Probing the nebular thermal structure using the he i recombination spectrum and evidence against temperature fluctuations and density inhomogeneities

Abstract

A long-standing problem in nebular astrophysics has been that heavy element abundances derived from collisionally excited lines (CELs) are systematically lower than those derived from optical recombination lines (ORLs). A chemically homogeneous nebula with temperature and density variations fails to account for the discrepancy (c.f. [3]). Instead detailed multi-waveband analyses of a large sample point to the presence of H-deficient condensations embedded in diffuse nebulae [2,8,9]. The 2-abundance model, first proposed by Liu et al. [4], predicts that 2006 Springer.