Astronomy and Astrophysics – Astrophysics
Scientific paper
Jan 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995apj...439l..17r&link_type=abstract
Astrophysical Journal, Part 2 - Letters (ISSN 0004-637X), vol. 439, no. 1, p. L17-L20
Astronomy and Astrophysics
Astrophysics
72
Emission Spectra, Fine Structure, H Ii Regions, Helium, Interstellar Matter, Line Spectra, H Alpha Line, Hydrogen Ions, O Stars
Scientific paper
The He I lambda 5876/hydrogen alpha line intensity ratio in the warm ionized component of the interstellar medium is significantly less than that observed in traditional H II regions surrounding O stars. The observations, carried out in two directions at low Galactic latitude, reveal that the helium is primarily neutral, n(He II)/n(Hetotal approximately less than 0.27, within warm, low-density regions in which the hydrogen is primarily ionized. This implies that the spectrum of the diffuse interstellar radiation field that ionizes the hydrogen is softer than that from the Galactic O star population in the solar neighborhood. Specifically, the ratio of helium ionizing to hydrogen ionizing photons Q(He0)/Q(H0) approximately less than 0.030, which corresponds to a spectral type of O8 or later and, if representative of the interstellar background, challenges the O star ionization models.
Reynolds Ron J.
Tufte Stephen Louis
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