Properties of Galactic early-type O-supergiants. A combined FUV-UV and optical analysis
Abstract
Aims: We aim to constrain the properties and evolutionary status of early and mid-spectral type supergiants (from O4 to O7.5). These posses the highest mass-loss rates among the O stars, and exhibit conspicuous wind profiles.
Methods: Using the non-LTE wind code cmfgen we simultaneously analyzed the FUV-UV and optical spectral range to determine the photospheric properties and wind parameters. We derived effective temperatures, luminosities, surface gravities, surface abundances, mass-loss rates, wind terminal velocities, and clumping filling factors.
Results: The supergiants define a very clear evolutionary sequence, in terms of ages and masses, from younger and more massive stars to older stars with lower initial masses. O4 supergiants cluster around the 3 Myr isochrone and are more massive than 60 M⊙, while the O5 to O7.5 stars have masses in the range 50-40 M⊙ and are 4 ± 0.3 Myr old. The surface chemical composition is typical of evolved O supergiants (nitrogen-rich, carbon- and oxygen-poor). While the observed ranges of carbon and nitrogen mass-fractions are compatible with those expected from evolutionary models for the measured stellar masses, the N/C ratios as a function of age are inconsistent with the theoretical predictions for the four earliest (O4 spectral type) stars of the sample. We question the efficiency of rotational mixing as a function of age for these stars and suggest that another mechanism may be needed to explain the observed abundance patterns. Mass-loss rates derived with clumped-models range within a factor of three of the theoretical mass-loss rates. The corresponding volume-filling factors associated with small-scale clumping are 0.05 ± 0.02. Clumping is found to start close to the photosphere for all but three stars, two of which are fast rotators.
- Publication:
-
Astronomy and Astrophysics
- Pub Date:
- August 2012
- DOI:
- 10.1051/0004-6361/201118594
- arXiv:
- arXiv:1205.3075
- Bibcode:
- 2012A&A...544A..67B
- Keywords:
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- stars: early-type;
- stars: atmospheres;
- stars: winds;
- outflows;
- stars: fundamental parameters;
- stars: evolution;
- Astrophysics - Solar and Stellar Astrophysics
- E-Print:
- 30 pages, 11 figures, accepted for publication in A&