The Ratio of Helium and Hydrogen Abundances in Planetary Nebulae.

Abstract

view Abstract Citations (16) References (19) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The Ratio of Helium and Hydrogen Abundances in Planetary Nebulae. Mathis, John S. Abstract In the first part of the paper the populations of seventeen of the lowest singlet terms of neutral helium are found by an approximate theory of statistical equilibrium. The 2'S term is assumed to be insufficiently metastable to affect the populations of the terms above it, and transitions to the ground state are assumed to be balanced by the inverse transitions The effects of variations in electron temperature and density along the line of sight in the nebula are next discussed. The error introduced by such fluctuations into the ratio of helium and hydrogen abundances is found to be probably less than 10 per cent. The abundance ratios Nz /]Voo for several nebulae are discussed. Except for NGC 6572, the photometric data are taken from previous publications. The derived abundances are not found to depend upon the nebular excitation. Probably these are intrinsic abundance differences in planetaries. The mean ratio of hydrogen to helium is 5.4. The ratio found previously for the Orion Nebula showed slightlv less helium but was within the spread of values obtained for the planetaries. Publication: The Astrophysical Journal Pub Date: November 1957 DOI: 10.1086/146422 Bibcode: 1957ApJ...126..493M full text sources ADS |