The Helium Spectrum of Cool Stars: Towards Atmospheric Abundances

Details The Helium Spectrum of Cool Stars: Towards Atmospheric Abundances The Helium Spectrum of Cool Stars: Towards Atmospheric Abundances

Dec 1992

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992aas...181.5115s&link_type=abstract

American Astronomical Society, 181st AAS Meeting, #51.15; Bulletin of the American Astronomical Society, Vol. 24, p.1208

Astronomy and Astrophysics

Astronomy

1

Scientific paper

Any spectral line of helium appearing in the spectrum of a F- to M-type star must be formed in conditions away from equilibrium. A combination of theoretical and observational complexity in the helium spectrum problem prevents so far direct determination of helium abundances in a range of objects. Old cool stars have the potential to yield accurate primordial helium abundance Y_P, clues to globular cluster evolution, stellar pulsation theory, etc. In a first step to a comprehensive theory of the helium spectrum in non-equilibrium stellar atmospheres we identify the major processes involved. Briefly these are: (a) radiative transfer effects due to the H I Lyman emission lines on the populations of He II; (b) photoionization of He I by the wings of the He II lambda 304 line; (c) photoionization by coronal incident radiation; (d) effects of transition region particle diffusion on He I lines. We explore the relative importance of these processes in a set of model calculations ranging from the quiet solar models to cool supergiants. The atmospheres are dynamic and non-LTE. We discuss abundance sensitivity of the He I lambda 10830 and He II lambda 1640 lines in view of their complex mechanisms of line formation. The accuracy of helium abundance determination is limited by the effects of atmospheric inhomogeneity and atomic data. However, it is higher than in the currently used direct methods (H II regions), and more reliable than most indirect ones.

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