[Paper Review] Properties of Galactic early-type O-supergiants: A combined FUV-UV and optical analysis

J.‐C. Bouret, D. J. Hillier+2 more|arXiv (Cornell University)|May 14, 2012

Stellar, planetary, and galactic studies119 references88 citations

TL;DR

This study combines FUV-UV and optical spectroscopy with non-LTE wind modeling (CMFGEN) to derive physical and wind parameters for 16 Galactic early-type O-supergiants (O4–O7.5). It finds that observed N/C ratios deviate from theoretical predictions, suggesting limitations in rotational mixing models, while mass-loss rates are consistent with theory when clumping is accounted for, with filling factors of 0.05 ± 0.02 and clumping initiating near the photosphere for most stars.

ABSTRACT

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. 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. 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 Msun, while the O5 to O7.5 stars have masses in the range 50 - 40 Msun 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.

Motivation & Objective

• To constrain the evolutionary status and physical parameters of early-type O-supergiants using multi-wavelength spectroscopy.
• To investigate the reliability of theoretical mass-loss rates by accounting for wind clumping in non-LTE models.
• To assess the role of rotational mixing in shaping surface abundances by comparing observed N/C ratios with theoretical predictions.
• To improve the accuracy of surface gravity and mass determinations by modeling both FUV-UV and optical lines simultaneously.
• To explore the radial stratification of clumping and its impact on spectral diagnostics in massive star winds.

Proposed method

• Simultaneous analysis of FUV-UV and optical spectra using the non-LTE radiative transfer code CMFGEN to model both photospheric and wind components.
• Use of FUSE and IUE FUV-UV data, combined with ELODIE and FEROS optical spectra, for comprehensive spectral coverage.
• Application of clumped-wind models with variable filling factors to correct for overestimation of mass-loss rates due to density inhomogeneities.
• Incorporation of microclumping effects via the porosity (or macroclumping) formalism to match observed line profiles and ionization fractions.
• Adoption of a non-void interclump medium model to better reproduce observed spectroscopic features.
• Use of ionization equilibrium diagnostics (e.g., P v) to validate model assumptions and adjust metal abundances, including sub-solar P abundance to match observations.

Experimental results

Research questions

• RQ1 How do the derived mass-loss rates of O-supergiants compare with theoretical predictions when clumping is accounted for?
• RQ2 To what extent do observed N/C abundance ratios in O4–O7.5 supergiants agree with predictions from stellar evolution models including rotational mixing?
• RQ3 Where and how does wind clumping begin in O-supergiants, and what is its radial distribution?
• RQ4 What is the impact of microclumping on the ionization structure and emergent spectrum of massive star winds?
• RQ5 Why do theoretical mass-loss rates systematically exceed empirical estimates, and is this discrepancy luminosity-dependent?

Key findings

• O4 supergiants are younger than 3 Myr and more massive than 60 M☉, while O5–O7.5 stars are 4.0 ± 0.3 Myr old with masses between 50 and 40 M☉.
• The surface composition is nitrogen-rich and carbon- and oxygen-poor, consistent with evolved O supergiants.
• Observed N/C ratios for the four O4 stars deviate from theoretical predictions, suggesting insufficient rotational mixing or need for alternative mixing mechanisms.
• Mass-loss rates derived with clumped models are within a factor of 3 of theoretical values, with clumping filling factors of 0.05 ± 0.02.
• Clumping begins close to the photosphere for all but three stars, with two fast rotators (λ Cep, ζ Puppis) and one additional fast rotator (HD 16691) showing delayed onset.
• Theoretical-to-observed mass-loss rate ratios increase with luminosity, indicating potential overestimation of wind driving in models for hotter, more massive stars.