HELIUM ENHANCEMENTS IN LUMINOUS OB-TYPE STARS - THE EFFECT OF MICROTURBULENCE

We present non-LTE analyses of high quality spectra for two B-type sup ergiants, kappa Orionis & epsilon Orionis (spectral types B0.5Ia & B0I a respectively). We investigate the effect of including a microturbule nt velocity in the H I, He II and He I line broadening on the derived stellar atmospheric parameters (T-eff, log g and y). Its inclusion has a significant effect on the profiles of the lines of neutral helium a nd leads to approximately normal estimates for the helium fractional a bundances for both supergiants. By contrast, adopting zero microturbul ence implies significant helium overabundances, if we adopt the mean a bundance of all the lines considered here. The He I lines at 4437, 438 7 and 5047 Angstrom are found to be rather insensitive to microturbule nce and hence are more reliable indicators of helium abundance. There are some remaining unresolved discrepancies, such as a systematic diff erence between singlet and tripler transitions which we attribute to t he neglect of line blocking, while the 5015 Angstrom 2(1)S-3(1)P trans ition remains much stronger than predicted. This latter problem we ten tatively attribute to the neglect of sphericity; the generalized dilut ion effect discussed by Voels et al. (1989). We suggest that a judicio us choice of He I lines, and the use of an appropriate microturbulent velocity, may contribute to resolving the 'helium discrepancy' in O-ty pe stars. The question of the origin of a perceived microturbulence in supergiants is also briefly discussed in the context of stellar winds . That two normal supergiants have close to solar helium abundances cl early implies that they have nor undergone dredge-up in a previous red supergiant phase of evolution, supporting the contention that massive stars do not perform blue loops in the HR diagram at solar metallicit y. Marginal evidence for a mild helium overabundance in kappa Ori howe ver could be interpreted as being due to mixing processes, perhaps dur ing its main sequence lifetime.