On the nature of the Be star HR 7409 (7 Vul)
HR 7409 (7 Vul) is a newly identified Be star possibly part of the Gould Belt and is the massive component of a 69-day spectroscopic binary. The binary parameters and properties of the Be star measured using high-dispersion spectra obtained at Ondřejov Observatory and at Rozhen Observatory imply the presence of a low mass companion (). If the pair is relatively young ( Myr), then the companion is a K V star, but, following another, older evolutionary scenario, the companion is a horizontal-branch star or possibly a white dwarf star. In the latter scenario, a past episode of mass transfer from an evolved star onto a less massive dwarf star would be responsible for the peculiar nature of the present-day, fast-rotating Be star.
keywords:stars: emission-line, Be – stars: individual: HR 7409
The B5 star HR 7409 (7 Vul, BD+19 4039, TD1 24807, HD 183537, HIC/HIP 95818) was noted for its broad helium lines and other unspecified spectroscopic peculiarities (Lesh, 1968). The broad helium and magnesium line profiles show that HR 7409 has a high rotation velocity km s (Wolff, Edwards, & Preston, 1982; Abt, Levato, & Grosso, 2002). Hill, Hilditch, & Pfannenschmidt (1976) found it to be photometrically variable ( mag) but with an unknown period, although Hipparcos photometry only shows possible variations of 0.009 mag and an “unsolved” periodicity of 0.59 d during the survey time line (Koen & Eyer, 2002). Using the same data Molenda-Żakowicz (2002) measured variations of 0.013 mag with a period of 2.72 d. The ultraviolet (UV) spectrum obtained with the TD-1 satellite (Jamar et al., 1976) shows a B5-type star intermediate to main-sequence and supergiant (Cucchiaro et al., 1980). The UV photometric measurements obtained with the Astronomical Netherlands Satellite (ANS, Wesselius et al., 1982) are consistent with TD-1 flux measurements (Thompson et al., 1978) and show the effect of interstellar reddening (Savage et al., 1985; Friedemann, 1992) consistent with its proximity to the Galactic plane (). The ANS photometry also shows evidence of variability.
HR 7409 originally figured among stars assembled as a likely open cluster around 4 and 5 Vul (Meyer, 1903, 1905) and later known as Collinder 399 (Collinder, 1931). After the Collinder 399 membership was reduced to six objects that excluded HR 7409 (Hall & Vanlandingham, 1970), the increased precision of parallax and proper motion measurements (Hipparcos, Tycho, Tycho2) allowed Baumgardt (1998) and Dias, Lépine, & Alessi (2001) to conclude that Collinder 399 is not a real cluster. In summary, HR 7409 is not part of a cluster, but it may belong to a local system or association referred to as the Gould Belt (Lesh, 1968).
Plaskett et al. (1921) initially listed HR 7409 as a new spectroscopic binary but additional radial velocity measurements (Plaskett & Pearce, 1931) contradicted their initial assessment. Plaskett et al. (1921) noted the “nebulous” helium lines while broad hydrogen lines were also noted by Plaskett & Pearce (1931). Both observations were evidence of a high rotation velocity. A few additional radial velocity measurements that differ from these early measurements are available in the literature (Duflot, Figon, & Meyssonnier, 1995; Fehrenbach et al., 1996) underlining difficulties in measuring radial velocities in fast rotating B stars. On the other hand, Abt & Cardona (1984) did not consider HR 7409 to be in a binary.
Although HR 7409 was known to be peculiar, a detailed spectroscopic investigation of a possible link to the Be phenomenon is, so far, lacking. Slettebak (1976) examined the role played by rotation on the Be phenomenon (see the review by Porter & Rivinius, 2003) and concluded that although Be stars are apparently rotating near critical velocity, other mechanisms are possibly responsible for episodes of mass loss. In fact, Frémat et al. (2005) found that the average rotation rate is 88% of the critical velocity, while Cranmer (2005) concluded that Be stars are sub-critical rotators.
We present in Section 2 a series of H high-dispersion spectra showing that HR 7409 is a Be star in a single-lined spectroscopic binary. The H emission appears variable on a time-scale of years. We constrain the parameters of the Be star using the ultraviolet-to-optical spectral energy distribution (Section 3.1) and Balmer line profiles (Section 3.2). Next, we measure the binary parameters (Section 4.1) and mass function of the companion (Section 4.2) while attempting to retrace the prior evolution of the system. We summarize and conclude in Section 5.
Table 1 lists astrometric and photometric measurements of HR 7409. The ultraviolet (ANS), optical (UBV), and infrared (2MASS) photometry and the Hipparcos parallax help calculate the extinction-corrected absolute luminosity. The ANS magnitudes are converted to flux measurements using
|Parameter||Value||Ref.111References: (1) Perryman et al. (1997); (2) van Leeuwen (2007); (3) Wesselius et al. (1982); (4) Crawford, Barnes, & Golson (1971); (5) Mermilliod (2006); (6) Skrutskie et al. (2006); (7) Savage et al. (1985); (8) Friedemann (1992); (9) Neckel & Klare (1980); (10) Guarinos (1992).|
|( mas yr)||2.950.46, 1.380.43||1,2|
|( mas yr)||16.890.54, 15.840.55||1,2|
|[15515 nm] (mag)||4.2610.014||3|
|[18015 nm] (mag)||4.4720.018||3|
|[22020 nm] (mag)||4.9280.014||3|
|[25015 nm] (mag)||5.1160.011||3|
|[33010 nm] (mag)||5.5370.010||3|
|(mag)||0.06, 0.07, 0.09, 0.08||7,8,9,10|
We observed HR 7409 using the coudé spectrograph attached to the 2m telescope at Ondřejov Observatory (Šlechta & Škoda, 2002). We obtained the spectroscopic series using the 830.77 lines per mm grating with a SITe CCD, with the slit width set at , resulting in a spectral resolution of and a spectral range from 6254 to 6763 Å. We verified the stability of the wavelength scale by measuring the wavelength centroids of Oi sky lines. The velocity scale remained stable within 1 km s. Table 2 presents our observation log.
|UT Date||UT Start||UT Date||UT Start||UT Date||UT Start||UT Date||UT Start|
We also obtained a series of spectra on UT 2010 Nov 15 using the coudé spectrograph attached to the 2m telescope at Rozhen National Astronomical Observatory (Bulgaria). We used the 632 lines per mm grating with a SITe CCD. We set the slit width at resulting in a spectral resolution of , 20 000, 17 000, and 16 000 at four tilt angles centred on H, H, H, and H, respectively.
All spectra were wavelength calibrated with a ThAr comparison arc spectra obtained shortly after each exposure. The telluric features in the H spectra were removed using a fast-rotating B star template (HR 7880). The data were reduced using standard IRAF procedures.
The first spectrum obtained on 2007 May 23 revealed a fast rotating star with a H emission core typical of Be stars. Spectra obtained in the following months and years show the emission component decreasing in strength. The Ondřejov spectra also show the rotationally broadened lines He i6678, Si ii6347.109,6371.371, and Ne i6402.246.
3 Properties of the Be star
We analysed the spectral energy distribution (SED) and line profiles using a grid ( K) of line-blanketed spectra in non-local thermodynamic equilibrium (non-LTE “BSTAR” grid, Lanz & Hubeny, 2007). We supplemented this grid with models ( K) from a grid of spectra in local-thermodynamic equilibrium (LTE, Castelli & Kurucz, 2003). We selected spectra with solar abundances from both model grids. However, the effect of gravitational darkening and geometric distortion on parameter measurements of fast-rotating B stars such as HR 7409 are sizeable. Frémat et al. (2005) and Lovekin, Deupree, & Short (2006) investigated the effect of near critical equatorial rotation velocity on mean effective temperature and surface gravity measurements. The magnitude of the effect depends on the projection angle as well as the fraction of the critical velocity attained (), and may amount to an underestimation of the temperature by 10% and surface gravity by 0.2 dex.
We estimated the mean stellar parameters by (1) fitting the infrared to ultraviolet SED, and (2) fitting the Balmer line series from H to H.
3.1 Spectral energy distribution
The SED is affected by extinction in the interstellar medium. The colour-excess measurements range from to mag (Table 1), i.e., to 0.28 mag assuming . The two lowest measurements () were obtained by modelling the 2200Å bump in the five-channel ANS ultraviolet photometry. The two highest measurements () were obtained assuming mag for a B5 V type star. HR 7409 lies toward the Vulpecula rift, an extinction wall associated with the Vulpecula molecular cloud and rising at a distance of 0.3 kpc (Fresneau & Monier, 1999), and in a region of colour-excess .
We fitted the SED with model spectra assuming and variable temperature and extinction. The SED comprises eleven data points (Table 1) with equal weights assigned to them. We employed the parametrized extinction curves () from Cardelli, Clayton, & Mathis (1989). A minimum effective temperature K is obtained by setting . Allowing the temperature to increase to 14 000, 15 000, and 16 000 K, we found that the best-fitting increased to 0.053, 0.087, and 0.117, respectively. Varying the temperature and the colour-excess simultaneously the best-fitting parameters are:
The LTE and non-LTE models at 15 000 K delivered consistent solutions for the colour-excess, and 0.087, respectively. Figure 1 shows the infrared to ultraviolet SED of HR 7409 using the non-LTE model fit at the lower edge of the non-LTE grid (15 000 K).
The extinction-corrected magnitude is . Therefore, adopting the revised Hipparcos parallax of van Leeuwen (2007), the distance modulus is
and the de-reddened absolute V magnitude is , somewhat brighter than for a normal B4 V star (). Lamers et al. (1997) found that absolute visual magnitudes of O and B stars based on Hipparcos parallaxes correlate with rotation velocity and may be up to 1.5 mag brighter than inferred from the apparent spectral types.
3.2 Line profile analysis: Balmer lines
Figure 2 shows H spectra obtained 3.4 years apart and illustrating extreme ranges in observed line profiles. Figure 3 shows the evolution of the line equivalent width (EW) during that period. The EW was measured using a Å window; the weakening of the emission wings over time results into deeper absorption and larger EW. HR 7409 was caught during a declining phase. Because disc variability may occur on time scales of years (Clark, Tarasov, & Panko, 2003) or decades (Hubert, 2007), continuing monitoring should assist us in capturing the next activity episode. The absence of central emission, but, instead, the broad shoulder and narrow line core suggest the onset of a “shell” phase (see Steele, Negueruela, & Clark, 1999).
Next, we fitted the Balmer line spectra observed at Rozhen Observatory using the “BSTAR” model grid with HeH. The model line profiles were convolved with a rotational broadening function with km s and limb darkening coefficient (see Cranmer, 2005). As discussed earlier, we neglected the effect of gravity darkening. Both model and spectra are normalized at km s and we excluded the line cores ( km s) from the fit. Figure 4 shows the best model-fit to the Balmer line series (H to H):
Parameter estimations based on the Balmer line wings and on the SED are both possibly affected by the fast rotation of the star. The small statistical errors in the measurements based on the line profiles are almost certainly underestimating the true errors and do not encompass systematic effects. Although the apparent parameters are estimated conservatively as
and are marginally consistent with the published spectral type B5 V, the true spectral-type is possibly earlier than this by at least one subtype. We propose the classification B4-5 III-IVe. Molenda-Żakowicz (2002) measured and using ubvy data, in agreement with our results. Following the isochrones of Schaller et al. (1992) for models in the () plane as well as the () plane, HR 7409 is a star with an age of 50-80 Myr old nearing the end of its main-sequence life. The estimated age ignores, at this stage, the possibility of past binary interaction.
4 Binary parameters and nature of the companion
We measured radial velocities using narrow H line core showing, as suspected in earlier investigations, that HR 7409 resides in a binary (Table 3). The velocities were obtained by fitting Voigt profiles to the central 10 pixels and we applied the heliocentric velocity correction. The H, H, and H velocities measured at Rozhen at a single epoch differ from the H velocity by up to km s. The zero point of the velocity scale appears uncertain although all measurements obtained with H are internally consistent. Figure 5 shows the periodogram and H radial velocity measurements phased on the orbital period. The velocity residual is 1.3 km s and is commensurate with the expected velocity accuracy. In the following we identify the Be star with the subscript “A” and the unseen companion with “B”.
4.1 Binary parameters
We fitted the H radial velocity measurements and simultaneously constrained the systemic velocity , the velocity semi-amplitude , the eccentricity , and the angle of passage of star A at (passage at periastron). Table 4 lists the best-fitting orbital parameters.
Adopting and , i.e., , the critical velocity is km s (assuming , see Cranmer, 2005). Therefore, the measured rotation velocity limits the inclination to , or .
|(2450000+)||(km s)||(2450000+)||(km s)||(2450000+)||(km s)||(2450000+)||(km s)|
4.2 Nature of the companion and evolutionary scenarios
We may now constrain the nature of the binary companion by calculating the mass function:
and solving iteratively for the secondary mass :
where . The orbit and the Be star rotation are almost certainly co-planar. Adopting for the inclination of the Be star rotation plane, i.e., assuming sub-critical rotation velocity, and adopting , we find and . A lower limit on the secondary mass is set by assuming resulting in , or . In summary, the secondary star has a mass within the range for a binary mass ratio . The semi-major axis is or au.
The mass of the companion is typical of M1 to K1 main-sequence stars, but also of the bulk of white dwarf stars (Shipman, 1979). If the companion is a main-sequence star, then the system is relatively young (young-scenario: 50-80 Myr; Schaller et al., 1992) and with a total systemic mass of . If the companion is a white dwarf then the current orbital separation necessarily implies past interaction and we must investigate plausible evolutionary scenarios (old-scenario, see Willems & Kolb, 2004). The outcome of the old-scenario is not necessarily a white dwarf plus main-sequence binary, but the evolved component of the system may also be caught at shorter-lived, intermediary stages.
The mass-accreting component of a close binary may acquire sufficient angular momentum to reach critical rotation velocity (Packet, 1981). Although it was originally proposed that Be stars are subjected to ongoing accretion (Kříž & Harmanec, 1975), the properties of Be stars rather suggest that many, but not all, are exhibiting the effect of past rather than current accretion events, more specifically a case-B mass transfer events while the evolving star climbed the giant branch (Waters et al., 1989; Pols et al., 1991; van Bever & Vanbeveren, 1997). Indeed, Waters, Cote, & Pols (1991) find evidence that the Be binary HR 2142 holds a white dwarf or He-star secondary, and Gies et al. (1998) show that the companion to the Be star Per (Poeckert, 1981) is an extreme-horizontal branch (EHB) star. HR 7409 shares a number of characteristics with the Be star 4 Her (Koubský et al., 1997). Both stars are fast rotating and reside in long period binaries. Moreover, their mass functions imply the presence of a low-mass companion. However, our interpretation of the phenomenon involves past interaction and mass transfer rather than on-going accretion as proposed by Koubský et al. (1997).
Population syntheses (see, e.g., Raguzova, 2001; Willems & Kolb, 2004) aim at predicting the binary period and final-mass distributions. Willems & Kolb (2004) described a likely scenario (labelled number “2”) for the formation of the present-day binary HR 7409: a pair in a close 5-day binary is expected to experience its first Roche lobe overflow (RLOF) event after the primary crosses the Hertzsprung gap and climbs the giant branch ( Myr). The process quickly results in a reversal of the mass ratio (from to ). With the cessation of mass transfer, the binary enters an extended quiet period while the evolved star, labelled a “naked” helium star, sits on the horizontal branch (HB) for Myr. After exhaustion of the helium fuel, the binary enters a second RLOF episode and becomes a detached white dwarf plus B-star binary with a final period of 93 days. This scenario is not only applicable to the formation of Be stars but to any fast-rotating B stars. Gies et al. (2008) found that the fast-rotating B star Regulus is a 40-day binary with a likely white dwarf companion. Rappaport, Podsiadlowski, & Horev (2009) reconstructed the past history of this system and concluded that the Regulus system will likely evolve into an AM CVn system.
HR 7409 appears to be a less massive version of scenario number 2 described by Willems & Kolb (2004). Assuming conservative mass-transfer throughout the evolution we may assume initial masses of ( Myr, Schaller et al., 1992) and for the original primary and secondary and an orbital period of several days. In this scenario, are transfered from the original primary to the secondary leaving a fast-rotating star with an evolved He-star or white dwarf companion. Considering that the present day Be star exhibits evidence of a wind, an accreting white dwarf would emit copious X-rays as in the case of Cas (Harmanec et al., 2000) or HD 161103 and SAO 49725 (Lopes de Oliveira et al., 2006). However, HR 7409 is not an X-ray source (Berghoefer, Schmitt, & Cassinelli, 1996), leaving us with the possibility that the unseen companion is a He-star with a shallower potential well.
Having been striped of its hydrogen envelope, the naked He-star may join other HB stars with and contribute up to 15% of the composite optical flux, or resemble a hot subdwarf on the EHB with absolute V magnitude (see Heber, 2009) and would contribute less than 1% of the composite flux. Further examination of the high-dispersion spectra may offer clues to the nature of the companion.
4.3 Line profile analysis: He i6678.15
Figure 6 shows co-added He i6678 spectra phased with the orbital period. In order to increase the signal-to-noise ratio and search for weak spectral lines tracing the orbit of the companion we regrouped the spectra in four bins and adjusted the velocity scale to the Be star rest-frame. The He i6678 line shows a broad red-shifted emission and its overall shape varies over time. A weak absorption feature ( mÅ) marked with vertical lines is moving in opposite directions to the Be star ( km s) and may belong to a subluminous B-type companion. The full amplitude of the motion, after correcting for orbital smearing in the co-added spectra, is km s and would imply a mass ratio for the companion in agreement with the mass function (Section 4.2).
A confirmation of the detection of a secondary star may be secured with optical or ultraviolet echelle spectroscopy at high-dispersion and very high-signal-to-noise ratio.
5 Summary and conclusions
Table 5 lists some stellar properties. We found that the Be star HR 7409 forms a 69.2 d binary with a low-mass companion. Although, we cannot exclude a low-mass main-sequence star, the companion is most likely a hot sub-luminous star. An evolutionary scenario involving an episode of conservative mass-transfer also offers a natural explanation for the fast rotation of the Be star. Using the distance, systemic velocity and proper-motion measurements (, , , and ) we confirm the peculiar kinematics of the Be star which imply that HR 7409 belongs to a group of runaway B stars (Hoogerwerf, de Bruijne, & de Zeeuw, 2001). Instead, we propose that its prior evolution and “rejuvenation” suggests more evolved kinematical properties such as those of white dwarf stars that show a lag in the Galactic -component of km s (Sion et al., 1988).
A relatively old age for the system would imply that it is not part of the Gould Belt. The main-sequence lifetime of the evolved star is estimated at 250 Myr for an initial mass of 3.4 (see Section 4.2), but increasing the progenitor mass above 5 ( Myr) would bring the system age closer to the estimated age of the Gould Belt (30-60 Myr, see Bekki, 2009, and references therein). Detailed evolutionary models for this particular system should help elucidate the age problem.
|Types||B4-5 III-IVe plus (EHB, HB, K V, or WD)|
S.V. and A.K. are supported by GA AV grant numbers IAA300030908 and IAA301630901, respectively, and by GA ČR grant number P209/10/0967. A.K. also acknowledges support from the Centre for Theoretical Astrophysics (LC06014). The visit of S.J. and I.P. at Ondřejov Observatory was supported by the Department of Astronomy, Faculty of Mathematics, University of Belgrade. We thank the referee, J. Zorec, for a prompt and informative review. We also thank P. Škoda, P. Koubský, M. Netolický, J. Polster, B. Kučerová, D. Korčáková, and P. Hadrava for obtaining some of the spectra used in the present study.
This research has made use of the SIMBAD database, operated at CDS, Strasbourg, France. This publication makes use of data products from the Two Micron All Sky Survey, which is a joint project of the University of Massachusetts and the Infrared Processing and Analysis Center/California Institute of Technology, funded by the National Aeronautics and Space Administration and the National Science Foundation.
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