The He-rich stars are the most massive CP stars. Their helium abundance ranges from nearly solar to larger than unity with respect to hydrogen. Spectroscopic and photometric variability is explained by an abundance distribution across the stellar surface. To explain the helium abundance itself a link between mass loss, abundance anomaly, radiative diffusion and magnetic field is suggested. Given standard atmospheric conditions in B-type stars (assuming no wind), diffusion is unable to support helium in the stellar atmosphere and helium sinks. However, Vauclair (1975) showed that diffusion could lead to He overabundance in the presence of stellar mass-loss. Models of abundance anomalies with selective mass-loss for He-rich stars suggest normal CNO abundances as a test (Michaud et al. 1987). More specifically, the model distinguishes between helium ionization degree, convective zone location, magnetic field configuration and stellar wind strength in the stellar atmosphere. Consequently, the CNO abundance pattern would depend on several parameters including effective temperature. In this contribution, we analyse both low and high-resolution ESO spectra to obtain light element abundances for He-rich stars and to put constraints on the theoretical model.
© European Southern Observatory (ESO) 1999
Online publication: April 12, 1999