Spectroscopic Determinations of Stellar Parameters and Photospheric Helium Abundances for Alpha Cygni and Beta Orionis

Abstract

A detailed non-LTE analysis of neutral helium lines was carried out for two early A--late B supergiants alpha Cyg and beta Ori in order to obtain information concerning the abundance of helium in their photospheres, the key element for studying evolution-induced mixing in the envelope of such massive stars. To minimize the systematic errors caused by uncertainties of model atmospheres, an elaborate study for determining their model parameters was first performed by using the Balmer line profiles (Hγ , Hdelta ) and the energy distributions from UV to IR, while attention was paid to the impact of the interstellar reddening effect. The best parameters were concluded to be: T_eff =~ 10000 K, log g =~ 1.5, and E_{B-V} =~ +0.15 for alpha Cyg; and T_eff =~ 13000 K, log g =~ 2.0, and E_{B-V} =~ +0.05 for beta Ori. This suggested somewhat higher T_eff's than those so far believed. These results, as well as the related fundamental stellar parameters (also derived), are discussed and compared along with other estimations. With these models, a reasonably good agreement was found between the theoretical predictions and the observations, except for the spectra in the far-UV or far-IR regions, which are presumably affected by the extended circumstellar envelope surrounding these stars. The resulting helium abundances based on these model atmospheres indicated an apparent He-deficiency by as much as ~ 0.8 dex in alpha Cyg and a moderate He-excess of ~ 0.4 dex in beta Ori. When combined with the evidence of N-excess for both stars, the former consequence for alpha Cyg is rather difficult to interpret, and further investigations are therefore needed concerning this matter; the latter implies that the surface layer of beta Ori is most likely contaminated by the CNO-cycled products dredged-up from the interior, though its evolutionary status is yet unsettled.