Study of a Sample of Faint Be Stars in the Exofield of CoRoT. Part III. Global Spectroscopic Characterization and Astrophysical Parameters of the Central Stars

TL;DR

This study provides a detailed spectroscopic analysis of faint Be stars observed by CoRoT, determining their physical parameters, circumstellar disk properties, and rotational characteristics, revealing they are subcritical rotators with implications for their mass ejection mechanisms.

Contribution

It offers the first comprehensive spectroscopic characterization of faint Be stars in the CoRoT field, including corrected stellar parameters and insights into their circumstellar disks and rotation.

Findings

Be stars are subcritical rotators with rotation ratios of 0.6-0.7.

Circumstellar disk extents and density distributions match interferometric data.

Hgamma and Hdelta lines form close to the star, indicating inner disk properties.

Abstract

The present work is devoted to the study of faint Be stars observed by CoRoT in the fourth long run (LRA02). The astrophysical parameters were determined from the spectra observed with the VLT/FLAMES instruments at ESO. Spectra were fitted with models of stellar atmospheres using our GIRFIT package. Spectra in the lambda-lambda 6400-7200$ AA domain enabled the confirmation or a first identification of Be star candidates. The apparent parameters (Teff,log g,Vsin i) for a set of 19 B and Be stars were corrected for the effects induced by the rapid rotation. These allowed us to determine: 1) masses that are in agreement with those measured for detached binary systems; 2) distances that agree with the GAIA parallaxes; and 3) centrifugal/gravity equatorial force ratios of ~0.6-0.7, which indicate that our Be stars are subcritical rotators. A study of the Balmer Halpha, Hgamma and Hdelta emission lines produced: extents of the circumstellar disk (CD) emitting regions that agree with the interferometric inferences in other Be stars; R-dependent exponents n(R) of the CD radial density distributions; CD base densities. The Hgamma and Hdelta emission lines are formed in CD layers close to the central star. These lines produced a different value of the exponent n(R) than assumed for Halpha. Further detailed studies of Hgamma and Hdelta emission lines could reveal the physical properties of regions where the viscous transport of angular momentum to the remaining CD regions is likely to originate from. The subcritical rotation of Be stars suggests that their huge discrete mass-ejections and concomitant non-radial pulsations might have a common origin in stellar envelope regions that become unstable to convection due to rotation. The errors induced on the estimated Teff by the possible presence of stripped sub-dwarf O/B companions are not likely to exceed their present uncertainties.