The He i lambda 10830 Line in Classical Cepheids. II. Mechanism of Formation

Details The He i lambda 10830 Line in Classical Cepheids. II. Mechanism of Formation The He i lambda 10830 Line in Classical Cepheids. II. Mechanism of Formation

Mar 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994apj...423..785s&link_type=abstract

Astrophysical Journal v.423, p.785

Astronomy and Astrophysics

Astronomy

12

Stars: Variables: Cepheids, Line: Formation, Radiative Transfer, Stars: Chromospheres

Scientific paper

We present the experimental approach to constrain the theory of formation of the He I 10830 line, and the helium spectrum in a cool stellar atmosphere. It consists of using the regularly pulsating atmosphere of a classical Cepheid as a modulating environment of the He I λ10830 line.
We study the radiative transfer problem for hydrogen and helium in detail, including all stages of ionization and compute time-dependent models of the dynamic non-LTE radiative transfer in H, He, Ca ii, and Mg ii in the pulsating Cepheid atmospheres. We conclude that the formation of the He I λ10830 absorption line is an interplay between the population number density of the 2s 3S state and the corresponding integrated optical depth in the 10830 Å transition. This interplay is complicated by the different mechanisms populating the 2s 3S state, because they are depth dependent. Our use of Cepheids solves the problem by providing model- independent depth discrimination due to their radial pulsation. Thus our combination of 1.08 microns and Hα observations constrain the technically unobservable EUV spectrum of He II, which must be considered (via its coronal irradiation) in the non-LTE formation of He I λ10830 as a potentially important mechanism for populating the 2s 3S state.
In the special case of Cepheids, we find that photoionization of He I by any coronal source is small; the triplet states are populated mostly collisionally. However, for luminous cool stars in general, photoionization of He I and H I by the wings of the He II λ304 line will have important consequences for He I λ10830. This applies even for stars with strong coronal X-ray flux. That is, of course, if the assumption, that the He II λ304 line is the dominant EUV emission line in luminous cool stars (as it is in the Sun), is true. We suggest that the more accessible He II λ1640 lines (and its multiplet structure) can provide access to the EUV spectrum of He II and constrain the formation of He I λ10830. Finally, we show that the He I λ10830 absorption line, when observable, can be a very suitable wind diagnostic in luminous cool stars.

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