Fundamental parameters of galactic luminous OB stars - III. Spectroscopic analysis of O stars in Cygnus OB2

We present the results of the spectral analysis of 11 OB stars in Cyg OB2, among them seven giants and supergiants. The projected rotational velocitie s of these stars are low or moderate. We find that only one of the stars (p reviously classified as luminosity class V) shows helium enhancement, and t hat this turns out to be compatible with standard evolutionary models witho ut rotation. Only a second object gives an abundance that could be larger t han solar. In addition, these two stars are found to be the most luminous i n the sample. In summary, no helium discrepancy is found for the stars anal ysed in Cyg OB2. The cause of this result is speculated to be due to: a) th e 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 fou nd to have ages between I and 5 Myr, and the most massive ones have initial evolutionary masses in excess of 100 M.. Thus we confirm that Cyg OB2 is a young association rich in very massive stars. We study a possible correlat ion between the helium abundance and the stellar rotation and conclude that present data are consistent with the hypothesis that mixing processes prob ably related to rotation are present in the massive stars and in some cases strongly influence their early evolutionary phases.