Elemental abundances of Galactic bulge planetary nebulae from optical recombination lines

TL;DR

This study analyzes the elemental abundances in Galactic bulge planetary nebulae using optical recombination lines, revealing significant discrepancies with collisional lines and suggesting a cooler plasma component influences ORL emissions.

Contribution

It provides a detailed comparison of plasma diagnostics from CELs and ORLs in planetary nebulae, highlighting the impact of a cool plasma component on abundance measurements.

Findings

Large Te discrepancies between CELs and ORLs are mainly due to low Te from ORLs.

Heavy element abundances from ORLs can be tens of times higher than from CELs.

ORLs originate from a cool plasma component not detectable by CELs.

Abstract

(abridged) Deep long-slit optical spectrophotometric observations are presented for 25 Galactic bulge planetary nebulae (GBPNe) and 6 Galactic disk planetary nebulae (GDPNe). The spectra, combined with archival ultraviolet spectra obtained with the International Ultraviolet Explorer (IUE) and infrared spectra obtained with the Infrared Space Observatory (ISO), have been used to carry out a detailed plasma diagnostic and element abundance analysis utilizing both collisional excited lines (CELs) and optical recombination lines (ORLs). Comparisons of plasma diagnostic and abundance analysis results obtained from CELs and from ORLs reproduce many of the patterns previously found for GDPNe. In particular we show that the large discrepancies between electron temperatures (Te's) derived from CELs and from ORLs appear to be mainly caused by abnormally low values yielded by recombination lines and/or continua. Similarly, the large discrepancies between heavy element abundances deduced from ORLs and from CELs are largely caused by abnormally high values obtained from ORLs, up to tens of solar in extreme cases. It appears that whatever mechanisms are causing the ubiquitous dichotomy between CELs and ORLs, their main effects are to enhance the emission of ORLs, but hardly affect that of CELs. It seems that heavy element abundances deduced from ORLs may not reflect the bulk composition of the nebula. Rather, our analysis suggests that ORLs of heavy element ions mainly originate from a previously unseen component of plasma of Te's of just a few hundred Kelvin, which is too cool to excite any optical and UV CELs.