Probing the Dark Secrets of PNe with ORLs

Abstract

There are two long-standing problems in nebular astrophysics: a) The dichotomy of abundance determinations whereby heavy element abundances relative to hydrogen from collisionally excited lines (CELs) are systematically lower than those from (much fainter) optical recombination lines (ORLs), a discrepancy that can be traced back to Wyse (1942) and Aller & Menzel (1945) who pioneered the ORL method; and b) The dichotomy of temperature determinations whereby Te derived from the collisionally excited [O III] forbidden line ratio is systematically higher than that derived from Balmer jump of the H I recombination spectrum, a discrepancy first discovered by Peimbert (1971) and attributed to temperature fluctuations in the nebula (Peimbert 1967). Thanks to the rapid progress in detector technology and recombination theory for multi-electron systems, significant progress has been made in recent years in our understanding of these two fundamental problems, which could potentially affect the derivation of abundances for a wide range of photoionized nebulae, both galactic and extragalactic. Both types of dichotomy are found to be real and are most likely caused by astrophysical effects rather than uncertainties in observations or in the basic atomic physics. The two problems are shown to be related and can be explained by assuming that PNe contain a small mass of extremely cold (Te ~ 103 K) inclusions, highly enriched in helium and heavy elements, embedded in diffuse material of “normal” abundances (~ solar) and temperature (Te ~ 104 K). The existence and nature of such H-deficient inclusions in PNe remains a mystery. A recent review on this subject is given by Liu (2002).