A Table of stellar parameters

New massive members of Cygnus OB2

Key Words.:
massive stars – OB-type – Cygnus OB2 – new members

Abstract

Context: The Cygnus complex is one of the most powerful star forming regions at a close distance from the Sun (1.4 kpc). Its richest OB association Cygnus OB2 is known to harbor many tens of O-type stars and hundreds of B-type stars, providing a large homogeneous population of OB stars that can be analyzed. Many studies of its massive population have been developed in the last decades, although the total number of OB stars is still incomplete.

Aims:Our aim is to increase the sample of O and B members of Cygnus OB2 and its surroundings by spectroscopically classifying 61 candidates as possible OB-type members of Cygnus OB2, using new intermediate resolution spectroscopy.

Methods:We have obtained intermediate resolution (R5000) spectra for all of the OB-type candidates between 2013 and 2017. We thus performed a spectral classification of the sample using HeI-II and metal lines rates, as well as the Marxist Ghost Buster (MGB) software for O-type stars and the IACOB standards catalog for B-type stars.

Results:From the whole sample of 61 candidates, we have classified 42 stars as new massive OB-type stars, earlier than B3, in Cygnus OB2 and surroundings, including 11 O-type stars. The other candidates are discarded as they display later spectral types inconsistent with membership in the association. We have also obtained visual extinctions for all the new confirmed massive OB members, placing them in a Hertzsprung-Russell Diagram using calibrations for and luminosity. Finally, we have studied the age and extinction distribution of our sample within the region.

Conclusions: We have obtained new blue intermediate-resolution spectra suitable for spectral classification of 61 OB candidates in Cygnus OB2 and surroundings. The confirmation of 42 new OB massive stars (earlier than B3) in the region allows us to increase the young massive population known in the field. We have also confirmed the correlation between age and Galactic longitude previously found in the region. We conclude that many O and early B stars at mag are still undiscovered in Cygnus.

1 Introduction

The Cygnus region is the most powerful nearby stellar complex, conspicuous at all wavelengths and very young, with several rich OB associations, numerous young open clusters and tens of compact H II and star formation regions in the field. Hosting the largest number of nearby massive stars and an intense star forming activity (Reipurth & Schneider, 2008), it provides an updated view of the high-mass stellar population in one of the largest groups of young stars in our Galaxy. It is an ideal place to study the process of massive star formation and evolution, individually and in stellar groups, and their interaction with the surroundings.

Its association Cygnus OB2 (d1.4 kpc, Rygl et al., 2012) has received a lot of attention and has been studied at all wavelengths with different spatial coverage since it hosts a high number of early spectral type stars (Walborn et al., 2002). First studies were carried out by Morgan et al. (1954), Schulte (1956, 1958) and Reddish (1968), but it was Massey & Thompson (1991) who developed an extensive survey of the massive population in the association, identifying 120 possible massive star members, 70 of which were classified as OB stars (42 O-type stars). Knödlseder (2000) proposed that this number should be much larger, around 100 O-type stars. In the last few decades, many other studies were carried out in the region updating continually this number (Comerón et al., 2002; Hanson, 2003; Kiminki et al., 2007; Comerón et al., 2008; Negueruela et al., 2008; Comerón & Pasquali, 2012). These surveys have allowed the global study of the massive population in the region, using photometry to place the stars in a Hertzprung-Russell diagram from which the star formation history and mass function of the association were assessed (Wright et al., 2015). In spite of the many photometric and spectroscopic surveys carried out in the region, only a small homogeneous group of early type stars have been spectroscopically analyzed (Herrero et al., 1999, 2002; Negueruela et al., 2008), and few stars have been observed in the UV range (Herrero et al., 2001). There is still a large number of stars that should be explored. The optical extinction of the region is high ( mag, Wright et al., 2015), but not so much as to prevent obtaining spectra of its most massive stars for a rough spectral classification. Therefore, new spectroscopy to search for previously undiscovered massive stars is mandatory to complete the last census of massive O and B-type stars in the association.

One of the most complete spectroscopic surveys in the Cygnus region was developed by Comerón & Pasquali (2012). They performed spectral classification of a magnitude-limited sample ( 16 mag and mag) selected with a homogeneous photometric criterion over a large area that includes Cygnus OB2 and its surroundings, providing a large sample of known and new OB stars, as well as a list of 61 OB candidates for which no spectral data is available and that are pending spectroscopic confirmation.

The main goal of this work is to complete the spectral classification of this latter sample, aiming a later determination of the stellar parameters. Thus, we have obtained intermediate-resolution spectra of all the list candidates, in order to confirm or reject them as true massive OB-type stars. In Sect. 2 we present the observations. In Sect. 3 we describe the spectral classification criteria used in this work, and in Sect. 4 visual extinctions and stellar parameters derived for the new OB-type members. The results are discussed in Sect. 5 where we show the Hertzsprung-Russell diagram (HRD) of the region and the age distribution found across Galactic longitude. Finally, we summarize our conclusions in Sect. 6.

2 Observations and data reduction

The study developed by Comerón & Pasquali (2012) produced a sample of O and early B stars (in a 6 x 4 region centered on Galactic coordinates 79.8 and +0.8 of Cygnus OB2) which were identified using two homogeneous reddening-free criteria {align} Q_BJK = 0.196 (B-J)-0.981 (J-K)-0.098 ¿ 0
Q_JHK = 0.447 (J-H)-0.894 (H-K)-0.089 ¡ 0 which allowed them to classify 60 new OB stars and produce a list of 61 candidates pending spectroscopic data. They used photometry tabulated in the USNO-B (Zacharias et al., 2010, 2004) and 2MASS all-sky (Skrutskie et al., 2006) catalogs setting limiting magnitudes of mag and mag. By combining these two magnitude cuts with the colors of OB stars, Comerón & Pasquali (2012) set up a selection method sensitive to main-sequence stars earlier than B1 and obscured by mag.

We have obtained new spectra for all the proposed OB candidates, whose location is shown in Fig. 1. The sample has been observed in five different runs between 2013 and 2017. For an accurate spectral classification of OB-type stars we need blue spectra (4000-5000 ) where the diagnostic He I-II and metal lines are located.

Figure 1: Inverse Spitzer 8 m image of the Cygnus region showing the location of the 61 OB candidates. The 42 confirmed massive OB-type stars earlier than B3 are indicated with red dots. The remaining stars are late B and foreground A-F-G stars which are indicated with green dots. The solid line circle delimits the 1 degree radius area of Cygnus OB2 adopted by Comerón & Pasquali (2012). For reference, the dash-dotted line circle shows the area considered by Wright et al. (2015).
Instrument Telescope Resol. Date Stars
and grating
ISIS - 600B ORM-WHT 2500 Oct 2013 1
WYFFOS - H2400B ORM-WHT 5000 Jul 2014 7
ISIS - H2400 ORM-WHT 7500 Jul 2015 2
OSIRIS - R2500U/V ORM-GTC 2500 May 2016 2
IDS - R1200B ORM-INT 5000 Jul 2016 47
ISIS - R1200B ORM-WHT 5000 Apr 2017 2
Table 1: Telescopes, instruments and settings used in this work.

The bulk of stars were observed in July 2016, obtaining spectra of 47 candidates. We have chosen the R1200B grating of IDS (EEV10) on the Isaac Newton Telescope (INT) in La Palma, which provides a resolution of 5000 at 4500 . The remaining stars were observed in different runs between 2013 and 2017, at the William Herschel Telescope (WHT) using the AF2/WYFFOS and ISIS instruments, and at the Gran Telescopio CANARIAS (GTC) using the OSIRIS instrument. Out of the whole sample, three of the stars (J20301097+4120088, J20315433+4010067, J20345785+4143543) belong to the Galactic O-Star Spectroscopic Survey (GOSSS) catalog (Maíz Apellániz et al., 2016).

The information on the different runs carried out in this work is shown in Table 1. The final spectra were reduced using the IRAF procedures, with standard routines for bias and flat-field subtraction and also for the wavelength calibration.

3 Spectral classification

Object RA (hhmmss) Dec ( ) Region SpT Binary star
J20423509+4256364 20 42 35.08 +42 56 36.43 c 14.480 8.304 9.211 O6IIIz
J20371773+4156316 20 37 17.73 +41 56 31.57 c 15.760 8.041 9.071 O7V
J20345785+4143543 20 34 57.84 +41 43 54.25 a 15.430 7.417 8.447 O7:Ib
J20293563+4024315 20 29 35.63 +40 24 31.45 b 12.468 8.268 8.831 O8IIIz
J20222481+4013426 20 22 24.81 +40 13 42.55 c 13.620 8.410 9.034 O8II
J20261976+3951425 20 26 19.75 +39 51 42.46 c 15.600 8.348 9.351 O8.5IV
J20275292+4144067 20 27 52.92 +41 44 06.65 b 13.330 7.277 8.144 O9.5II
J20262484+4001413 20 26 24.84 +40 01 41.25 c 13.290 8.330 9.021 O9.2III
J20291617+4057372 20 29 16.17 +40 57 37.19 b 15.030 7.899 8.855 O9.7III
J20382173+4157069 20 38 21.72 +41 57 06.89 c 15.810 7.682 8.760 O9.7II
J20181090+4029063 20 18 10.89 +40 29 06.29 c 14.940 8.399 9.343 O9.7Ib yes*
J20273787+4115468 20 27 37.87 +41 15 46.79 b 14.570 8.263 9.146 B0II
J20301097+4120088 20 30 10.97 +41 20 08.82 b 15.690 8.882 9.855 B0:II:
J20323968+4050418 20 32 39.68 +40 50 41.83 a 14.410 8.913 9.631 B0II
J20395358+4222506 20 39 53.58 +42 22 50.62 c 15.890 5.822 7.345 B0I yes*
J20281176+3840227 20 28 11.75 +38 40 22.73 c 11.805 7.944 8.349 B0Ib
J20323882+4058469 20 32 38.82 +40 58 46.85 a 15.430 8.821 9.701 B0Ib
J20225451+4023314 20 22 54.50 +40 23 31.39 c 13.107 8.601 9.175 B0Iab
J20253320+4048444 20 25 33.19 +40 48 44.38 c 13.112 7.648 8.340 B0Iab
J20272099+4121262 20 27 20.99 +41 21 26.15 b 13.830 8.730 9.448 B0.5V yes
HDE229258 20 24 25.51 +39 49 28.30 c 10.235 8.689 8.833 B0.7V
J20330526+4143367 20 33 05.26 +41 43 36.74 a 13.940 8.634 9.286 B0.5III
J20361806+4228483 20 36 18.06 +42 28 48.30 c 15.650 8.855 9.814 B0.7III
J20233816+3938118 20 23 38.16 +39 38 11.84 c 11.293 8.731 8.996 B0.7Ib
HD228973 20 20 07.35 +41 07 46.72 c 10.34 7.684 7.922 B1V yes
J20201435+4107155 20 20 14.34 +41 07 15.45 c 12.412 8.240 8.627 B1V
J20230290+4133466 20 23 02.90 +41 33 46.59 c 14.780 7.809 8.762 B1V
BD+404193 20 29 13.55 +40 41 03.38 b 10.412 8.812 8.945 B1V
BD+404208 20 30 49.97 +40 44 18.53 b 10.654 8.664 8.869 B1V
J20314341+4100021 20 31 43.40 +41 00 02.07 a 15.940 8.957 9.885 B1V yes*
J20315898+4107314 20 31 58.98 +41 07 31.41 a 15.490 8.832 9.773 B1V yes*
J20330453+3822269 20 33 04.53 +38 22 26.91 c 11.383 8.790 9.021 B1V
J20230183+4014029 20 23 01.83 +40 14 02.90 c 13.465 8.060 8.579 B1III
J20274925+4017004 20 27 49.25 +40 17 00.42 b 13.460 8.104 8.713 B1III
J20315433+4010067 20 31 54.33 +40 10 06.71 b 15.990 8.884 9.742 B1III
J20382889+4009566 20 38 28.88 +40 09 56.63 c 9.996 8.816 8.896 B1III
J20440752+4107342 20 44 07.51 +41 07 34.18 c 15.470 8.703 9.562 B1III
LSII+3797 20 33 35.52 +38 01 36.73 c 11.838 5.900 6.691 B1Ia yes*
J20211924+3936230 20 21 19.24 +39 36 22.98 c 12.732 7.886 8.488 B1Ib yes*
BD+394179 20 25 27.28 +40 24 00.15 c 11.066 6.667 7.210 B1Ib
J20290247+4231159 20 29 02.46 +42 31 15.91 c 13.330 8.426 9.090 B1Ib yes*
J20381289+4057169 20 38 12.88 +40 57 16.86 c 12.870 8.675 9.183 B2V
1
Table 2: Basic data of the confirmed massive new members (earlier than B3).

The accuracy of the spectral classification depends on the effects of spectral resolution as well as the signal-to-noise ratio . The main diagnostic method for O-type stars is the comparison of He II 4542/He I 4471 ratio (Sota et al., 2011). These lines are similar for a O7 type star. For later O types the relative strengths of He II 4542/He I 4387 and He II 4200/He I 4144 are normally used, and represent the main criteria for types O8-B0. Spectral types earlier than O8 were classified using the criteria described by Gray & Corbally (2009). The presence of metal lines in different ionization stage, such as Si III or Mg II, indicates early B-type stars. The relative strength of HeII 4471/MgII 4481 is a useful indicator for B1-late B stars. As secondary indicators we used the criteria described by Gray & Corbally (2009), which were also used to classify stars of spectral types A, F and G.

Regarding the luminosity class, the criteria used for early O-type stars were introduced by Walborn (1971, 1973), taking into account the emission effects in the He II 4686 line and N III 4634-4640-4642. For late O-type stars, we have used the criteria described by Sota et al. (2011) and for B-type stars the criteria described by Gray & Corbally (2009) and the Balmer lines width.

Although the classification for O-type stars was based on the described He and metal lines diagnostic criteria, we have also used for O-type stars the Marxist Ghost Buster (MGB) code developed by Maíz Apellániz et al. (2012) in order to obtain a more accurate result. This tool compares the observed spectra with a grid of O standards (in this work the GOSSS library), allowing us to vary spectral type, luminosity class, velocity and resolution until obtaining a best match. Furthermore, for B-type stars we have used a sample of IACOB standards (Simón-Díaz et al., 2015) to improve their classification.

Object RA (hhmmss) Dec ( )
late B-type stars
CCDMJ20323+4152AB 20 32 20.81 +41 52 00.78
BD+423785a 20 34 15.39 +43 09 35.28
A-type stars
J20252497+3934030 20 25 24.96 +39 34 03.02
J20285874+4013302 20 28 58.74 +40 13 30.22
J20315984+4120354 20 31 59.84 +41 20 35.41
J20320734+3828586 20 32 07.34 +38 28 58.62
BD+413801 20 33 30.39 +42 04 17.35
J20364336+3906145 20 36 43.36 +39 06 14.53
CCDM J20429+4311AB 20 42 54.24 +43 10 38.71
F-type stars
J20193232+4042447 20 19 32.32 +40 42 44.72
J20253116+4005508 20 25 31.16 +40 05 50.82
J20275204+4131200 20 27 52.03 +41 31 19.98
CCDM J20420+4015 20 42 01.18 +40 14 42.70
J20500396+4300118 20 50 03.96 +43 00 11.76
J20504551+421012.6 20 50 45.51 +42 10 12.64
G-type stars
J20300022+4337553 20 30 00.21 +43 37 55.29
J20340430+4136507 20 34 04.29 +41 36 50.67
J20432737+4308525 20 43 27.37 +43 08 52.47
J20472235+4220523 20 47 22.34 +42 20 52.30
Table 3: Candidates classified as late B and A-F-G type stars.

4 New confirmed OB type stars

The observed spectra have high enough and resolution for a spectral classification of the stars. All the candidate spectra are plotted in Appendix B (Fig. 12) where the main diagnostic lines used are also indicated.

Out of the 61 candidates 42 are OB type stars, earlier than B3, including 11 O-type stars. Two more are late B-type stars. The location of these new confirmed OB stars is shown in Fig. 1 while their names, coordinates, magnitudes and the derived spectral classification are listed in Table 2. We have also included the region in which they are located: (a) the Cygnus OB2 area considered by Wright et al. (2015) which is the youngest core of Cyg OB2 at present; (b) the Cygnus OB2 area considered by Comerón & Pasquali (2012) which is the extended Cyg OB2 area containing older stars, on average, and (c) the surrounding area which includes part of the Cygnus OB9 association and field population. The remaining observed stars are late B and foreground A-F-G stars, which are globally listed in Table 3. The selection criteria success rate obtained by Comerón & Pasquali (2012) along with the number of the confirmed OB type obtained in this work (), support the success at identifying reddened massive OB stars with this method.

We have also detected nine possible or confirmed SB2 binaries in the sample of new OB stars (see Table 2). In some cases, indicated with asterisks, we can only suggest possible binary nature mainly due to noisy spectra. This number represents a of our massive OB sample. We assign to these stars the spectral classification of their primary component. An example is shown in Fig. 2. Sana & Evans (2011) found that at least of the O star population in clusters and OB associations is comprized of spectroscopic binaries, which indicates that it is highly likely that more binaries are undetected in our sample.

Figure 2: Example of SB2 detection: HeI lines in star HD228973.

4.1 Extinction

Figure 3: , diagram of the confirmed O-type stars in Cygnus OB2 within the 1 deg. radius area adopted by Comerón & Pasquali (2012). Blue dots indicate stars previously known, and red dots represent the new ones confirmed in this work. The upper left vertical line shows the position of the unreddened main sequence based on intrinsic magnitudes from Martins & Plez (2006) at a distance modulus mag. The dash-dotted lines represent the locus expected for different spectral type stars using the reddening law of Wright et al. (2015) with = 2.91. The solid line marks the limits imposed by the selection criteria at magnitudes mag, mag. Vertical dotted lines indicate color bins.

We used the colors and the recent extinction law derived by Wright et al. (2015) () to obtain individual extinctions for all the new confirmed massive OB type stars (see Table 4). Most of them have visual extinctions in the range mag, which agrees with previous studies in the region (Comerón & Pasquali, 2012; Wright et al., 2015). But, as in Comerón & Pasquali (2012), this is partly consequence of the imposed and magnitude limits.

In Fig. 3 we present the , diagram of the confirmed O-type stars in Cygnus OB2. We observe that some of them are aligned below the O9.5V reddening vector, which could suggest a small shift in the adopted intrinsic color calibration since most of them are already known dwarf late O-type members. In spite of this we can assess the incompleteness of the O population in the region by taking into account the ratios of stars in different spectral (O3-O5, O5-O7 and O7-O9.5) and color bins.

All the stars of our sample with colors mag should be bright enough to have been detected by our selection criteria. Figure 3 shows that the population is expected to be complete for mag. But for mag we can not see late O-type stars because they are fainter than mag, which is the imposed magnitude limit. Assuming that the ratios of spectral types are independent of extinction, we can conclude we are loosing the fainter or more obscured O-type stars.

The extinction distribution for the 42 new confirmed OB stars earlier than B3 in Cygnus OB2 and boundaries is shown in Fig. 4 (top). Again, the spectral classification for those stars classified as possible or confirmed SB2 binaries is taken from the primary component. Although O-type stars are clearly more obscured, a median value of mag was found for the whole sample. However, this value is also affected by completeness. O-type stars are more obscured on average because we can detect them up to higher foreground extinctions thanks to their intrinsic brightness. In the bottom histogram the same sample is differentiated by location: the whole sample that belongs to Cygnus OB2, Cygnus OB9 and boundaries, and as a sub-sample, those stars located within the Cygnus OB2 area (from Comerón & Pasquali, 2012) for which a median value of mag is derived.

Figure 4: Top: Extinction distribution of the new classified OB type stars earlier than B3 in Cygnus OB2 and boundaries. Gray indicates B-type stars while dark gray are O-type stars. Bottom: Green indicates all the confirmed stars in the Cygnus region, which includes Cyg OB2, Cyg OB9 and field. Red indicates only those new OB stars located in Cygnus OB2. Green and red dashed lines indicate the median values for the stars in the whole Cygnus region and only Cygnus OB2 respectively.

We see an abrupt cut for mag. Stars with such high visual extinctions shall be intrinsically extremely bright to be seen. An O9V star with a visual extinction of mag will have an apparent magnitude mag, beyond the selection criteria limits and probably this is the reason why we do not find stars beyond this value. However, for the star J20395358+4222506 classified as B0I, we have obtained a visual extinction of 11.0 mag. It has a magnitude in the B band of 15.89 mag but in the Ks band of only 5.82 mag which indicates a very bright star. These cases show again that the sample does not provide a complete census. The magnitude-limited sample of Cygnus OB2 is made incomplete due to extinction.

In Fig. 5 we present the spatial location of the new classified OB stars, where each star is color-coded according to its derived visual extinction . The extinction distribution varies smoothly across the region, increasing from the south-west (Cygnus OB9) to the northeast (Cygnus X-North) where the most extinguished star of our sample, J20395358+4222506, is located.

Figure 5: Location of the new confirmed OB stars in the region colored according to the derived extinction () and over-plotted on an inverse Spitzer 8 m image.

4.2 Stellar parameters

Effective temperature

For each classified O-type star and luminosity classes V, III and I, we have used the effective temperature () scale developed by Holgado et al. (in prep.) based on a revision of the O-type standards in the IACOB database. We have preferred it over the SpT- calibration suggested by Martins et al. (2005) since the latter gives too low values for late O-type stars (Simón-Díaz et al., 2014).

For early B-type stars and luminosity class V, we decided to use the spectral type compilation of Nieva (2013). An excellent agreement with the calibration of Holgado et al. is found. For late B-type dwarf stars we have used the calibration of Pecaut & Mamajek (2013). Regarding B stars with luminosity class I, we have used the scale of Markova & Puls (2008), and for luminosity class III we have interpolated between classes I and V. In Fig. 6 are represented the different scales adopted in this work, and the derived values for all the new OB type stars are shown in Appendix A (Table 4). We see that all scales for B-types fit smoothly the Holgado et al. scale. Temperatures were also obtained for those stars classified as SB2 binaries by taking into account the primary component.

Comerón & Pasquali (2012) used the ‘observational’ effective temperature based on the versus spectral type calibration of Martins et al. (2005). For B-type stars, they used the spectral type compilation of Tokunaga (2000) but applying a scaling factor to force the agreement with Martins et al. (2005) temperature scale. In order to be consistent with them we have recalculated temperatures for all of their known OB stars sample in the Cygnus Region using our criteria. New values are shown in Appendix A (Table 5).

Bolometric correction and luminosity

As Comerón & Pasquali (2012), we have adopted a distance modulus of mag, and used the intrinsic magnitudes derived by Martins & Plez (2006) for O stars and those compiled by Tokunaga (2000) for B stars. Thus we could derive absolute magnitudes (see Comerón et al., 2008). To derive luminosities we adopted bolometric corrections (BCs) from Lanz & Hubeny (2003, 2007). There is an excellent agreement with Martins et al. (2005) for temperatures higher than 32000 K, and also with the bolometric corrections from Nieva (2013) for cooler stars (see Nieva, 2013). The derived luminosity values for the new and already known OB stars are shown in Table 4 and Table 5 respectively.

Figure 6: scales used in this work.
Figure 7: HR diagrams of OB stars in Cygnus OB2 assuming a mag. Known OB stars from Comerón & Pasquali (2012) are also included to complete the sample. Black and orange dots indicate the already known and new OB-type stars respectively. Blue crosses indicate those new stars classified as possible SB2 stars. Top left hand panel: Isochrones (dotted lines) and evolutionary stellar tracks (solid lines) for non-rotating models from Ekström et al. (2012). Top right hand panel: Isochrones (dotted lines) and evolutionary stellar tracks (solid lines) for rotating models (V/V = 0.4) from Ekström et al. (2012) . Bottom left hand panel: Isochrones (dotted lines) and evolutionary stellar tracks (solid lines) for non-rotating models from Brott et al. (2011). Bottom right hand panel: Isochrones (dotted lines) and evolutionary stellar tracks (solid lines) for rotating models (V = 330 km/s) from Brott et al. (2011).

5 Discussion

Some of the new confirmed OB type stars (see Fig. 1) are located at the boundaries assigned to Cygnus OB2. However, the limits of the association are not strongly defined. The first surveys in the association assumed a smaller area where the most luminous members are located (Münch & Morgan, 1953; Massey & Thompson, 1991). Then, Knödlseder (2000) provided wider limits showing that extinction was a limiting factor in previous studies. This area was thus extended with the identification of new early-type members (Comerón et al., 2002, 2008; Comerón & Pasquali, 2012; Wright et al., 2009, 2015). If we assume the extension of Cygnus OB2 as the area adopted by Comerón & Pasquali (2012) (1 deg. radius centered on Galactic coordinates 79.8 and +0.8), as well as the most recent census in the area developed by Wright et al. (2015), we can now update the census of O and B-type stars in Cygnus OB2 from 204 to 221, and the number of confirmed O-type members increases from 66 to 70 stars.

5.1 Hertzsprung-Russell diagram

Some studies about the age of Cygnus OB2 have been developed in the last two decades (Massey et al., 2001; Hanson, 2003; Drew et al., 2008; Negueruela et al., 2008; Wright et al., 2010; Comerón & Pasquali, 2012; Comerón et al., 2016). However, different evolutionary stellar models were used. We decided to explore four different stellar models (two families with two initial rotational velocities) to construct the Hertzsprung-Russell Diagrams (HRD) in order to assess uncertainties on the age distribution.

We have placed the new confirmed OB stars in the HRD to study the evolutionary status of the association. For this aim we have considered the stars within the area adopted by Comerón & Pasquali (2012). We have also added the already known OB stars compiled in their work to complete the sample. Two of them, classified as B0 stars (J20325964+4115146 and J20333700+4116113) were recently reclassified as O9.7III and O9.5IV by Maíz Apellániz et al. (2016). We thus decided to assume this last classification for both. We have used the Geneva evolutionary tracks and isochrones with and without rotation as calculated by Ekström et al. (2012) (Fig. 7 top), and for comparison, the Bonn evolutionary tracks and isochrones with and without rotation as calculated by Brott et al. (2011) (Fig. 7 bottom).

In the Geneva case, we found a difference up to 2 Myr in the stellar ages for those most massive members ( 20 M) depending on whether we consider rotation or not. Larger stellar lifetimes are derived from rotating stellar models. A similar result was found by Wright et al. (2015) by comparing both non-rotating and rotating Geneva stellar models. They derived an age range of 1 - 7 Myr for the association, but the results from rotating stellar models suggested an age of 4 - 5 Myr, while non-rotating models hinted at a younger age (2 - 3 Myr). On the contrary, the Bonn stellar models included on this work do not exhibit a large difference in the estimated age of the stars when they include or not stellar rotation. This different behavior is most likely to be ascribed to their different treatment of rotation. Thus, the positions of the isochrones in both Bonn HR diagrams are very similar. Moreover, in the Bonn case we see an extended terminal age main sequence (TAMS), and therefore, all the OB stars are located on the main sequence (MS). In spite of this, all models suggest that most of the stars are in the age range of 1 - 6 Myr, indicating a continuous (but not necessarily constant) star formation activity. However, a combination of different scenarios (rotating and non-rotating stellar models) would help to narrow the possible ages. Slow rotators with initial masses of 30 M or less would give similar ages as more massive stars born with larger rotational velocities.

In view of the results, we can conclude that uncertainties due to rotation and adopted models affect mainly the most massive members, suggesting larger ages of about 1 - 2 Myr for them. This makes the age determination very uncertain and strengthens the need for additional diagnostics, in particular individual rotation velocity measurements, to better constrain them.

5.2 Age distribution across Galactic longitude

Comerón & Pasquali (2012) studied the correlation between ages and Galactic longitudes in Cygnus OB2. They suggest that star formation has proceeded from lower to higher Galactic longitudes, finding most of the old stars located at low Galactic longitudes while the youngest ones lie at higher Galactic longitudes. In order to corroborate it, we have obtained ages by comparing isochrones from the HRDs, and divided the sample of OB stars in three age groups: the young group contains stars in the range of 0 - 5 Myr, the intermediate group contains stars in the range of 5 - 10 Myr, and the old one contains stars with ages Myr.

Figures 8 and 9 show the histograms of the relative frequency of our OB type sample located at low (78.5- 79.5), central (79.5- 80.5) and high (80.5- 81.5) Galactic longitudes in each age group and for each model considered (Geneva (Fig. 8) and Bonn (Fig. 9) rotating and non-rotating models). In all cases, most of the stars at high Galactic longitudes belong to the young-intermediate age group, while those stars located at low Galactic longitudes belong to the old age group. We only obtain significant age differences at using the rotating Geneva models since in this case rotation gives older ages for the most massive stars. Thus, we can see a change in the relative frequencies of old and intermediate stars. On the other hand, we do not see these age differences at using rotating Bonn models because of their lower sensitivity to rotation as noted earlier.

This analysis thus supports the correlation between ages and Galactic longitudes in Cygnus OB2 as previously suggested by Comerón & Pasquali (2012). Massive star formation in Cygnus OB2 seems to have proceeded from lower to higher Galactic longitudes, regardless of the details of the models used.

Figure 8: Relative frequency histograms of the stars located at low (78.5- 79.5), central (79.5- 80.5) and high (80.5- 81.5) Galactic longitudes in Cygnus OB2 using Geneva non-rotating (top) and rotating (bottom) models. Orange, purple and yellow colors indicate those stars located in the young (0 - 5 Myr), intermediate (5 - 10 Myr) and old (¿ 10 Myr) age groups.
Figure 9: Relative frequency histograms of the stars located at low (78.5- 79.5), central (79.5- 80.5) and high (80.5- 81.5) Galactic longitudes in Cygnus OB2 using Bonn non-rotating (top) and rotating (bottom) models. Orange, purple and yellow colors indicate those stars located in the young (0 - 5 Myr), intermediate (5 - 10 Myr) and old (¿ 10 Myr) age groups.

5.3 The whole Cygnus region

In order to obtain a big picture of the age distribution across the Galactic longitude, we have performed the same analysis as in Cygnus OB2 but now in a wider area which includes Cygnus OB2, Cygnus OB9 and boundaries. We have placed the whole sample of OB stars, including late B-type stars, in an HRD (see Fig. 10). Due to the similar results obtained in Cygnus OB2 by using different models, we have now used isochrones and evolutionary stellar tracks for non-rotating models from Ekström et al. (2012). We derived ages by comparison with isochrones and divided again the sample in three age groups: the young group (0 - 5 Myr), the intermediate group (5 - 10 Myr), and the old group ( Myr). In Fig. 11 we show the spatial distribution of the different age groups, where most of the younger stars (left) are concentrated at higher Galactic longitudes, while the older ones (right) are located at central-lower Galactic longitudes. These results suggest that massive star formation has proceeded from lower to higher Galactic longitudes, from Cygnus OB9 to Cygnus OB2, with a strong peak in the northern part of the association.

6 Conclusions

We have carried out several observing runs between 2013 and 2017 to obtain new blue intermediate-resolution spectra suitable for spectral classification for the magnitude-limited ( mag, mag) candidate list compiled by Comerón & Pasquali (2012). Out of 61 candidates, we confirm 42 new massive OB-type stars, earlier than B3, including 11 new O-type members. A of this sample results on confirmed or possible SB2 binaries. Two other stars are late-B, seven are A-type, six are F-type, and the remaining four are G-type stars. Therefore, the spectral classification of the homogeneously selected sample is now completed. The selection criteria success rate obtained by Comerón & Pasquali (2012) along with this work is , supporting the success at identifying reddened massive OB stars with the reddening-free parameters based on photometry (see Sect. 2). However, the magnitude cutoff and dust extinction introduce an incompleteness. We loose the faintest and more obscured late O-type members in Cygnus OB2, and we are still far to obtain a complete census of the early-type population in the association.

Figure 10: HR diagram of OB stars in the Cygnus region (Cygnus OB2, Cygnus OB9 and field population) assuming a mag. Known OB stars from Comerón & Pasquali (2012) are also included to complete the sample. Black and orange dots indicate the already known and new OB-type stars respectively. Blue crosses indicate those new stars classified as possible or confirmed SB2 stars. Isochrones (dotted lines) and evolutionary stellar tracks (solid lines) for non-rotating models are from Ekström et al. (2012). Late-B stars are also included.
Figure 11: Spatial distribution of stars of different age in the Cygnus area: the young population (0 - 5 Myr) on the left, the intermediate age one (5 - 10 Myr) in the middle, and the old population (¿ 10 Myr) on the right.

We have also estimated individual visual extinctions () for the new confirmed OB-type stars using the extinction law derived by Wright et al. (2015). We found a median value of 5.5 mag for the stars in the whole Cygnus region, and as we expect, a large median value of 6.5 mag for those stars within the Cygnus OB2 association.

We have placed the new sample of OB stars in the HR diagram using both rotating and non-rotating models calculated by Ekström et al. (2012) and Brott et al. (2011) in order to assess uncertainties. We have also placed the already known OB type stars compiled by Comerón & Pasquali (2012) for completeness. To this aim, we derived effective temperatures and luminosities using spectral type calibrations for the whole sample. Although coming from different sources, our adopted calibrations fit very well with each other. Uncertainties about rotation and adopted models affect only the most massive members, suggesting larger ages for them of 1-2 Myr. Even so, all models support the previous age spread observed in Cygnus OB2 of 1 - 6 Myr.

In order to check the correlation between age and Galactic longitude found by Comerón & Pasquali (2012) in Cygnus OB2, we have divided the sample of OB stars in different location and age groups by comparing with isochrones: the young group (0 - 5 Myr), the intermediate group (5 - 10 Myr), and the old group ( Myr). Despite the differences in physical process treatment (rotation and physics of the stellar interior) done by the Geneva and Bonn groups, the spatial distribution of each age group is globally similar. Therefore, the result obtained by Comerón & Pasquali (2012) can not be an effect of using the Geneva stellar models without rotation when age dating the Cygnus region stars. Most of the stars at high Galactic longitudes belong to the young-intermediate age group, while stars at low Galactic longitudes belong to the old group. Assuming a wider area which includes Cygnus OB2, part of Cygnus OB9 and boundaries, we obtain similar results suggesting that massive star formation in the region proceeds from Cygnus OB9 to Cygnus OB2, with a strong peak in the northern part of the association. Therefore the correlation between age and Galactic longitude is confirmed, regardless of the details of the models used.

Acknowledgements.
We acknowledge financial support from the Spanish Ministry of Economy and Competitiveness (MINECO) under the grants AYA2012-39364-C02-01, AYA 2015-68012-C2-01 and Severo Ochoa SEV-2015-0548. A.S. also acknowledges support from MINECO through grants AYA2013-40 611-P and AYA2016-75 931-C2-2-P. AP and CBM acknowledge support from the Sonderforschungsbereich SFB 881 ”The Milky Way System” (subproject B5) of the German Research Foundation (DFG). We also thank the WHT and its service programme (SW2017a04), J. Maíz Apellániz for his useful MGB software, and N. Wright for helpful discussion and comments in this paper.

Appendix A Table of stellar parameters

Table 4 shows the derived stellar parameters for the new classified OB stars in a large area which includes Cygnus OB2 and its surroundings. We have also included the derived individual visual extinctions. Table 5 shows the updated temperatures and luminosities for the already known OB stars from Comerón & Pasquali (2012) assuming the same spectral calibrations used in this work (see Sect. 4.2).

Object SpT (K) log () (mag)
Cygnus OB2
J20345785+4143543 O7:Ib 34990 5.25 8.2
J20293563+4024315 O8IIIz 33961 4.75 5.1
J20275292+4144067 O9.5II 30626 5.09 7.1
J20291617+4057372 O9.7III 28518 4.78 7.7
J20273787+4115468 B0II 25932 4.62 6.8
J20301097+4120088 B0:II: 25932 4.39 7.4
J20323968+4050418 B0II 25932 4.32 5.7
J20323882+4058469 B0Ib 27800 4.33 6.7
J20272099+4121262 B0.5V 26308 4.33 5.8
J20330526+4143367 B0.5III 24164 4.27 5.2
BD+404208 B1V 23590 4.14 2.2
J20314341+4100021 B1V 23590 4.21 7.0
J20315898+4107314 B1V 23590 4.26 7.1
BD+404193 B1V 23590 4.06 1.7
J20274925+4017004 B1III 21557 4.27 4.9
J20315433+4010067 B1III 21557 4.02 6.5
CCDMJ20323+4152AB B9V 10700 2.96 -0.2
Surroundings
J20423509+4256364 O6IIIz 38192 4.97 7.4
J20371773+4156316 O7V 36872 5.06 8.2
J20222481+4013426 O8II 33570 4.69 5.5
J20261976+3951425 O8.5IV 33391 4.81 8.0
J20262484+4001413 O9.2III 31317 4.65 5.9
J20382173+4157069 O9.7II 30859 4.99 8.5
J20181090+4029063 O9.7Ib 28644 4.58 7.6
J20395358+4222506 B0I 25094 5.57 11.0
J20281176+3840227 B0Ib 25094 4.43 3.6
J20290247+4231159 B1Ib 19979 4.03 5.0
J20225451+4023314 B0Iab 25094 4.21 4.7
J20253320+4048444 B0Iab 25094 4.62 5.5
HDE229258 B0.7V 24949 4.13 1.8
J20361806+4228483 B0.7III 22860 4.14 7.2
J20233816+3938118 B0.7Ib 21514 3.87 2.3
HD228973 B1V 23590 4.45 2.4
J20201435+4107155 B1V 23590 4.27 3.4
J20230290+4133466 B1V 23590 4.68 7.2
J20330453+3822269 B1V 23590 4.09 2.4
J20230183+4014029 B1III 21557 4.26 4.3
J20382889+4009566 B1III 21557 3.84 1.4
J20440752+4107342 B1III 21557 4.09 6.5
LSII+3797 B1Ia 19979 5.07 5.8
J20211924+3936230 B1Ib 19979 4.23 4.6
BD+394179 B1Ib 19979 4.70 4.2
J20381289+4057169 B2V 20549 3.98 4.2
BD+423785a B9V 10700 2.90 0.6
Table 4: Temperatures, luminosities, and individual visual extinctions derived for the new classified massive OB stars.
Object RA (hhmmss) Dec ( ) SpT (K) log ()
Cygnus OB2
J20331411+4120218 20:33:14.110 +41:20:21.91 O3If 40910 5.66
J20330879+4113179 20:33:08.818 +41:13:17.93 O4III 41070 5.88
J20360451+4056129 20:36:04.500 +40:56:13.01 O5V((f)) 41250 5.31
J20331798+4118311 20:33:17.982 +41:18:31.19 O5III 39440 5.62
J20340850+4136592 20:34:08.514 +41:36:59.39 O5I 37630 5.80
J20331074+4115081 20:33:10.735 +41:15:08.22 O5If 37630 6.04
J20332346+4109130 20:33:23.471 +41:09:12.90 O5.5V 40440 5.96
J20331326+4113287 20:33:13.264 +41:13:28.67 O6V 39630 5.14
J20303980+4136506 20:30:39.805 +41:36:50.63 O6V 39630 4.97
J20344410+4051584 20:34:44.146 +40:51:58.67 O6.5III(f) 37845 5.31
J20283203+4049027 20:28:32.027 +40:49:02.88 O7 38010 5.65
J20310019+4049497 20:31:00.204 +40:49:49.70 O7V((f)) 38010 5.06
J20341350+4135027 20:34:13.511 +41:35:02.86 O7V 38010 5.04
J20331748+4117093 20:33:17.483 +41:17:09.35 O7V 38010 5.17
J20334086+4130189 20:33:40.863 +41:30:18.95 O7V 38010 4.77
J20342959+4131455 20:34:29.599 +41:31:45.49 O7V 38010 5.58
J20315961+4114505 20:31:59.609 +41:14:50.45 O7V 38010 4.87
J20321383+4127120 20:32:13.822 +41:27:12.01 O7IIIf 36900 5.29
J20313690+4059092 20:31:36.911 +40:59:09.06 O7Ib(f) 34350 5.48
J20323154+4114082 20:32:31.531 +41:14:08.18 O7.5Ib-II(f) 33530 5.52
J20323857+4125137 20:32:38.571 +41:25:13.79 O8V(n) 36390 5.02
J20274361+4035435 20:27:43.616 +40:35:43.51 O8V: 36390 4.72
J20331369+4113057 20:33:13.688 +41:13:05.77 O8V 36390 4.87
J20324545+4125374 20:32:45.450 +41:25:37.57 O8V 36390 5.16
J20331803+4121366 20:33:18.035 +41:21:36.67 O8V 36390 4.90
J20323486+4056174 20:32:34.865 +40:56:17.35 O8V 36390 4.77
J20300788+4123504 20:30:07.877 +41:23:50.44 O8V 36390 4.91
J20330292+4117431 20:33:02.913 +41:17:43.16 O8V 36390 5.06
J20325002+4123446 20:32:50.016 +41:23:44.70 O8V 36390 4.95
J20325919+4124254 20:32:59.057 +41:24:24.79 O8V 36390 4.81
J20333030+4135578 20:33:30.316 +41:35:57.88 O8V 36390 5.17
J20342193+4117016 20:34:21.934 +41:17:01.66 O8III+O8III 35010 5.11
J20323843+4040445 20:32:38.441 +40:40:44.48 O8III 35010 5.24
J20330292+4047254 20:33:02.928 +40:47:25.29 O8II((f)) 33860 5.65
J20314540+4118267 20:31:45.403 +41:18:26.73 O8I 32710 5.16
J30332557+4133269 20:33:25.569 +41:33:26.88 O8.5V 35580 5.12
J20331634+4119017 20:33:16.256 +41:19:00.16 O8.5V 35580 4.82
J20332674+4110595 20:33:26.756 +41:10:59.42 O8.5V 35580 4.90
J20313749+4113210 20:31:37.506 +41:13:20.99 O9: 34770 5.83
J20335842+4019411 20:33:58.417 +40:19:41.13 O9: 34770 5.43
J20321656+4125357 20:32:16.563 +41:25:35.67 O9V 34770 4.79
J20311833+4121216 20:31:18.329 +41:21:21.65 O9V 34770 5.06
J20340486+4105129 20:34:04.851 +41:05:11.76 O9V 34770 4.59
J20311055+4131535 20:31:10.543 +41:31:53.53 O9V 34770 5.14
J20332101+4117401 20:33:21.016 +41:17:40.11 O9V 34770 4.63
J20331571+4120172 20:33:15.685 +41:20:18.75 O9V 34770 4.60
J20301839+4053466 20:30:18.391 +40:53:46.56 O9V 34770 4.99
J20314965+4128265 20:31:49.658 +41:28:26.50 O9III 33120 4.53
J20345606+4038179 20:34:56.057 +40:38:17.92 O9.7Iab 29922 5.33
J20305772+4109575 20:30:57.727 +41:09:57.51 O9.5V 33960 4.85
J20340601+4108090 20:34:06.017 +41:08:09.13 O9.5V 33960 4.87
J20335952+4117354 20:33:59.527 +41:17:35.46 O9.5V 33960 4.91
J20341605+4102196 20:34:16.046 +41:02:19.59 O9.5V 33960 4.73
J20272428+4115458 20:27:24.282 +41:15:45.82 O9.5V 33960 4.51
J20293480+4120089 20:29:34.798 +41:20:08.93 O9.5V 33960 4.76
J20323033+4034332 20:32:30.310 +40:34:33.22 O9.5IV 33067 5.25
J20333700+4116113 20:33:36.994 +41:16:11.31 O9.5IV 33067 4.88
J20334610+4133010 20:33:46.112 +41:33:01.00 O9.5Ia 30250 5.71
Table 5: Temperatures and luminosities derived for the known massive OB stars from Comerón & Pasquali (2012).
Object RA (hhmmss) Dec ( ) SpT (K) log ()
J20325964+4115146 20:32:59.633 +41:15:14.66 O9.7III 31797 4.80
J20283039+4105290 20:28:30.385 +41:05:29.04 OC9.7Ia 29922 5.52
J20302730+4113253 20:30:27.300 +41:13:25.13 Ofpe 38612 5.79
J20281547+4038196 20:28:15.471 +40:38:19.81 B0V: 32816 4.79
J20323951+4052475 20:32:39.507 +40:52:47.46 B0:V: 32816 5.19
J20331050+4122224 20:33:10.502 +41:22:22.44 B0V 32816 4.45
J20305552+4109575 20:30:55.516 +40:54:54.03 B0V 32816 4.66
J20295701+4109538 20:29:57.010 +41:09:53.84 B0V 32816 4.64
J20305111+4120218 20:30:51.115 +41:20:21.78 B0V 32816 4.53
J20331130+4042337 20:33:11.300 +40:42:33.73 B0:III: 30308 4.72
J20333821+4041064 20:33:38.213 +40:41:06.35 B0Ia 27800 5.64
J20344471+4051465 20:34:44.716 +40:51:46.73 B0Ia 27800 5.59
J20323904+4100078 20:32:39.057 +41:00:07.78 B0Ia 27800 5.79
J20322774+4128522 20:32:27.738 +41:28:52.26 B0Ib 27800 4.24
J20345878+4136174 20:34:58.781 +41:36:17.35 B0Ib(n)sb 27800 5.31
J20333822+4053412 20:33:38.218 +40:53:41.19 B0Ib 27800 4.94
J20333910+4119258 20:33:39.102 +41:19:25.98 B0Iab 27800 5.23
J20323498+4052390 20:32:34.848 +40:52:39.46 B0.2V 31906 4.58
J20292449+4052599 20:29:24.485 +40:52:59.85 B0.2IV 30588 4.71
J20321568+4046170 20:32:15.679 +40:46:17.00 B0.2IV 30588 4.39
J20294666+4105083 20:29:46.672 +41:05:08.32 B0.5V(n)sb 30288 4.50
J20282772+4104018 20:28:27.723 +41:04:01.80 B0.5V 30288 4.47
J20314605+4043246 20:31:46.053 +40:43:24.61 B0.5IV 28969 4.55
J20331870+4059379 20:33:18.696 +40:59:37.92 B0.5IIIe 27651 4.64
J20303970+4108489 20:30:39.701 +41:08:48.80 B0.7Ib 24014 5.30
J20294060+4109585 20:29:40.601 +41:09:58.54 B1[e] 27173 4.27
J20313338+4122490 20:31:33.378 +41:22:49.02 B1V 27173 4.25
J20340435+4108078 20:34:04.349 +41:08:07.91 B1V 27173 4.27
J20303833+4010538 20:30:38.329 +40:10:53.84 B1V 27173 4.34
J20293473+4020381 20:29:34.728 +40:20:38.09 B1V 27173 4.38
J20273982+4040384 20:27:39.821 +40:40:38.35 B1V 27173 4.28
J20303297+4044024 20:30:32.965 +40:44:02.41 B1V 27173 4.31
J20310464+4030568 20:31:04.659 +40:30:56.93 B1III:e 24903 5.95
J20334783+4120415 20:33:47.831 +41:20:41.37 B1III 24903 4.80
J20281539+4044046 20:28:15.392 +40:44:04.57 B1III 24903 4.58
J20310700+4035537 20:31:07.003 +40:35:53.73 B1III 24903 4.08
J20314885+4038001 20:31:48.848 +40:38:00.05 B1II 23767 4.04
J20333078+4115226 20:33:30.791 +41:15:22.70 B1I 22632 5.41
J20312203+4131284 20:31:22.026 +41:31:28.40 B1Ib: 22632 4.45
J20322734+4055184 20:32:27.339 +40:55:18.25 B2V 21092 4.39
J20312210+4112029 20:31:22.101 +41:12:02.87 B2V 21092 4.10
J20354703+4053012 20:35:47.026 +40:53:01.17 B2V 21092 3.91
J20284657+4107069 20:28:46.566 +41:07:06.86 B2II 19482 3.83
J20320689+4117570 20:32:06.877 +41:17:56.97 B3V 18546 3.91
Cygnus OB9
HD229196 20:23:10.784 +40:52:29.85 O5 41250 5.89
J20223777+4140292 20:22:37.766 +41:40:29.23 O5If 37630 5.48
BD+394177 20:25:22.122 +40:13:01.09 O6.5 38820 5.55
HD229250 20:24:11.733 +39:40:41.54 O7 38010 5.44
BD+394168 20:24:21.475 +39:46:03.90 O7 38010 5.48
HD229202 20:23:22.840 +40:09:22.53 O8V: 36390 5.14
BD+404159 20:25:06.521 +40:35:49.78 O9V 34770 4.61
BD+404148 20:23:14.549 +40:45:19.07 O9.5:V 33960 4.88
J20194916+4052090 20:19:49.156 +40:52:08.99 O9.5V 33960 4.86
J20190610+4037004 20:19:06.102 +40:37:00.39 O9.7Iab 29922 4.74
HD193945 20:21:25.823 +41:11:39.56 B0Vnn 32816 4.92
BD+384058 20:23:28.531 +39:20:59.05 B0V 32816 4.83
LSII+4032 20:25:28.893 +40:12:54.13 B0III 30308 4.36
J20243872+3930301 20:24:38.720 +39:30:30.10 B0I: 27800 4.58
J20183413+4025045 20:18:34.130 +40:25:04.47 B0.2IV 30588 4.86
NGC6910-14 20:23:07.575 +40:46:08.87 B0.5V 30288 4.56
Object RA (hhmmss) Dec ( ) SpT (K) log ()
J20240515+4046035 20:24:05.154 +40:46:03.51 B0.5V 30288 4.60
HD194092 20:22:05.443 +40:59:08.17 B0.5III 27651 4.39
HD228882 20:18:57.784 +40:42:18.52 B0.5Ia 25014 5.25
HD228929 20:19:36.542 +39:54:41.80 B0.5Ib 25014 5.13
J20234624+3937078 20:23:46.238 +39:37:07.83 B0.7IV 27818 4.54
J20241767+3920326 20:24:17.666 +39:20:32.56 B1V 27173 4.25
J20214868+4043005 20:21:48.682 +40:43:00.45 B1V 27173 4.32
HD228919 20:19:27.908 +40:27:42.09 B1IV 26038 4.34
J20233375+4045199 20:23:33.752 +40:45:19.93 B1III 24903 4.03
J20223944+3935420 20:22:39.442 +39:35:42.02 B1III 24903 4.42
J20220454+4042487 20:22:04.541 +40:42:48.73 B1III 24903 4.28
J20215593+4110129 20:21:55.930 +41:10:12.92 B1III 24903 4.03
J20220879+3958161 20:22:08.793 +39:58:16.07 B1II 23767 4.38
J20214410+4012529 20:21:44.103 +40:12:52.91 B1Ia 22632 5.18
J20215160+3959496 20:21:51.600 +39:59:49.61 B1Ib 22632 3.94
J20203933+4031176 20:20:39.334 +40:31:17.64 B1.5V 24132 4.08
J20204933+4033027 20:20:49.333 +40:33:02.73 B1.5V 24132 4.15
HD228911 20:19:21.712 +40:53:16.46 B2 21092 4.25
HD194194 20:22:44.760 +40:42:52.63 B2III 20019 3.91
J20211677+4023162 20:21:16.773 +40:23:16.19 B2III 20019 3.94
J20250591+4020124 20:25:05.912 +40:20:12.44 B2III 20019 3.75
HD228928 20:19:32.709 +40:39:13.75 B2Ib:nn 18945 4.38
HD228941 20:19:40.169 +40:53:19.19 B3 18546 3.98
BD+404146 20:23:10.464 +40:45:52.34 B3 18546 4.35
NGC6910-16 20:23:07.301 +40:46:55.25 B3 18546 4.05
J20215115+3934215 20:21:51.149 +39:37:51.47 B3V 18546 4.18
J20221729+3946035 20:22:17.286 +39:34:21.50 B5Ia 14012 4.01
HD228821 20:18:04.930 +40:06:06.80 B8 12449 3.03
HD193426 20:18:39.749 +40:13:36.89 B9Ia 11457 4.85
Boundaries
BD+433654 20:33:36.079 +43:59:07.38 O4If 39270 5.87
LSII+3953 20:27:17.572 +39:44:32.60 O7V: 38010 5.07
BD+423760 20:28:40.812 +43:08:58.46 O8.5V 35580 4.96
BD+423835 20:42:06.863 +43:11:03.72 O9p… 34770 5.25
J20342894+4156171 20:34:28.941 +41:56:17.09 O9V 34770 4.86
J20462826+4223417 20:46:28.255 +42:23:41.74 O9V 34770 4.88
J20272553+3929246 20:27:25.529 +39:29:24.58 O9.5V 33960 4.88
J20325571+4307583 20:32:55.713 +43:07:58.26 O9.5V 33960 4.97
J20310838+4202422 20:31:08.376 +42:02:42.25 O9.7II 30859 5.15
HD199021 20:52:53.207 +42:36:27.87 B0V 32816 5.05
J20382040+4156563 20:38:20.413 +41:56:56.51 B0II 29054 5.29
J20385918+4202395 20:38:59.181 +42:02:39.45 B0Ib 27800 4.67
J20294195+3859342 20:29:41.952 +38:59:34.16 B0.2V 31906 4.61
J20352227+4355305 20:35:22.266 +43:55:30.46 B0.2IV 30588 4.90
BD+413794 20:32:02.204 +42:12:26.15 B0.2III 29270 4.97
HD194839 20:26:21.545 +41:22:45.65 B0.5Iae 25014 5.73
J20313693+4201218 20:31:36.921 +42:01:21.79 B0.7Ib 24014 4.96
J20301273+3904216 20:30:12.732 +39:04:21.59 B1V 27173 4.24
LSIII+4217 20:35:10.623 +42:20:22.83 B1III 24903 4.69
J20374323+4232334 20:37:43.232 +42:32:33.40 B1III 24903 4.13
J20314215+4225532 20:31:42.151 +42:25:53.26 B1Ib 22632 5.67
BD+373976 20:33:49.752 +38:17:00.06 B1.5Vn 24132 4.17
J20313853+4152585 20:31:38.532 +41:52:58.46 B1.5V 24132 4.07
J20312725+4304227 20:31:27.253 +43:04:22.67 B1.5V 24132 4.33
BD+394189 20:26:20.922 +39:40:10.06 B2p?e? 21092 4.85
BD+413762 20:28:15.212 +42:25:39.14 B2V 21092 4.32
J20452110+4223514 20:45:21.103 +42:23:51.37 B2V 21092 3.99
J20264025+4233221 20:26:40.251 +42:33:22.09 B2II 19482 3.67
HD196489 20:36:24.259 +39:11:40.70 B3V 18546 3.92
J20462289+4212311 20:46:22.892 +42:12:31.07 B3V 18546 4.06
HD194779 20:25:55.077 +41:20:11.73 B3II 16983 4.12
BD+384098 20:27:33.010 +38:46:19.62 B9Ib 11457 3.92

Appendix B Candidate spectra

In Fig. 12 are plotted the normalized spectra of the 61 OB candidates from the list of Comerón & Pasquali (2012). The spectra have been corrected for the stellar radial velocities and diagnostic lines are also indicated.

Figure 12: Spectra of the 61 OB candidate stars where dotted vertical lines indicate H, He, Si and Mg lines in the wavelength range.

Footnotes

  1. magnitudes from the USNO-B catalog. and magnitudes from 2MASS catalog. Region a indicates the 1 deg. circular area centered on Cyg OB28 trapezium adopted by Wright et al. (2015) for the Cygnus OB2 association. Region b indicates the 1 deg. radius area adopted by Comerón & Pasquali (2012) for the same Cygnus OB2 association. Region c indicates the surrounding area outside the 1 deg. radius adopted by Comerón & Pasquali (2012). For binary stars asterisks indicate possible SB2 stars, whose spectral types are refered to the primary component.

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