Astronomy and Astrophysics – Astronomy
Scientific paper
Apr 1994
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994pasj...46..181t&link_type=abstract
PASJ: Publications of the Astronomical Society of Japan (ISSN 0004-6264), vol. 46, no. 2, p. 181-203
Astronomy and Astrophysics
Astronomy
18
Abundance, Cygnus Constellation, Helium, Infrared Spectra, Interstellar Extinction, Light (Visible Radiation), Orion Constellation, Photosphere, Stellar Atmospheres, Stellar Envelopes, Stellar Evolution, Stellar Models, Supergiant Stars, Ultraviolet Spectra, Balmer Series, Energy Distribution, Infrared Astronomy, Local Thermodynamic Equilibrium, Spectroscopy, Stellar Temperature, Ultraviolet Astronomy
Scientific paper
A detailed non-local thermodynamic equilibrium (non-LTE) analysis of neutral helium lines was carried out for two early A-late B supergiants Alpha Cygni and Beta Orionis 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 (H(gamma), H(delta)) and the energy distributions from ultraviolet (UV) to infrared (IR), while attention was paid to the impact of the interstellar reddening effect. The best parameters were concluded to be: Teff approximately equals 10000 K, log g approximately equals 1.5, and EB-V approximately equals +0.15 for Alpha Cygni; and Teff approximately equals 13000 K, log g approximately equals 2.0, and EB-V approximately equals +0.05 for Beta Orionis. This suggested somewhat higher Teff'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 approximately 0.8 dex in Alpha Cygni and a moderate He-excess of approximately 0.4 dex in Beta Orionis. When combined with the evidence of N-excess for both stars, the former consequence for Alpha Cygni is rather difficult to interpret, and further investigations are therefore needed concerning this matter; the latter implies that the surface layer of Beta Orionis is most likely contaminated by the CNO-cycled products dredged-up from the interior, though its evolutionary status is yet unsettled.
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