Astron. Astrophys. 348, 542-552 (1999)

Fundamental parameters of galactic luminous OB stars

III. Spectroscopic analysis of O stars in Cygnus OB2 ^*

A. Herrero ^1,2, L.J. Corral ¹, M.R. Villamariz ¹ and E.L. Martín <sup>1

1</sup> Instituto de Astrofísica de Canarias, c/Vía Lactea s/n, E-38200 La Laguna, Tenerife, Spain
² Universidad de La Laguna, Departamento de Astrofísica, Avda. Astrofísico Francisco Sánchez, s/n, E-38071 La Laguna, Spain

Received 26 January 1999 / Accepted 4 May 1999

Abstract

We present the results of the spectral analysis of 11 OB stars in Cyg OB2, among them seven giants and supergiants. The projected rotational velocities of these stars are low or moderate. We find that only one of the stars (previously classified as luminosity class V) shows helium enhancement, and that this turns out to be compatible with standard evolutionary models without rotation. Only a second object gives an abundance that could be larger than solar. In addition, these two stars are found to be the most luminous in the sample. In summary, no helium discrepancy is found for the stars analysed in Cyg OB2. The cause of this result is speculated to be due to: a) the youth of the stars studied, b) the low rotational velocity of the sample observed, or c) a combination of both. The massive stars in Cyg OB2 are found to have ages between 1 and 5 Myr, and the most massive ones have initial evolutionary masses in excess of 100 . Thus we confirm that Cyg OB2 is a young association rich in very massive stars. We study a possible correlation between the helium abundance and the stellar rotation and conclude that present data are consistent with the hypothesis that mixing processes probably related to rotation are present in the massive stars and in some cases strongly influence their early evolutionary phases.

Key words: stars: early-type stars: atmospheres stars: fundamental parameters stars: evolution stars: individual: Cyg OB2

* The Isaac Newton Telescope is operated on the island of La Palma by the RGO in the Spanish Obervatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias.

Present address: University of California, 601 Campbell Hall, Berkeley, CA 94270, USA

Send offprint requests to: ahd@ll.iac.es

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Contents

• 1. Introduction
• 2. Observations
• 3. Spectral analysis
  • 3.1. Comments on individual stars
• 4. Discussion
• 5. Conclusions and future work
• Acknowledgements
• References

© European Southern Observatory (ESO) 1999

Online publication: July 26, 1999
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