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Hydrogen and helium ionization structure of gaseous nebulae

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Abstract

In this paper we discuss the ionization equilibrium of hydrogen and helium in a nebula with an arbitrary gas density distribution.

If we consider the spectral characteristics of hot stars, a power law is found to provide a good approximation to the Lyman continuum spectrum for stars withT eff≤100 000 K. With this simplification the ionization equilibrium equation is analytically solved first for a pure hydrogen nebula, then for the general case of a nebula containing H, He, and heavy elements. A simple and quite general formula for the determination of the size and the emission of the He+ zone is obtained. Finally, the ionization equilibrium He++−He+ is considered. This problem can be decoupled from that of the ionization of H0 and He0 if the stellar spectrum is steeper thanv −0.9 or, equivalently, if the star effective temperature is lower than 200 000 K. Within this limit, which surely includes all classicalHii regions and the low-medium excitation planetary nebulae, an analytical solution of the problem can be used.

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Authors and Affiliations

  1. Osservatorio Astrofisico di Arcetri, Firenze, Italy

    E. Oliva

  2. Istituto di Radioastronomia CNR, Bologna, Italy

    N. Panagia

Authors
  1. E. Oliva
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  2. N. Panagia
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Oliva, E., Panagia, N. Hydrogen and helium ionization structure of gaseous nebulae. Astrophys Space Sci 94, 437–461 (1983). https://doi.org/10.1007/BF00653733

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  • DOI: https://doi.org/10.1007/BF00653733

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