Fundamental parameters of Galactic luminous OB stars VI. Temperatures, masses and WLR of Cyg OB2 supergiants

Abstract

Abstract.We have analyzed six OB supergiants and one giant covering spectral types from O3 to B1 in the Galactic OB association Cyg OB2 by means of an updated version of FASTWIND (Santolaya-Rey et al. 1997) that includes an approximate treatment of metal line blocking and blanketing. This large coverage in spectral type allows us to derive a new temperature scale for Galactic O supergiants that is lower than the one obtained by using pure H-He models, either plane-parallel and hydrostatic or spherical with mass-loss. The lower temperatures are thus a combined effect of line blanketing and the large mass-loss rates. In some cases, the newly derived effective temperature is reduced by up to 8000 K. Changes are larger for earlier stars with large mass-loss rates. As a consequence, luminosities are modified as well, which results in a lower number of emerging ionizing photons and reduces the mass discrepancy. Although there are still significant differences between spectroscopic and evolutionary masses, we do not find any obvious systematic pattern of those differences. We derive mass-loss rates and the corresponding wind momentum-luminosity relation for the analyzed stars. Although consistent with previous results by Puls et al. (1996) for Galactic stars, our relation is better defined due to a reduction of errors related to stellar distances and points to a possible separation between extreme Of stars (Of + , Of * ) and stars with more moderate morphologies. However this finding is only tentative, as the statistics are still scarce.

Bright OB stars in the Galaxy

“…(Admittedly, most of the early type (high L) objects are Ia's, whereas the later types concentrate around Iab's with few Ia/Ib's.) This finding becomes even more apparent when the WLR is extended towards higher luminosities by including Galactic O supergiants (from Repolust et al 2004;Markova et al 2004;Herrero et al 2002), as done on the right of the same figure. KPL99 were the first to point out that the offsets in the corresponding WLR of OBA-supergiants depend on spectral type, being strongest for O-SGs, decreasing from B0-B1 to B1.5-B3 and increasing again towards A supergiants.…”

“…While the number of Galactic O and early B stars with reliably determined stellar and wind parameters has progressively increased during the last few years (e.g., Herrero et al 2002;Repolust et al 2004;Garcia & Bianchi 2004;Bouret et al 2005;Martins et al 2005;Crowther et al 2006), mid and late B supergiants (SGs) are currently under-represented in the sample of stars investigated so far. Given the fact that B-SGs represent an important phase in the evolutionary sequence of massive stars, any study aiming to increase our knowledge of these stars would be highly valuable, since it would allow several important issues to be addressed (see below).…”

“…During the last years, the quantitative analyses of spectra in the far-UV/UV and optical domains (e.g., Herrero et al 2002;Bianchi & Garcia 2002;Crowther et al 2002;Bouret et al 2003;Repolust et al 2004;Massey et al 2004;Heap et al 2006) have unambiguously shown that the inclusion of line-blocking and blanketing and wind effects (if present) significantly modifies the temperature scale of O-stars (for a recent calibration at solar metallicity, see Markova et al 2004;Martins et al 2005). Regarding B-SGs, particularly of later subtype, this issue has not been addressed so far, mostly due to lacking T eff estimates.…”

Investigation of Cyg OB2 #11(O5 Ifc) by Modelling its Atmosphere

“…For comparison, Table 1 gives the parameters of Cyg OB2 #11 derived by Herrero et al (2002) and Mokiem et al (2005). The temperature difference is within error limits.…”

“…Atmospheric parameters of Cyg OB2 #11, derived in this work -(1) and in the works of Herrero et al (2002) - (2) and Mokiem et al (2005) -(3). Determination of carbon fraction in atmospheres of O-stars is a very important and difficult task.…”