6 Conclusions

We have analyzed a homogeneous set of spectra of Cyg OB2 supergiants with spectral types ranging from O3 to B1. The analysis has been performed using a new version of the code presented by Santolaya-Rey et al. (1997) that includes an approximate treatement of metal line-blocking/blanketing. A test applied to the O9 V star 10 Lac resulted in an excellent fit, at temperatures lower than those obtained previously (Herrero et al. 1992) and in agreement with other findings (Hubeny et al. 1998; Martins et al. 2002).

The fits to the supergiants are also satisfactory, except for the discrepancy between the fits of H $_{\rm\alpha}$ and other higher Balmer lines for stars with dense winds (see discussion in Sect. 6.1 of Herrero et al. 2000) and the pure absorption profile predicted for He II $\lambda$ 4686 in Cyg OB2 #8C instead of the P-Cygni profile observed.

We obtain temperatures that are cooler than in our previous analyses, as a consequence of the effects of sphericity, mass-loss and metal-line blocking/blanketing. Thanks to our coverage in spectral type, we can construct a temperature scale for O-supergiants. This temperature scale is 4000 to 8000 K cooler at early types than the one presented by Vacca et al. (1996), mainly based on pure H-He analyses. An important additional result is that at the same spectral subtype, Cyg OB2 #8C and #11 have a difference of 3000 K in their effective temperatures, reflecting their very different mass-loss rates. For the hotter stars, additional information based on a nitrogen temperature scale is needed, as the He lines become progressively either insensitive to changes in stellar parameters or simply too weak.

Models including metal line opacities result in cooler temperatures and similar radii than those using pure H/He opacities, therefore giving smaller luminosities. Together with the blocking produced in the (E)UV, the corresponding ionizing luminosities may change drastically, which will have a large impact on studies of the surrounding regions. However, each particular case has to be treated individually.

The results obtained indicate that Cyg OB2 stars are very massive. Only for two of the seven stars we found spectroscopic or evolutionary masses below 40 $M_{\odot}$. Although there are still significant differences between spectroscopic and evolutionary masses, we do not find any obvious systematic pattern. Thus, at present we cannot conclude that there is a discrepancy between both sets of masses in Cyg OB2. However, this association is a very young one and the stars analyzed show only moderate projected rotational velocities: it might be possible that rotationally induced effects simply have had no sufficient time to become apparent. It is remarkable that we found an enhanced He abundance for only one of the stars, Cyg OB2 #7.

Finally, we derived a new calibration of the wind momentum-luminosity relationship for O supergiants, including errors resulting from our analysis. Our data indicate that there might be a different relation for extreme Of and for moderate O, Of stars, respectively, perhaps indicating different ionization conditions or clumping in the wind. Considering only the moderate O, Of stars, we obtain a very low scatter in the relation, but this might reflect only the low number statistics. In fact, our still limited sample is also consistent with a unique relation including all stars, even the B supergiant. Clearly, more data are needed to disentangle whether the large modified wind momentum rate we derive for the extreme Of stars is a real, physical effect or just the result of our poor statistics.

Acknowledgements

We thank an annonymous referee for very valuable suggestions that helped to improve the paper. FNP thanks the spanish MCyT for grants ESP98-1351 and PNAYA2000-1784 within the Ramón y Cajal Program. AHD thanks support from the spanish MCyT under project PNAYA2001-0436 that includes EU FEDER funds.