Properties of dwarf stars in Cygnus OB2

We present the results of investigation of five stars, originally classified as dwarfs, belonging to Cyg OB2 association, their stellar and wind properties. Using both TLUSTY and CMFGEN codes we derived effective temperatures, surface gravities, chemical abundances, mass-loss rates and projected rotation velocities. Due to the fact that distance to the stars is well known, we were able to estimate their luminosities. Using evolutionary models we estimated the ages of these sample stars and find that lower mass ones - MT282 and MT343 - belong to older population of the association. Their ages are greater than 10 Myr. The ages of three other stars - MT317, MT299, MT259 - are between 4-6 Myr.Comment: 16 pages, 10 figures, accepted for publication in PAS

GR 290 (Romano's Star): 2. Light history and evolutionary state

We have built the historical light curve of the luminous variable GR 290 back to 1901, from old observations of the star found in several archival plates of M 33. These old recordings together with published and new data show that for at least half a century the star was in a low luminosity state, with B ~18. After 1960, five large variability cycles of visual luminosity were recorded. The amplitude of the oscillations was seen increasing towards the 1992-1994 maximum, then decreasing during the last maxima. The recent light curve indicates that the photometric variations have been quite similar in all the bands, and that the B-V color index has been constant within +/-0.1 m despite the 1.5m change of the visual luminosity. The spectrum of GR 290 at the large maximum of 1992-94, was equivalent to late-B type, while, during 2002-2014, it has varied between WN10h-11h near the visual maxima to WN8h-9h at the luminosity minima. We have detected, during this same period, a clear anti-correlation between the visual luminosity, the strength of the HeII 4686 A emission line, the strength of the 4600-4700 A lines blend and the spectral type. From a model analysis of the spectra collected during the whole 2002-2014 period we find that the Rosseland radius R_{2/3}, changed between the minimum and maximum luminosity phases by a factor of 3, while T_eff varied between about 33,000 K and 23,000 K. The bolometric luminosity of the star was not constant, but increased by a factor of ~1.5 between minimum and maximum luminosity, in phase with the apparent luminosity variations. In the light of current evolutionary models of very massive stars, we find that GR 290 has evolved from a ~60 M_Sun progenitor star and should have an age of about 4 million years. We argue that it has left the LBV stage and is moving to a Wolf-Rayet stage of late nitrogen spectral type.Comment: Accepted on The Astronomical Journal, 10 figures. Replaced because the previous uploaded file was that without the final small corrections requested by the refere

On the nature of high reddening of Cygnus OB2 #12 hypergiant

© 2016 The Authors.To explain the nature of the high reddening (AV ≃ 10 mag) towards one of the most luminous stars in the Galaxy - Cyg OB2 #12 (B5 Ia-0), also known as MT304, we carried out spectrophotometric observations of 24 stars located in its vicinity. We included five new Bstars among the members of Cygnus OB2, and for five more photometrically selected stars we spectroscopically confirmed their membership. We constructed the map of interstellar extinction within 2.5 arcmin radius and found that interstellar extinction increases towards MT304. According to our results the most reddened OB-stars in the association after MT304 are J203240.35+411420.1 and J203239.90+411436.2, located about 15 arcsec away from it. Interstellar extinction towards these stars is about 9 mag. The increase of reddening towards MT304 suggests that the reddening excess may be caused by the circumstellar shell ejected by the star during its evolution. This shell absorbs 1 mag, but its chemical composition and temperature are unclear. We also report the detection of a second component of MT304, and discovery of an even fainter third component, based on data of speckle interferometric observations taken with the Russian 6-m telescope

Analysis of Interstellar Extinction towards the Hypergiant Cyg OB2 No. 12

The Cyg OB2 stellar association hosts an entire zoo of unique objects, and among them – an enigmatic star Cyg OB2 No. 12 (Schulte 12, MT 304). MT 304 is enigmatic not only due to its highest luminosity (according to various estimates, it is one of the brightest stars in the Galaxy), but also because its reddening is anomalously large, greater than the mean reddening in the association. To explain the nature of anomalous reddening (AV ≃ 10 mag) of MT 304, we carried out spectrophotometric observations of 25 stars located in its vicinity. We mapped interstellar extinction within the 2.5 arcmin radius and found it to increase towards MT 304. According to our results, the two most reddened stars in the association after MT 304 are J203240.35+411420.1 and J203239.90+411436.2, both located about 15 arcsec from it. Interstellar extinction AV towards these stars is about 9 mag. Our results favor the hypothesis of the circumstellar nature of reddening excess. In the second part of the paper we present the results of our modeling of MT 282 (B1 IV) and MT 343 (B1 V), which belong to the older population of the association and have ages greater than 10 Myr

On the nature of high reddening of Cygnus OB2 #12 hypergiant

© 2016 The Authors.To explain the nature of the high reddening (AV ≃ 10 mag) towards one of the most luminous stars in the Galaxy - Cyg OB2 #12 (B5 Ia-0), also known as MT304, we carried out spectrophotometric observations of 24 stars located in its vicinity. We included five new Bstars among the members of Cygnus OB2, and for five more photometrically selected stars we spectroscopically confirmed their membership. We constructed the map of interstellar extinction within 2.5 arcmin radius and found that interstellar extinction increases towards MT304. According to our results the most reddened OB-stars in the association after MT304 are J203240.35+411420.1 and J203239.90+411436.2, located about 15 arcsec away from it. Interstellar extinction towards these stars is about 9 mag. The increase of reddening towards MT304 suggests that the reddening excess may be caused by the circumstellar shell ejected by the star during its evolution. This shell absorbs 1 mag, but its chemical composition and temperature are unclear. We also report the detection of a second component of MT304, and discovery of an even fainter third component, based on data of speckle interferometric observations taken with the Russian 6-m telescope