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

Abstract

Attention is drawn to new, 19-state quantal calculations for collisionalexcitation by electron impact in neutral helium. Recommended empiricalformulas are given for the collisional contribution to He I recombinationlines such as 4471, 5876 A in gaseous nebulae. Collisional ionizationof metastable (2 3S) He I is significant for high-temperature nebulae.Collisional transfers provide significant cooling in nebulae withlow heavy-element abundances. Revised mean He/H ratios for threelarge 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 effectsis only 10 percent, but it indicates that PN are very little self-enrichedin He. The He I line used to indicate optical-depth effects in Hetriplets, 7065 A, is actually mostly excited by collisions: correctionsto He abundances in PN for self-absorption effects are shown nowto be negligible. It is shown that the hypothesis that He abundancesin PNs should not depend on the nebular temperature and density isbetter satisfied when corrections for collisional effects are made.