Study of a sample of faint Be stars in the exofield of CoRoT

Abstract

Context. The search and interpretation of non-radial pulsations from Be star light curves observed with the CoRoT satellite requires high-quality stellar astrophysical parameters. Aims. The present work is devoted to the spectroscopic study of a sample of faint Be stars observed by CoRoT in the fourth long run (LRA02). Methods. The astrophysical parameters were determined from the spectra in the λλ4000–4500 Å wavelength domain observed with the VLT/FLAMES instruments at ESO. Spectra were fitted with models of stellar atmospheres using our GIRFIT package. Spectra obtained in the λλ6400–7200 Å wavelength domain enabled the confirmation or, otherwise, a first identification of Be star candidates. Results. 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) stellar masses that are in agreement with those measured for detached binary systems; (2) spectroscopic 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 Hα, Hγ and Hδ emission lines produced: (1) extents of the circumstellar disk (CD) emitting regions that agree with the interferometric inferences in other Be stars; (2) R– dependent exponents n(R) = ln[ρ(R)/ρo]/ln(Ro/R) of the CD radial density distributions; and (3) CD base densities ρo similar to those inferred in other recent works. Conclusions. The Hγ and Hδ 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 Hα. Further detailed studies of Hγ and Hδ 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. If it is proven that the studied Be stars are products of binary mass transfer phases, the errors induced on the estimated Teff by the presence of stripped sub-dwarf O/B companions are not likely to exceed their present uncertainties.