-old stars present within NGC6231 (Fig. 13 and Appendix B; noting that the low of Sco may be understood as a combination of spindown and/or an unfavourable inclination) or a somewhat younger ( Myr) population of less rapidly rotating objects that may also be present (Appendix B). This would imply a progenitor mass of for Sco, depending on age and initial rotation, broadly consistent with the current spectroscopic mass of .
Finally, we address Cyg OB2 #12. As highlighted previously, this star appears difficult to accomodate in the current evolutionary scheme due to the combination of high luminosity and low temperature displacing it from any theoretical isochrones applicable to Cyg OB2 (Fig. 14; Negueruela et al. iggy ()). While its co-location in the HR diagram with bona-fide LBVs such as the Pistol Star, FMM 362 and AFGL 2298 (Fig. 12) suggests a similar nature, the long-term stability combined with its relatively unevolved chemsitry - and critically solar H/He ratio - appear to exclude such an identification. We are thus currently unable to place Cyg OB2 #12 into a coherent evolutionary scheme, noting that with a current spectroscopic mass of - significantly in excess of both the Pistol Star and FMM 362 (Najarro et al. paco09 ()) - one might expect it to evolve through a much hotter O4-6I/Iaf/WNLha evolutionary sequence.
In this respect, the results of a quantative analysis of HD 80077 would be of particular interest, given its apparent similarly extreme luminosity (Marco & Negueruela marco ()). Such an analysis has been performed for the M33 B1 Ia star [HS80] 110A by Urbaneja et al. (urbaneja ()) which yielded stellar parameters that, like Cyg OB2 #12, both place it above the HD-limit and appear to distinguish it from known LBVs888log, kK, kms and yr.. This finding appears to suggest that there might indeed be an hitherto unidentified (but rarely traversed?) evolutionary pathway that leads to the production of very luminous but comparately cool BHGs at very high stellar masses.
In order to determine the physical properties and hence evolutionary state of Galactic early-B hypergiants, we have undertaken new quantitative analyses of CygOB2 #12, Sco and HD 190603 and employ previous model results for BP Cru. Synthetic spectra and SEDs were calculated and compared to comprehensive UV/optical–radio spectroscopic and photometric datasets compiled from the literature and supplemented by new and previously unpublished observations. Building on this effort, we also constructed exhaustive spectroscopic and photometric histories for all Galactic BHGs in order to test the assertion that they are physical identifiable with LBVs.
Turning first to stellar variability and both the early- and late-B HGs demonstrated the rapid (days) spectroscopic and photometric variability that is symptomatic of the aspherical wind substructure and photospheric pulsations that characterises luminous early stars. While low-luminosity, late-B HGs have long be known to undergo LBV excursions (e.g. HD 160529) we found no evidence for such secular variability amongst the early, high luminosity BHGs, with the possible exception of Sco in the sparse photometric data obtained prior to the 20 Century. In this regard it is interesting that both Cyg OB2 #12 and HD 80077 appear to lie above the empirical HD limit.
Model results reveal that the early BHGs have physical properties - luminosity, wind terminal velocity, mass loss rate and chemical abundances - intermediate between Galactic BSGs and LBVs of comparable temperature, suggesting they provide a link between both evolutionary phases for stars of inital mass between . In this respect, they would play a similar role to the more massive and luminous O4-6Iaf objects for stars in the range (Najarro et al. paco04 (), Martins et al. martins08 ()). The simultaneous presence of populations of early B1-3 Ia supergiants and mid-late B5-9 Ia BHGs within Wd 1 suggest that BHGs are potentially the immediate descendents of stars with initial masses as low as . However, the presence of early BHGs within this cluster indicates that a parallel channel may also lead to this phase via close binary evolution.
Nevertheless, Wd 1 and the Quintuplet provide direct observational tests of the evolutionary sequence proposed; in the former cluster the BHGs should be more chemically evolved than the BSG population but less than the RSGs that are present, while in the latter cluster one would expect to find a population of early BHGs; observations aimed at verifying both hypotheses will be undertaken later this year.
The apparent physical association of Sco with NGC 6231/Sco OB1 also allows us to test this scenario. Encouragingly, we find that the physical properties derived from our analysis are consistent with the observed stellar population of this region and in turn with theoretical isochrones for stars of between 3.2-5 Myr age - further discrimination is difficult in the absence of an absolute rather than projected rotational velocity.
Conversely, the combination of extreme luminosity and cool temperature of CygOB2 #12 is inconsistent with theoretical isochrones, assuming membership of Cyg OB2. Given its co-location with a handful of bona fide LBVs above the HD limit it might be supposed to be a similar object, but a combination of (i) a lack of the characteristic LBV variability, (ii) extreme current stellar mass and (iii) lack of chemical evolution differentiate it from these. We are therefore currently unable to place this star in a consistent evolutionary scheme, noting that Negueruela et al. (iggy ()) also find that a number of the hottest and most luminous stars within CygOB2 are difficult to accommodate in a formation scenario for the association where the most recent epoch of star formation occurred 2.5 Myr ago.
Nevertheless, we note that the stability of CygOB2 #12 for the last century would appear to indicate that the HD limit does not appear to define a region of the HR diagram utterly inimical to the presence of massive stars, but a combination of (pulsational?) instabilities and extreme mass loss rates presumably prevent stars residing above it for large fractions of their lifetime. Indeed, from Fig. 12 we may see that in addition to highly luminous BHGs such as Cyg OB2 #12 the empirical HD limit appears to be delineated by LBVs and other closely related (variable) stars such as the cool hypergiants and the WNVLs, further suggesting that it should not be regarded as a firm barrier to stellar stability, nor the location at which the onset of instabilities occurs. In this regard the B1 Ia star [HS80] 110A and the cool F-hypergiant B324 (both located in M33) are of considerable interest; like Cyg OB2#12, the combination of temperatures and luminosities they demonstrate are not replicated by current evolutionary tracks and place both stars well above the HD limit (Urbaneja et al. urbaneja (), Clark et al. clark12 ()). Quantitative modeling of the Galactic BHG HD 80077 would therefore be of considerable interest to see whether this too violated the HD limit.
Moreover, despite residing above the HD limit, we find an Eddington parameter, , of only (and at for Cyg OB2 #12; below the Eddington limit. Unfortunately, in the absence of an inclination we may not determine how close Cyg OB2 #12 is to the rotationally modified Eddington limit, although we note that the Eddington parameter for it is lower than Groh et al. (groh11 ()) found for AG Car at any epoch. Given that AG Car is also a known rapid rotator but has not been observed at, or to have exceeded, the rotationally modified Eddington limit we suspect the same to be true for Cyg OB2 #12. From this we might therefore conclude that the HD-limit is not the direct result of a star (b)reaching the rotationally modified Eddington limit. Indeed, Groh et al. (groh11 ()) show that AG Car most closely approaches the rotationally modified Eddington limit during its hot, compact phases when, in contrast to its cool extended state, it lies beneath the HD limit (Fig. 12).
Acknowledgements.JSC acknowledges support from an RCUK fellowship. This research is partially supportedby the Spanish Ministerio de Ciencia e Innovación (MICINN) under grants AYA2008-06166-C03-02/03, AYA2010-21697-C05-01/05 and CSD2006-70. MAU acknowledges support by NSF under grant AST-10088798 We also thank John Hillier for providing the CMFGEN code and Dan Kiminki, Otmar Stahl, Philip Dufton and Sergio Simón-Díaz for providing electronic versions of published and unpublished stellar spectra.
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Appendix A Summary of historical data for Galactic BHGs
In order to understand the role of the BHG phase in the lifecycles of massive stars it is invaluable to contrast the long term (in)stability of such stars to other, potentially related evolutionary phases such as the LBVs. In order to accomplish this we undertook a literature review of all known Galactic examples. This has allowed the construction of the comprehensive spectroscopic and photometric datasets used in the quantitative modeling descibed in Sect. 4. To the best of our knowledge there are 16 known or candidate BHGs within the Galaxy, of which 8 have early (B1-4) spectral types. In constructing this list we have explicitly included 3 high luminosity stars of late-B spectral type which, although classified as luminosity class Iae, have prominent Balmer emission lines that appear characteristic of bona fide BHGs.
a.1 The early-B HGs: Cyg OB2 #12
a.1.1 Photometric variability
Gottlieb & Liller (gottlieb ()) provide an extensive optical photometric history for Cyg OB2 #12, comprising, where available, annual mean band magnitudes from 1890-1980. They report low level annual variability (mag.) albeit with no apparent secular evolution. Despite being somewhat limited, the compilation of multi-epoch photometry presented in Table A.1 is consistent with little photometric activity being present999We note that in common with early observations of all the BHGs considered, interpretation is complicated by the lack of a reported date for some observations - a problem also present for the spectroscopic observations..
While systematic photometric monitoring of Cyg OB2 #12 ceased after 1980 - fortuitously at a point when spectroscopic observations increased in frequency (Sect. A.1.2) - two further long-term photometric datasets are available. The first, acquired via the Hipparcos mission (ESA ESA (), Perryman et al. perry ()), provides data from 1989 November to 1993 March, with the passband, , spanning 3400-9000Å (van Leeuwen et al. vanL97 ()). Subsequently, the Northern Sky Variability Survey (NSVS; Woźniak et al. wozniak ()) yielded a further year long dataset (1999 April - March 2000); this operated without filters, resulting in a wide optical band with a response defined by that of the CCD, extending from 450 to 1000nm.
These data are summarised in Fig. 3 and although the non-standard passbands do not allow a direct comparison to previous data they do constrain variability over the periods in question. As with the results of Gottlieb & Liller (gottlieb ()), these datasets indicate that low level (0.3mag) non secular and aperiodic variability is present, which occurs over rather short (days) timescales.
While only single epoch mid-IR observations are available (Table A.2), multiple epochs of radio continuum observations are available in the literature, and are summarised in Table A.3. While the spectral indices derived from these data are consistent with thermal emission from a partially optically thick stellar wind, these fluxes demonstrate significant variability. As with the short period photometric variability, a possible interpretation of this behaviour is provided in Sect. 3.
a.1.2 Spectral classification and variability
A summary of the multiwavelength spectroscopy obtained for Cyg OB2
#12 over the past half century is presented in Table A.4. The
spectral classifications given are those reported by the works in
question; reclassification is provided only for those occasions where
the source papers assumed an earlier result (this being the case for
the IR studies, where Cyg OB2 #12 was typically included as a
spectral standard). Regarding reclassification, a luminosity class of
Ia is implictly assumed throughout the following
Souza & Lutz (souza ()), Massey & Thompson (massey ()), Klochkova & Chentsov, (klochkova ()) and Kiminiki et al. (kiminki ()) all provide a detailed discussion of the classification of Cyg OB2 #12 from 4000-5000Å spectra at high resolution and S/N, using the He i 4471Å:Mg ii 4481Å line ratio as the primary spectral-classification criterion. Following this methodology, spectral types in the range B3-B8 have been inferred by these authors, with variability in the line ratio (e.g. Kiminiki et al. kiminki ()) taken as evidence for changes in the spectral type of Cyg OB2 #12.
However, we caution that extreme care must be taken in inferring changes
in the physical properties of the star from this diagnostic.
Firstly, the Mg ii 4481Å transition shows a dependence on stellar luminosity
and so callibrations of spectral type versus the He i 4471Å:Mg ii 4481Å
line ratio derived from lower luminosity stars may not a priori be directly
applied to Cyg OB2 #12. More critically and as demonstrated in Sect. 4.1,
this ratio is highly sensitive to the properties of the
photosphere/wind transitional zone. Consequently, changes in
the structure of the inner wind - which we may infer from line profile
variability in H (see below) - at constant stellar temperature will
cause this line ratio to vary, and in turn lead to the erroneous conclusion that
because the spectral type has varied then the temperature has also changed.
Given the quality of their spectra - and in conjunction with data
Walborn & Fitzpatrick (walborn90 ()) - Kiminki et
al. (kiminki ()) also employed the weak absorption feature at
4542Å as a classification criterion, which they attributed
to a blend of He ii and Fe ii. While an absorption
feature at 4542Å is observed in B1.5-4 supergiants, it is
unlikely to be due to He ii (seen only for B0 and earlier
supergiants) or Fe ii (B5 and later). Given these
uncertainties, we instead choose to employ the Si iii
4552Å/Si ii 4128Å line ratio as a primary temperature
diagnostic. Thus, upon consideration of the above, we suggest a
spectral classification of B5 for the 1992 July 22 spectrum and B4 for
the 1998 August, 2000 September and 2008 July spectra (Fig. A.1),
noting that the marginally later classification in 1992 is likely to
be a result of the lower S/N and spectral resolution of these data. We
thus find no evidence for changes in the spectral type of Cyg OB2 #12
in these data.
The spectral region surrounding H
is largely devoid of classification criteria (e.g. Negueruela et
al. iggy10 ()). However (i) the strong central H emission
peak superimposed on broad emission wings, (ii) lack of He ii
6527, 6683Å absorption, (iii) absence of S iv 6668,
6701Å emission and (iv) the presence of strong C ii 6578,
6582Å photospheric lines are uniformly present in spectra from
1992 onwards101010The low resolution and S/N of previously
published spectra are insufficient to comment on the presence, or
otherwise, of photospheric features. and are all consistent with a
highly luminous B supergiant (Fig. 1). As found by Klochkova &
Chentsov (klochkova ()), the central peak of the H profile
appears to be variable at low projected velocities
Despite spanning a period of over 16 years, the 3 I band spectra are essentially identical (a representative spectrum from 2008 is presented in Fig. A.2). To classify Cyg OB2 #12 from these data we employed the scheme described in Negueruela et al. (iggy10 ()), supplemented by spectra of other early-mid BSGs and BHGs of known spectral type. While the BHG spectra show a similar morphological progression to those of normal BSGs (Negueruela et al. iggy10 (), Fig A.2), there is no indication of emission in the Paschen series (noting that the lower Balmer transitions are seen in emission). While such a finding precludes the separation of BHGs from BSGs in the spectroscopic window in which GAIA - the Global Astrometric Interferometer for Astrophysics - will operate, the lack of wind contamination aids in the quantitative determination of the underlying stellar parameters (Sect. 4).
Regarding the classification of Cyg OB2 #12, the presence of N i
absorption lines indicates a spectral type of B3 or later, with their
comparative weakness favouring B3. This finding is consistent with
both the strength of the O i 8446Å line, which clearly lies
between B2.5Ia and B4Ia (see Fig. A.2), and the temperature sensitive
Pa15/Pa11 and Pa16/Pa11 line ratios. However, the differences between
B3 and B4 Ia stars are small - hence we suspect that the slight
discrepancies between classifications based on the 4-5000Å
and 8-9000Å windows evident for data obtained in 1992 and
2008 are unlikely to be real. Likewise, distinguishing between B5 and
B8 supergiants on the basis of I band spectra alone is particularly
difficult (Negueruela et al. iggy10 ()); hence we caution against
using the results of Sharpless (sharpless ()) and Zappala
(zappala ()) as evidence for spectroscopic variability prior to
1970. Echoing Sect. 4, changes in the physical properties of the
photosphere/wind transitional zone have negligible impact on the I
band spectral morphology - hence explaining potential discrepancies
between the spectral type of Cyg OB2 #12 determined from the I band
and the He i 4471Å:Mg ii 4481Å
Two epochs of observations from 1990
and 1992 are presented by Conti & Howarth (conti ()), who report
no variability between the spectra. Unfortunately a paucity of
spectral features and little calibration data complicate
classification from these data, although the presence of weak He i 1.031m suggests a classification of B8 Ia or earlier and
the absence of He ii 1.0124m provides an upper limit of B3
Ia. Such a result is consistent with that derived from our spectra
obtained on 1992 July 22.
A single epoch of H band spectroscopy
was presented by Hanson et al. (hansonh ()), with Br11 and
He i 1.700m lines in absorption and of equal
strength and He ii 1.693m absent. The lack of He ii
1.693m is characteristic of a B spectral type, with
EW(Br11)EW(He i) occuring for B1.5 Ia and later (at
the resln. of the spectrum in question) and only becoming 1 at B8
Ia, hence providing upper and lower limits to the spectral type
(Hanson et al. hansonh (), hansonh ()).
Hanson et al. (hansonK ()) present a
K band spectrum from 1994, in which Br and He i
2.112m are seen in absorption. For a star of the expected
luminosity of Cyg OB2 #12 and with the low S/N and resolution of the
data in question, only broad constraints are possible, with the lack
of He ii 2.189m and the presence of He i
2.112m implying a classification between B0-9 Ia.
Finally, Lenorzer et
al. (lenorzer ()) and Whittet et al. (whittet ()) present
2.35-4.1m spectra, which we may supplement with an additional
spectrum from 1996 April 4. All four display emission in Br,
Pf and , with the higher transitions seen in
absoprtion. No variability is apparent between any of these spectra;
hence we do not reproduce them here. Unfortunately, as with the
1-2m window, there is a dearth of classification features
in this region and we may only infer a spectral type between B3-9
Ia from the presence and strength of the Brackett and Pfund series
and the lack of He i photospheric lines.
To summarise - we find that the available data provides no substantive evidence for the long-term evolution of the spectral type of Cyg OB2 #12 over the past half century, although this conclusion is necessarily weaker prior to the 1990s, given the sparser dataset available. Nevertheless, this finding is entirely consistent with the long-term lightcurve. A classification of B3-4 Ia appears appropiate thoughout this period, although short term variability is present in both wind (e.g. H…) and photospheric lines (e.g. Kiminki et al. kiminki ()).
Both phenomena are common in other luminous B super-/hypergiants (e.g. Clark et al. clark10 ()) where they are assumed to be due to, respectively, time-dependent wind structure and photospheric pulsations resulting in changes in photospheric temperature. Regarding the latter, changes in the He i 4471Å:Mg ii 4481Å flux ratio have led to the conclusion that the spectral type is variable (B3-8 Ia), from which changes in the photospheric temperature (16-12kK) of Cyg OB2 #12 have been inferred; such a range is fully consistent with models of the pulsating B supergiant HD64760 (Kaufer et al. kaufer ()) and the BHGs Wd1-7 and 42 (Clark et al. clark10 ()). However we caution that the detailed non-LTE model atmosphere analysis reveals that variation in the physical structure of the photosphere/wind transitional zone can also lead to this behaviour; hence concluding that the photospheric temperature has been variable based on the current spectroscopic dataset is clearly premature.
a.2 The early-B HGs: Sco
a.2.1 Photometric variability
Sterken (sterken77 ()) and Sterken et al. (sterken ()) have previously undertaken long-term(differential) uvby Stromgren and V BLUW photometric campaigns between 1973-4 and 1982-95 respectively. They report low level (mag) quasi-periodic variability over a wide range of timescales ( days), with additional stochastic variability (mag) superimposed. (Aperiodic) low level (mag) photometric variations were also present in the Hipparcos dataset between 1989-1993 (Lefèvre et al. lef ()) and between 1979-80 (Burki et al. burki ()).
In addition to these data Sterken et al. (sterken ()) also present historical observations from the 10-19 Century, to which we may add a number of more recent data from the 20 Century (Table A.1). Sterken et al. (sterken ()) suggest that Sco is a long-term variable, reporting in the middle of the 18 Century, m4.3 and 5.4 in 1875 and 1878 and between 1890-1900. The latter values are broadly consistent with the photometric data presented in Table A.1, which span the 20 Century and provide no evidence for the long term secular variability that characterises e.g. the LBV phase. Moreover, these data also demonstrate that the near-IR continuum is similarly stable, albeit over a shorter, yr baseline. We therefore conclude that the sole evidence for significant (mag) variability is provided by historical visual estimates dating from before 1890, for which we are unfortunately unable to quantify the observational uncertainty.
Finally, we turn to the radio data, for which Bieging et al. (bieging ()) report possible variability. This mirrors the findings for Cyg OB2 #12 and we provisionally associate this behaviour with the same physical cause (Sect. 3).
a.2.2 Spectroscopic classification and variability
Given the brightness of Sco, spectroscopic observations dating from the turn of the 19 Century are reported in the literature, although a lack of accurate dates for the earlier observations complicates their interpretation. Nevertheless, a summary of these observations are presented in Table A.5; we note that the lower reddening to Sco facilitates a more homgeneous dataset of blue end spectroscopy in comparison to Cyg OB2 #12. Fortuitously, the period between 1900-1950 is well sampled and hence complements the sparse photometric dataset in this period.
This compilation reveals an absence of long-termvariability or secular evolution of spectral type over a 110 yr interval. The description of the main features of the optical spectra over this whole period are remarkably consistent; the spectra presented in Fig. A.1 for the period 1994-2009 demonstrating this stability. Nevertheless, numerous authors report line profile variability in the wind dominated P Cygni profile of H and , with the higher H i transitions repeatedly varying between emission and absorption over at least a 50 yr interval (cf. Table A.5 and refs. therein). Rivinius et al. (rivinius ()) report the rapid variability of both photospheric and wind lines between 1992-5; we consider it likely that the variability observed in the H P Cygni line between 1998-2009 (Fig. 1) reflects a continuation of this behaviour.
a.3 The early-B HGs: HD 190603
Unfortunately, photometric datasets for HD 190603 are somewhat sparse in comparison to the previous two stars, with no photometric observations reported over the last decade and a 16 yr gap in the 1980s-90s (Table A.1). Nevertheless, we find no evidence for secular variability over the 46 yr period from 1952-1998. Conversely, rapid photometric variability has been reported on several occassions by both Percy & Welch (percy ()) and Koen & Eyer (koen ()).
Likewise comparatively few spectral observations have been reported, a problem exacerbated by the lack of reduced spectra being presented. Consequently we are forced to simply present the spectral classifications reported for HD 190603 in Table A.6. These data indicate a corresponding lack of spectroscopic evolution over a 50 year period. Indeed, the description of the spectrum by both Beals (beals ()) and Merrill & Birwell (merrill33 ()) as, respectively, that of a P Cygni supergiant and a Be star, suggest a similar morphology prior to 1933. The latter authors further described variable emission lines; similar findings being reported by Rosendhal (rosendahl ()) and Rivinius et al. (rivinius ()).
a.4 The early-B HGs: HD 80077, HD 169454 and BP Cru
We next turn to the remaining early-B HGs. We refrain from tabulating their sparse photometric datasets but note that there is no evidence for secular variability in any of the three stars; for instance comparison of the optical (Hiltner hiltner (), Kilkenny et al. kilkenny ()) and near-IR (Whittet et al. whittet (), Skrutskie et al. skrutskie ()) data for HD 169454 reveal constancy over 35 and 26 yr intervals respectively. However, following the previous discussions, rapid, low amplitude photometric variability appears ubiquitous (van Leeuwen et al. vanL98 (), Sterken sterken77 () and Hammerschlag-Hensberge et al. HH76 () respectively).
As with HD 190603, Table A.6 summarises their spectral types as reported in the literature. No evidence for the long-term (secular) evolution of spectral type over timescales in excess of 30 yrs was found, with the description of HD 169454 as a B star with H i P Cygni emission lines (Merrill & Burwell merrill33 ()) suggesting spectral stability for nearly a century. In contrast line profile variability (LPV) on a timescale of days-weeks is present in all 3 stars (Knoechel & Moffat knoechel (), Rivinius et al. rivinius () and Kaper et al. kaper06 () respectively).
a.5 The early-BHGs/WNVL stars Wd1-5 and 13
The remaining early-B HGs are found within the massive young cluster Wd1, and on the basis of a restricted wavelength range (5800-8900Å) were classified as borderline BHG/very late WN stars, forming an evolutionary sequence from Wd1-5 through -13 to the WN9h star Wd1-44 (Clark et al. clark08 ()). Unfortuntely, these stars have only been observed over the past decade, but no evidence for long term variability has been found (Clark etal. clark10 ()). However, Wd1-13 is a confirmed 9.27 day massive binary, while pronounced LPV in Wd1-44 also argues for such an identification. Consequently, we suspect that all three stars to have formed as the result of close binary evolution.
a.6 The late-B HGs
Finally, we examine the eight BHGs with spectral types of B5 and later that have been identified within the Galaxy. Three - W7, W33 (both B5 Ia) and W42a (B9 Ia) - are located within the massive young cluster Wd 1 and are discussed in detail in Clark et al. (clark10 ()). Unfortunately, the long-termspectroscopic and photometric datasets for these stars are less complete than those of the early-B HGs described above, although they are sufficient to confirm the presence of rapid LPV. This behaviour, as well as short term low amplitude photometric pulsations is also present in the other examples; HD 160529 (A9-B8 Ia; Stahl et al. 160529 ()), HD 168607 (B9 Ia; Chentsov et al. chentsov (), Sterken et al. sterken99 ()), HD 168625 (B8 Ia; Sterken et al. sterken99 (); Chentsov et al. chentsov ()), HD 183143 (B7 Iae; Adelman et al. adelman00 (), Chentsov et al. chentsov ()) and HD 199478 (B8 Iae Percy et al. percy08 (), Markova & Valchev mar00 ()), suggesting that the Cygni instabilities are ubiquitous across the complete temperature and luminosity range spanned by BHGs.
This phenomenon is also found to extend to cooler temperatures, having been identified in a number of early-A (A0-2.5 Ia/Iae) stars with similar spectral morphologies to the BHGs - e.g. HD 92207 (A0 Iae; Sterken sterken77 (), Kaufer et al. kaufer97 ()), HD 223385 (A2.5 Ia; Adelman & Albayrak adelman97 (), Chentsov chentsov ()) and AS 314 (A0 Ia; Miroshnichenko miro00 ()).
However, unlike the early-B HGs, HD 160529 and 168607 demonstrate characteristic LBV photometric modulation, with the former also exhibiting correlated spectral type variability, while both HD 168625 and HD92207 show evidence for a complex, dusty circumstellar environment (Roberto & Herbst rob (); Clarke et al. clarke ()) implying recent enhanced mass loss, possibly associated with an LBV phase.
|Cyg OB2 #12|
|1954||16.670.1||14.660.04||11.470.04||-||-||-||-||-||Sharpless (sharpless ())|
|1962||17.21||14.71||11.49||8.36||6.20||4.55||-||2.81||Johnson & Borgman (johnson63 ())|
|1964||17.200.01||14.700.01||11.480.01||8.260.01||5.940.01||4.320.01||-||2.660.1||Johnson (johnson ())|
|1967||17.15||14.70||11.48||8.26||5.95||4.42||-||2.70||Wisniewski et al. (wisniewski ())|
|1968||-||-||-||8.26||5.94||4.32||-||2.66||Johnson (johnson68 ())|
|1971||-||-||-||-||-||-||-||2.60||Stein & Gillett (stein ())|
|1973||-||-||-||-||-||-||3.340.06||2.770.05||Chaldu et al. (chaldu ())|
|1972 Autumn||-||-||-||-||-||4.590.05||3.370.05||2.820.05||Voelcker (voelcker ())|
|1975-6||-||-||-||-||-||4.380.02||3.280.02||2.720.02||Harris et al. (harris ())|
|1980 Aug 26||-||-||-||-||-||-||2.800.01||Abbott et al. (ab84 ())|
|1981 July||-||-||-||-||-||3.330.01||2.720.01||Leitherer et al. (leitherer ())|
|1989 May 26-30||-||-||-||-||-||4.390.04||3.340.04||2.750.04||Torres-Dodgen et al. (torres ()|
|1990 June 6||17.180.01||14.810.01||11.460.01||-||-||-||-||-||Massey & Thompson (massey ())|
|1998 June 21||-||-||-||-||-||4.70.3||3.50.3||2.700.4||Skrutskie et al.(skrutskie ())|
|1949-1952||-||-||4.88||-||-||-||-||-||Schilt & Jackson (schilt ())|
|1954||-||-||4.9||-||-||-||-||-||Bidelman (bidelman ())|
|1958||-||-||4.86||-||-||-||-||-||Code & Houck (code ())|
|1958||-||5.03||4.3||-||-||-||-||-||Evans et al. (evans ())|
|1958 March-April||4.54||5.20||4.82||-||-||-||-||-||Westerlund (westerlund ())|
|1961||-||-||4.80||-||-||-||-||-||Buscombe (buscombe ())|
|1963||-||-||4.74||-||-||-||-||-||Andrews et al. (andrews ())|
|1963 June||4.57||5.19||4.72||-||-||-||-||-||Feinstein (f68 ())|
|1964 March||4.64||5.20||4.72||-||-||-||-||-||Feinstein (f68 ())|
|1967-68||-||-||4.71||-||-||-||-||-||Schild et al. (schild ())|
|1968 July||-||-||-||-||-||-||3.30||3.11||Schild et al. (schild71 ())|
|1973||-||-||4.734.83||-||-||-||-||-||Jaschek & Jaschek (jj ())|
|1974 April||-||-||-||-||-||3.470.03||3.270.03||3.110.03||Whittet et al. (whittet ())|
|1963 to 1976||-||-||4.73||-||-||-||-||-||Feinstein & Marraco (feinstein79 ())|
|1979 Apr. to 1980 Sept||-||-||4.664.78||-||-||-||-||-||Burki et al. (burki ())|
|1981 August 9||-||-||-||-||-||-||-||3.140.05||Abbott et al. (ab84 ())|
|1974-82||4.76||5.29||4.77||4.31||3.85||-||-||-||Fernie (fernie ())|
|1982 August||-||-||-||-||-||3.580.03||3.320.03||3.180.03||Leitherer & Wolf (lw ())|
|1984||-||-||-||-||-||3.530.02||3.310.02||3.130.03||Lopez & Walsh (lopez ())|
|2003||4.61||5.18||4.78||-||-||-||-||-||Reed et al. (reed ())|
|1999 May 22||-||-||-||-||-||3.60.3||3.30.2||3.30.2||Skrutskie et al.(skrutskie ())|
|1952-4||5.72||6.18||5.65||-||-||-||-||-||Hiltner (hiltner ())|
|1958||5.59||6.13||5.56||-||-||-||-||-||Golay (golay ())|
|1961||-||6.14||5.59||-||-||-||-||-||Bouigue et al. (bouigue ())|
|1962-70||5.66||6.14||5.62||-||-||-||-||-||Crawford et al. (crawford ())|
|1964||5.720.01||6.180.01||5.650.01||5.120.01||4.720.01||4.500.01||-||4.110.1||Johnson (johnson ())|
|1966||5.73||6.19||5.65||-||-||-||-||-||Johnson et al. (1996)|
|1968||5.69||6.15||5.60||-||-||-||-||-||Blanco et al. (blanco ())|
|1968||-||6.14||5.62||-||-||-||-||-||Lesh (lesh ())|
|1968-9||5.81||6.25||5.70||-||-||-||-||-||Burnichon & Garnier (burnichon ())|
|1972 Autumn||-||-||-||-||-||4.500.05||4.140.05||4.110.05||Voelcker (voelcker ())|
|1974||5.72||6.18||5.64||-||-||-||-||-||Nicolet (nicolet ())|
|1974 Sept.||-||-||-||-||-||4.430.03||4.240.03||4.100.03||van Breda & Whittet (vb ())|
|1974-82||5.64||6.13||5.56||5.01||4.62||-||-||-||Fernie (fernie ())|
|1975 Oct.||-||-||-||-||-||-||-||4.10.2||Sneden et al. (sneden ())|
|1976-78||-||6.180.02||5.630.02||5.080.02||4.710.02||-||-||-||Moffett & Barnes (moffett ())|
|1979-82||-||6.18||5.64||-||-||-||-||-||Percy & Welch (percy ())|
|1998 May 11||-||-||-||-||-||4.420.26||4.260.20||4.040.03||Skrutskie et al. (skrutskie ())|
Note that where available errors and exact dates of observations are presented. For brevity we have not replicated the B band lightcurve of Gottlieb & Liller (gottlieb ()) for Cyg OB2#12 here. Observations between 1945-1951 by Cousins (cousins ()) suggest that Sco was variable with low amplitude (mag) between 1945-51, although details of the filter set used are not described. Sterken (sterken77 ()) report 35 epochs of differential ubvy photometry between 1973-1974 for Sco, with a further 399 epochs between 1982-95 presented by Sterken et al. (sterken ()); throughout these periods minimal variability was observed ( ubvy 0.1mag.). Finally Percy & Welch (percy ()) report 52 epochs of BV band photometry between 1979 May 30 and 1982 July 1 indicating t to be a short period variable. 3 observations in this period; no details of band pass Phot. magnitude possibly sourced from HD catalogue? 3 observations over this period. 43 observations over this period.
Cyg OB2 #12
|2.72 0.01||Leitherer et al. (leitherer ())|
|2.345||2.6350.02||Harris et al. (harris ())|
|3.013||2.4050.02||Harris et al. (harris ())|
|3.108||2.3750.02||Harris et al. (harris ())|
|3.420||2.2810.02||Harris et al. (harris ())|
|3.45||2.370.05||Abbott et al. (ab84 ())|
|3.50||2.280.05||Chaldu et al. (chaldu ())|
|3.57||2.280.02||Leitherer et al. (leitherer ())|
|3.57||2.1700.05||Torres-Dodgen et al. (torres ())|
|3.58||2.2170.02||Harris et al. (harris ())|
|3.820||2.1780.02||Harris et al. (harris ())|
|4.29||2.0570.118||MSX (Egan et al. egan ())|
|4.80||2.370.10||Abbott et al. (ab84 ())|
|4.97||2.060.02||Leitherer et al. (leitherer ())|
|8.28||1.9460.044||MSX (Egan et al. egan ())|
|10.2||2.030.10||Abbott et al. (ab84 ())|
|10.9||1.950.07||Leitherer et al. (leitherer ())|
|12.0||1.6420.00||IRAS PSC (1985)|
|12.13||1.9000.056||MSX (Egan et al. egan ())|
|14.65||1.7130.067||MSX (Egan et al. egan ())|
|20.0||1.540.10||Abbott et al. (ab84 ())|
|21.34||1.5750.079||MSX (Egan et al. egan ())|
|25.0||1.204||IRAS PSC (1985)|
|3.45(L)||2.980.03||Leitherer & Wolf (lw ())|
|4.7(M)||2.900.03||Leitherer & Wolf (lw ())|
|8.28||2.624||MSX (Egan et al. egan ())|
|12.13||3.416||MSX (Egan et al. egan ())|
|14.65||3.560||MSX (Egan et al. egan ())|
|3.6||3.960.18||Sneden et al. (sneden ())|
|4.9||3.890.20||Sneden et al. (sneden ())|
|8.28||3.680.04||MSX (Egan et al. egan ())|
|12.13||3.470.08||MSX (Egan et al. egan ())|
|14.65||3.460.09||MSX (Egan et al. egan ())|
Cyg OB2 #12
|1980 March 22||-||-||213||-||-||-||-||-||-||Wendker & Altenhoff (wendker ())|
|1980 May 22+23||-||-||-||-||-||-||-||3.20.3||-||Abbott et al. (abbott ())|
|1981 Oct. 16||-||-||-||6.02.0||-||-||-||3.40.2||-||Bieging et al. (bieging ())|
|1982 March 27||-||-||-||4.50.3||-||-||-||-||-||White & Becker (white ())|
|1982 Aug. 26||-||-||-||-||-||-||-||2.20.2||2.00.2||Bieging et al. (bieging ())|
|1983 Aug./Oct.||-||-||-||-||-||-||4.00.3||-||2.10.3||Becker & White (becker ())|
|1993 May 1||-||-||-||-||-||6.060.07||3.940.07||-||Waldron et al. (waldron ())|
|1994 Sept.||-||-||-||7.70.3||4.740.14||-||-||4.000.20||-||Scuderi et al. (scuderi ())|
|1994 Oct.||-||-||-||12.00.2||7.400.08||-||-||4.000.20||-||Scuderi et al. (scuderi ())|
|1995 April 27||-||22.90.6||-||11.30.1||7.180.04||-||3.640.12||-||-||Contreras et al. (contreras ())|
|1999 June 28||-||22.90.6||-||9.01.5||5.90.1||-||4.20.1||-||-||Contreras et al. (contreras04 ())|
|1981 May 7||-||-||-||-||-||-||1.20.3||-||-||Bieging et al. (bieging ())|
|1984 March 9||-||-||-||-||-||-||2.00.2||-||-||Bieging et al. (bieging ())|
|1984 April 3||-||-||-||4.30.1||-||-||1.90.2||-||-||Bieging et al. (bieging ())|
|1990 Sept. 12-14||23.02.4||-||-||-||-||-||-||-||-||Leitherer & Robert (leitherer91 ())|
|2006||-||-||-||-||-||2.4||-||-||-||Benaglia et al. (benaglia ())|
|1979 July 13||-||-||-||-||-||-||0.5||-||-||Abbott et al. (abbott ())|
|1994 Oct.||-||-||-||0.70.2||0.70.1||-||-||0.580.06||-||Scuderi et al. (scuderi ())|
Note that for brevity the wavelengths are presented to the nearest 0.1cm, with exact values given in the papers in question.
|1954||4100-6600Å||A0 I||H in emission||Morgan et al. (morgan ())|
|1957||8-9000Å||B5 Ia||-||Sharpless (sharpless ())|
|1970||near IR||B8 Ia||-||Zappala (zappala ())|
|1971||?||-||H in emission||Bromage (bromage ())|
|1973||5-8700Å+||-||H in emission||Chaldu et al. (chaldu ())|
|1977 July||3840-4480Å||B8 Ia||Variable H||Souza & Lutz (souza ())|
|1980 Autumn||H||-||H in emission||Leitherer et al. (leitherer ())|
|1981||4000-6600Å||-||H+H in emission||Hutchings (hutchings ())|
|1989-90 summer +||encompasses H||B5 Ia||H+H in emission,||Massey & Thompson (massey ())|
|‘few years earlier’||+ 4089-4686Å||B5 Ia||H+H infilled|
|1990 August||0.98-1.10m||B3 Ia,B8 Ia||-||Conti & Howarth (conti ())|
|1992 July||0.98-1.10m||B3 Ia, B8 Ia||-||Conti & Howarth (conti ())|
|1992 July 17||4036-4836Å||B4-5 Ia||H+H in absorption||This work|
|5765-9586Å||B3 Ia||H in emission|
|1994 Sept.||2-2.2m||B0.5 Ia, B9 Ia||-||Hanson et al. (hansonK () )|
|1995 July 17||6366-6772Å||-||H in emission||This work|
|1995 Dec. 23||2.35–8m||B3 Ia, B9 Ia||Br, Pf+ in emission||Whittet et al. (whittet ())|
|1996 April 4||2.35–8m||B3 Ia, B9 Ia||Br, Pf+ in emission||This work|
|1996 Oct. 17||2.35–8m||B3 Ia, B9 Ia||Br, Pf+ in emission||Whittet et al. (whittet ())|
|1997 June||1.66-1.72m||B1.5 Ia, B8 Ia||-||Hanson et al. (hansonh ())|
|1998 Apr-May||2.35–8m||B3 Ia, B9 Ia||Br, Pf+ in emission||Lenorzer et al. (lenorzer ())|
|1998 Aug. 09||3940-5737Å||B4 Ia||H in emission||This work|
|6366-6772Å||-||H in emission|
|1999 July 10||5500-7700Å||B Ia||H in emission||Kiminki (2010, priv. comm.)|
|2000 July 10||5500-7700Å||B Ia||H in emission||Kiminki (2010, priv. comm.)|
|2000 Sept. 18||3600-5200Å||B3 Iae||H infilled||Kiminki et al. (kiminki ())|
|2000 Sept. 23||4000-4750Å||B4 Ia||H+H in absorption||This work|
|6340-6740Å||-||H in emission|
|2001 June 21||4542-7939Å||B5 Ia||Balmer line emission||Klochkova & Chenstov (klochkova ())|
|2001 August 24||3800-4500Å||B6 Iae||H+H in absorption||Kiminki et al. (kiminki ())|
|2001 Sept. 01||3800-4500Å||B8 Iae||H+H in absorption||Kiminki et al. (kiminki ())|
|2003 April 12||5273-6764Å||B5 Ia||H emission||Klochkova & Chenstov (klochkova ())|
|2007 Aug. 22||7600-8900Å||B3 Ia||-||This work|
|2007 Aug. 30||5600-6800Å||B Ia||H in emission||Kiminki (2010, priv. comm.)|
|2008 July 22||4000-5270Å||B4 Ia||H in emission||This work|
|H+H in absorption|
|6450-7150Å||-||H in emission|
Note that a parameter is listed in italics if not explicitly given (wavelength of observation) or the data from which it is derived was not presented. Parameters given in bold are from this work. H is variable between the two epochs of observations presented by Klochkova & Chenstov (klochkova ()) as H and are between the 2000-8 spectra presented here, while Conti & Howarth (conti ()) report no variability between 1990-2 in the m region. Selected spectra between 1992-2008 are plotted in Fig. A.1.
|1896||-||-||H in emission||Pickering (pickering ())|
|1891-1899||4000-4900Å||B1p||P Cygni profiles in H,||Cannon (cannon ())|
|all other lines in absorption|
|(He i 4009, O ii 4349, C ii 4267)|
|1924||4100-4900Å||B(2)eq||P Cygni profiles in H,||Merrill et al. (merrill25 ())|
|H in absorption|
|1903-1929||-||B1e Ia||-||Bidelman (bidelman ())|
|1926-1930||3950-4900Å||B1pe||-||Rimmer (rimmer ())|
|30/07/34||-||-||P Cygni profiles in H and D3 He i||Merril & Burwell (merrill43 ())|
|1938||-||-||Listed as variable||Payne-Gaposchkin &|
|Gaposchkin (pg ())|
|1954-58||3800-6600Å||B1.5 Ia||No appreciable difference||Code & Houck (code ())|
|from Cannon (cannon ())|
|26/04/58 to 03/09/60||3900-4500Å||B1e Ia||Buscombe et al. (buscombe ())|
|22/08/61||6200-6600Å||-||H in emission||Jaschek et al. (jaschek ())|
|7/08/66||3800-4700Å||B1 Iae||H in emission, H,||Buscombe & Kennedy|
|& He i 4471 in absn.||(buscombe68 ())|
|6/07/60||3100-6750Å||B1 Ia||P Cygni profiles in||Hutchings (hutchings68 ())|
|9/08/60||H, & He i5876, 6678|
|10/08/60||Remaining lines in absorption|
|28/06/66||Variability in H profile, but|
|27/07/66||no global spectral evolution|
|1969||blue end||B1.5Ia||-||Schild et al. (schild ())|
|21/04/65, 20/05/67||3500-4900Å||B1 Ia||P Cygni profiles in H,||Jaschek & Jaschek|
|18/03/68, 14/08/70||Variable He i, N ii & O ii phot. lines||(jj ())|
|6/08/68||5800-6700Å||-||P Cygni profiles in||Rosendhal (rosendahl ())|
|H, He i5876, 6678|
|16/05/73 to 10/07/73||blue end||-||No apparent variability in spec. type||Hutchings et al. (hutchings76 ())|
|28/04/74 to 1/05/74||3800-4400Å||B1.5 Ia||No variability above 20% in||Walborn (walborn ())|
|28/04/75||line strength reported|
|1974 Oct. & 1975 Mar.||H||-||Variation in H P Cygni profile||Dachs et al. (dachs ())|
|26/06/72 to 14/08/75||3400-6700Å||B1Ia||Variable P Cygni profiles in||Sterken & Wolf|
|H, & He i5876, 6678||(sterken78 ())|
|No secular evolution in spec. type|
|1984||4300-4900Å||B1.5 Ia||Variable P Cygni profiles in||Lopez & Walsh|
|5500-6800Å||H, & He i6678||(lopez ())|
|1995||3450-8630Å||B1.5 Ia||Variable P Cygni profiles in||Rivinius et al. (rivinius ())|
|1990 to 1994||4000-6740Å||H, , He i 6678 & Fe iii|
|H, in absorption|
|No changes in He i:Si iii ratio|
|& hence spec. type|
|1998 Oct. 7||4000-8950Å||B1.5 Ia||H, , He i 6678 P Cygni, higher||This work|
|Balmer series in absorption|
|1999 June-July||4000-8950Å||B1.5 Ia||H, , He i 6678 P Cygni, higher||This work|
|Balmer series in absorption|
|2002 March-May||5810-7205Å||-||Variable H P Cygni||Morel et al. (morel ())|
|2003 May-June||4000-7000Å||B1.5 Ia||-||Crowther et al. (pacBSG ())|
|2005 April 24||3800-6800Å||B1.5 Ia||H, & He i6678 P Cygni, higher||This work|
|Balmer series in absorption|
|2006 Feb. 16||3933-7985Å||B1.5 Ia||H, & He i6678 P Cygni, higher||This work|
|Balmer series in absorption|
|2009 March 06||3060-5600Å||B1.5 Ia||H P Cygni, higher Balmer||This work|
|series in absorption|
Note that a parameter is listed in italics if not explicitly given (wavelength of observation) or the data from which it is derived was not presented. Selected spectra between 1992-2009 are plotted in Fig. A.1. 6 observations in this period, 44 observations in this period, 233 observations in this period, 57 observations in this period, 14 observations in this period. 6 observations in this period.
HD 80077 (B2.5 Ia)
Morgan et al. (morgan55 ())
|Buscombe & Kennedy (buscombe69 ())||1963-67|
|Nordstrom (nord ())||1975|
|Moffat & Fitzgerald (moffat ())||1977|
|Knoechel & Moffat (knoechel ())||1977|
|Negueruela et al (in prep.)||2008|
|HD 169454 (B1 Ia)|
|Morgan et al. (morgan55 ())||1942-55|
|Hiltner (hiltner ())||1956|
|Code & Houck (code ())||1958|
|Botto & Hack (botto ())||1962|
|Hutchings (hutchings70 ())||1968|
|Wolf & Stahl (wolf85 ())||1972-4|
|Walborn (walborn80 ())||1973-5|
|Hutchings (hutchings76 ())||1973-5|
|Rivinius et al. (rivinius ())||1992-5|
|Hanson et al. (hansonK ())||1994|
|Hanson et al. (hansonh ())||1997|
|Groh et al. (groh ())||2001-4|
|HD 190603 (B1.5 Ia)|
|Morgan et al. (morgan55 ())||1942-55|
|Ahmad (ahmad ())||1952|
|Hiltner (hiltner ())||1956|
|Slettebak (slettebak ())||1956|
|Lesh (lesh ())||1968|
|Hutchings (hutchings70 ())||1968|
|Walborn (walborn71 ())||1969|
|Hutchings (hutchings76 ())||1973-5|
|Bisiacchi et al. (bisiacchi ())||1975-6|
|Andrillat et al. (andrillat ())||1988-93|
|Rivinius et al. (rivinius ())||1990-1|
|Lennon et al. (lennon ())||1990|
|Hanson et al. (hansonK ())||1994|
|Blum et al. (blum ())||1996|
|Hanson et al. (hansonh ())||1998|
|Crowther et al. (pacBSG ())||1992-2003|
|Markova & Puls (markova ())||2007|
|BP Cru (B1 Ia)|
|Vidal (vidal ())||1973|
|Bord (bord ())||1974|
|Hammerschlag-Hensberge et al. (HH ())||1975|
|Parkes et al. (parkes ())||1977-78|
|Kaper et al. (kaper95 ())||1984|
|Kaper et al. (kaper06 ())||1996|
All classifications made in the optical(4-6000Å) band unless otherwise noted - marginally earlier spectral type of B2 Iae given. multiple spectra in this period. observations within the near-IR (1-2.2m) window. marginally later spectral type of B1.5 or B2 Iae given.
Appendix B Spectropolarimetry
Considering a simple ‘core–halo’ wind model for heuristic purposes, electron scattering of photospheric radiation in an asymmetric outflow will generate a grey intrinsic linear polarization, while emission lines formed in the wind will see a smaller scattering optical depth, and so are expected to be less polarized (e.g., McLean mclean79 ()). This will result in depolarization through the line, although the addition of an interstellar-polarization vector means that this ‘line effect’ often manifests in other ways, including an observed increase in degree of polarization, .
Previously unpublished spectropolarimetric observations of Cyg OB2 #12 and Sco were obtained as part of the investigations reported by Harries, Howarth & Evans (harries02 ()). Cyg OB2 #12 was observed on 1995 July 17 (using the WHT with ISIS spectrograph; ), and Sco on 1997 Jun 5 (AAT with RGO spectrograph; ), with the data acquisition and reduction essentially as in the manner described by Harries & Howarth (harries96 ()). Results are displayed in Fig. B.1.
For Cyg OB2 #12 there is no compelling evidence of a change in polarization through the H emission line (although there is a hint of a possible increase in ), and hence no strong indication of large-scale asymmetry in the electron-scattering envelope. Similarly, there is no strong evidence for temporal polarimetric variability; although the spread in published -band photopolarimetric measurements is somewhat larger than their quoted formal errors, the results presented by Schmidt et al. (schmidt92 (); %, ) Whittet et al. (whittet92 (); %, 117), and Kobulnicky, Molnar & Jones (kobulnicky94 (); %, ) are all broadly consistent with the present results, as are earlier O-band measurements reported by Kruszewski (kruszewski71 ()) and by McMillan & Tapia (mcmillan ()). We note, however, an anomalous result reported by Schulz & Lenzen (schulz (); %, ); their observations of two further stars agree well with measurements presented by Schmidt et al. (schmidt92 ()).
In spite of its brightness, Sco has been much less extensively observed, with the only available point of direct comparison being the narrow-band (Å) H polarimetric measurement of %, reported by McLean & Clarke (mcleanclarke ()).111111Matthewson & Ford (mathewson ()) report a blue-band measurement of %, .) This is in agreement with our results; however, our spectropolarimetry clearly reveals a significant, position-angle rotation through the emission line.
Physically, the line effect may be associated with large-scale, axisymmetric structures (such as might result from rapid rotation), or irregular ‘clumps’. For a star with parameters similar to those of Sco, summarized in Table 3, the critical equatorial rotation velocity121212 The velocity at which the outward centrifugal force equals the inward gravitational force. is kms. With a measured of 45 kms (Table 3), the intrinsic rotation is probably substantially subcritical ( with 95% confidence). A rotationally-induced axisymmetric departure from spherical symmetry (i.e. a ‘disc’ or ‘polar’ wind) therefore seems a priori improbable as the origin of the line effect in this star. A more likely cause of the observed change in polarization is the presence of transient large-scale inhomogenities, or ‘clumps’, as has been proposed for P Cygni and AG Car on the basis of spectropolarimetric observations by e.g. Nordsieck et al. (nordsieck ()) and Davies et al. (davies ()); indeed CMFGEN model atmosphere analysis of both these stars confirms the presence of the significant wind clumping (Najarro et al. paco01 (), Groh et al. agcar ()). Therefore, by direct analogy, an additional observational test of this hypothesis for Sco would be the detection of time-dependent variations in polarization (under the assumption that there is no prefered geometry for the clumping), noting that spectroscopic observations are already strongly indicative of the presence of transient wind structure (Appendix A).
Appendix C The age of NGC 6231 and Sco OB1
Given its proximity (1.64kpc; Sana et al. sana08 ()), NGC 6231, its host association Sco OB1, and the massive stars located within both have been the subject of numerous multiwavelength studies. Here we review these data in order to constrain the evolutionary history of both cluster and association to better understand the nature of Sco. Photometric studies by Baume et al. (baume ()) suggest a significant age spread within NGC6231, with star formation apparently commencing Myr ago, culminating in the formation of the massive stellar cohort Myr ago. The latter age is consistent with the findings of Sana et al. (sana06 (), sana07 ()), who report a similar value of Myr from an analysis of the photometric data of (X-ray selected) pre-MS stars. An analagous study of the wider Sco OB1 association by Perry et al. (per ()) supports a non coeval star formation history for this region, yielding an age of Myr.
Catalogues of the OB stellar content of both NGC 6231 and Sco OB1 are provided by Sana et al. (sana06b ()) and Ankay et al. (ankay ()) respectively, and allow for the individual placement of stars on the HR diagram presented in Fig. 13. For those stars in Sana et al. we adopted the luminosities given by these authors and utilised the spectral type/temperature relation of Martins et al. (martins05 ()). The temperatures and luminosities of the stars in Ankay et al. were again determined via the callibrations of Martins et al., with reddening for individual stars calculated via the intrinsic colours of Martins & Plez (martins06b ()). Finally, given their rather evolved nature, we adopted the results of the tailored non-LTE analysis of Crowther & Evans (pac6231 ()) for HD 151804 and 152408.
Comparison of these data to non-rotating and rotating Geneva isochrones clearly indicate that the region as a whole appears non coeval, at first glance being consistent with a spread of ages of between Myr. Such a conclusion is supported by inspection of the properties of individual stars in both association and cluster, although interpretation is complicated by possible contamination of the latter by the former if, as seems likely, Sco OB1 hosts a younger population than is found in NGC 6231.
A large number of Main Sequence (MS) stars are found within NGC 6231, with the O8 V companion in the binary HD 152234 apparently being the earliest and defining the MS turnoff; consistent with an age of 5 Myr. While an O6 V companion to the WC7 star WR79 has been reported (Hill et al. hill ()) the 126 day binary period of HD 152234 will have ensured that neither component in this system will have interacted, while the 8.89 day period of WR79 indicates that significant mass transfer to the secondary may have occured (e.g. Petrovic et al. pet ()). Encouragingly, the O9.7Ia primary of HD152234 also lies on the 5 Myr isochrone for rapid initial rotation, with the current vsini also being consistent with this placement (Fraser et al. fraser ()).
Building on this approach, we find that HD 152219 (O9.5 III + B1-2 III-V) and HD 152134 (B0.5 Ia) lie upon the 5 Myr evolutionary track for non-rotating stars, with - sequentially - HD 326331 (O8 III((f))), HD 152247 (O9 III), HD 152249 (O9 Ib((f))) and HD 152234 (O9.7 Ia + O8 V) following the rotating track. In both cases the systematic progression to later spectral types with increasing luminosity class suggest these are real evolutionary sequences (cf. Cyg OB2; Negueruela et al. iggy ()).
However, there are a number of stars within NGC 6231 and Sco OB1 that appear incompatable with a Myr population: the O8 Iafpe/WN9ha stars HD 151804 and 152048 and the O5.5-7.5 III stars HD 151515, 152723, 152233 and 152248. Regarding the former pair, Bohannan & Crowther (bohannan ()) suggest a close physical kinship between these objects, apparently precluding an evolution through the cooler late-O/early-BSG phase present in NGC 6231. Of the mid-O giants, HD 151515 and 152723 appear somewhat subluminous for their spectral type but the properties of the O5.5 III(f) + O7.5 and O7.5 III(f) + O7 III(f) binaries HD 152233 and 152248 (Sana et al. sana08 (), sana01 ()) clearly support the assertion that a younger population is present; although the dymanical masses for both components of the latter system appear somewhat lower than expected ( and respectively).
We therefore conclude that comparison of NGC 6231 to Sco OB1 implies that the respective stellar populations are non coeval. However, with the exception of HD 152233 and HD 152248, the massive stellar population of NGC6231 appears consistent with a single burst of star formation 5 Myr ago. Indeed, no early-mid O MS stars consistent with a younger population appears present within NGC 6231, while the population of late-O/early-B MS stars that are present are systematically displaced redwards from the 3 Myr isochrones. Therefore, if they are members of NGC 6231, we cannot exclude the possibility that, with spectral types O8, HD 152233 and HD 152248 are in fact bona fide blue stragglers; having evolved via a different pathway from the majority of stars and hence that NGC 6231 is truly coeval. In this respect it would therefore closely resemble Cyg OB2, for which Negueruela et al. (iggy ()) arrived at a comparable conclusion.
Appendix D Spectral fits to Sco and HD190603
We present additional model fits to Sco based on the FEROS (3700-8850Å) optical data (not presented in the paper version due to reasons of space).