The effect of EUV and X-ray radiation on stellar atmospheres and the problem of helium abundance in B-stars

Details The effect of EUV and X-ray radiation on stellar atmospheres and the problem of helium abundance in B-stars The effect of EUV and X-ray radiation on stellar atmospheres and the problem of helium abundance in B-stars

Jul 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992a%26a...260..303m&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 260, no. 1-2, p. 303-310.

Astronomy and Astrophysics

Astronomy

10

B Stars, Extreme Ultraviolet Radiation, Helium, Stellar Atmospheres, X Ray Astronomy, Abundance, Far Ultraviolet Radiation, H Lines

Scientific paper

A method for computing stellar atmosphere models irradiated externally is described. The results of calculations of equivalent widths and line profiles for He I lines in the atmospheres of B-stars are presented, taking UV radiation into account. It is shown that such coronal radiation significantly decreases the equivalent widths of the lines under consideration due to He I 'superionization'. The effect is significant for stars with effective temperature less than 25,000 K. The computed He I lines are used in a classical LTE analysis to estimate the effect of XUV radiation on derived helium abundances. Helium underabundances of 0.4 dex in the blue spectral range and of up to 2 dex in the red are predicted if the X-ray to bolometric luminosity ratio is 10 exp -8 to 10 exp 7. We suggest an interpretation of the phenomena of the 'He-weak' B-stars and sdOB stars, which represents an alternative to an interpretation in terms of chemical diffusion. We propose tests to discriminate between the two models on the basis of optical and X-ray observations.

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