Deep long-slit optical spectrophotometric observations are presented for 25 Galactic bulge planetary nebulae (GBPNe) and six Galactic disc planetary nebulae (GDPNe). The spectra, combined with archival ultraviolet (UV) spectra obtained with the International Ultraviolet Explorer and infrared spectra obtained with the Infrared Space Observatory, 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 ORLs reproduce many of the patterns previously found for GDPNe. In particular we show that the large discrepancies between electron temperatures (Te values) derived from CELs and 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 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 values of just a few hundred kelvins, which is too cool to excite any optical and UV CELs.
We find that GBPNe are on the average 0.1–0.2 dex more metal-rich than GDPNe but have a mean C/O ratio that is approximately 0.2 dex lower. By comparing the observed relative abundances of heavy elements with recent theoretical predictions, we show that GBPNe probably evolved from a relatively metal-rich environment of initial Z∼ 0.013, compared to an initial Z≲ 0.008 for GDPNe. In addition, we find that GBPNe tend to have more massive progenitor stars than GDPNe. GBPNe are found to have an average magnesium abundance about 0.13 dex higher than GDPNe. The latter have a mean magnesium abundance almost identical to the solar value. The enhancement of magnesium in GBPNe and the large [α/Fe] ratios of bulge giants suggest that the primary enrichment process in the bulge was Type II supernovae. PN observations yield a Ne/O abundance ratio much higher than the solar value, suggesting that the solar neon abundance may have been underestimated by 0.2 dex.