Collisional effects in He I lines and helium abundances in planetary nebulae

Details Collisional effects in He I lines and helium abundances in planetary nebulae Collisional effects in He I lines and helium abundances in planetary nebulae

Nov 1987

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1987mnras.229p..31c&link_type=abstract

Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711), vol. 229, Nov. 1, 1987, p. 31P-39P. Research supported by th

Astronomy and Astrophysics

Astronomy

215

Abundance, Collisional Plasmas, Helium, Planetary Nebulae, Stellar Spectra, Cooling, Recombination Reactions, Stellar Composition, Stellar Models

Scientific paper

Attention is drawn to new, 19-state quantal calculations for collisional excitation by electron impact in neutral helium. Recommended empirical formulas are given for the collisional contribution to He I recombination lines such as 4471, 5876 A in gaseous nebulae. Collisional ionization of metastable (2 3S) He I is significant for high-temperature nebulae. Collisional transfers provide significant cooling in nebulae with low heavy-element abundances. Revised mean He/H ratios for three large samples of planetary nebulae (PN) are given. The mean ratio, excluding Type I PN and objects with neutral He, is 0.099 + or - 0.007. The downward revision due to allowance for collisional effects is only 10 percent, but it indicates that PN are very little self-enriched in He. The He I line used to indicate optical-depth effects in He triplets, 7065 A, is actually mostly excited by collisions: corrections to He abundances in PN for self-absorption effects are shown now to be negligible. It is shown that the hypothesis that He abundances in PNs should not depend on the nebular temperature and density is better satisfied when corrections for collisional effects are made.

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