Selective absorption by dust in H II regions
A summary is given of evidence supporting the hypothesis that the helium abundance is constant throughout the Galaxy and that variations in observed ionized helium abundance are due to selective absorption of the helium ionizing photons by dust in the H II regions. New support comes from mapping of the recombination lines in the Orion Nebula and from detection of He+ in the region of the galactic center.
The absorption cross section per H-atom for photons λ<504 Å increases by a factor of 5 from the solar neighbourhood to the galactic center. This probably reflects a change in the dust to gas ratio.
In the solar neighbourhood, the cross section per H-atom for 912>λ>504 is less than the minimum predicted if the number of large grains per H-atom is the same as in the diffuse IS medium. A depletion of large grains in the H II regions by a factor of about 3 is needed to make the cross sections campatible with theory.
KeywordsAbsorption Cross Section Galactic Center Selective Absorption Recombination Line Solar Neighbourhood
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