Extinction properties of dust in the ultraviolet and the ionization of helium in galactic H II regions.
Abstract
Some observational evidence is found in NGC 2024 to suggest that the presence of dust grains in an H ii region affects the He+IH+ abundance ratio in the region. The evidence is presented by an anticorrelation between the He/H line ratio, as measured in different points in the nebula, and the He I A5876/Hfl line ratio in these points. No such relationship is found in NGC 6334. The observations in this nebula, however, are not in conflict with the above suggestion. Instead, they probably indicate a large amount of interstellar dust along the line of sight to this nebula. The observations of weak radio helium recombination lines in H ii regions may thus be explained on the basis of selective absorption of helium ionizing photons by dust grains in those nebulae. Subject headings: abundances, nebular - nebulae, individual - radio lines
- Publication:
-
The Astrophysical Journal
- Pub Date:
- April 1973
- DOI:
- 10.1086/152055
- Bibcode:
- 1973ApJ...181..369L