The stellar radiation field and the ionization of H and He in the local interstellar medium

Details The stellar radiation field and the ionization of H and He in the local interstellar medium The stellar radiation field and the ionization of H and He in the local interstellar medium

Dec 1988

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1988apj...335..188b&link_type=abstract

Astrophysical Journal, Part 1 (ISSN 0004-637X), vol. 335, Dec. 1, 1988, p. 188-196.

Astronomy and Astrophysics

Astrophysics

29

Helium, Hydrogen, Interstellar Matter, Ionized Gases, Photoionization, Ultraviolet Radiation, Abundance, B Stars, H I Regions, White Dwarf Stars

Scientific paper

Detailed photoionization calculations for the local interstellar medium (LISM) are presented based on constraints imposed by H I column density estimates derived from IUE and Copernicus data toward nearby B stars and hot white dwarfs. It is found that the nearby hot white dwarfs dominate the stellar contributions to the EUV radiation field. Considering stellar contribution to the EUV background alone, the resulting lower limits to the hydrogen and helium fractional ionization in the local diffuse cloud, at the sun, are 0.091 and 0.089, respectively. This result is insensitive to the absorbing geometry of the local cloud. The comparable ionizations of H and He near the sun are due to the lack of absorption from photospheric helium in the hot DA white dwarfs, to the difference in optical depths at wavelengths shortward of the H I and He I ionization edges, and to the fact that the hydrogen ionization controls the electron number density. The derived H and He ionization limits have important implications for the interpretation of the He I and H I backscattering results, for which uncertainties in charge-exchange interactions occurring in or near the heliopause reflect additional significant uncertainties in determining the ionization in the LISM.

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