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. Based on observations made with the NASA-CNES-CSA Far Ultraviolet Spectroscopic Explorer (FUSE) and by the NASA-ESA-SERC International Ultraviolet Explorer (IUE), and retrieved from the Multimission Archive at the Space Telescope Science Institute (MAST). Based on observations collected with the ELODIE spectrograph on the 1.93-m telescope (Observatoire de Haute-Provence, France). Based on observations collected with the FEROS instrument on the ESO 2.2 m telescope, program 074.D-0300 and 075.D-0061.Appendix A is available in electronic form at http://www.aanda.org
Astronomy and Astrophysics
- Pub Date:
- August 2012
- stars: early-type;
- stars: atmospheres;
- stars: winds;
- stars: fundamental parameters;
- stars: evolution;
- Astrophysics - Solar and Stellar Astrophysics
- 30 pages, 11 figures, accepted for publication in A&