Observations of [S [CSC]iv[/CSC]] 10.5 μm and [N[CLC]e[/CLC] [CSC]ii[/CSC]] 12.8 μm in Two Halo Planetary Nebulae: Implications for Chemical Self-Enrichment

We have detected the [S IV] 10.5 micron and [Ne II] 12.8 micronfine-structure lines in the halo population planetary nebula (PN) DdDm 1, andset upper limits on their intensities in the halo PN H 4-1. We also present newmeasurements of optical lines from various ions of S, Ne, O, and H for DdDm 1,based on a high-dispersion spectrum covering the spectral range 3800 A to 1micron. These nebulae have similar O/H abundances, (O/H) = 1e-4, but S/H andNe/H are about half an order of magnitude lower in H 4-1 than in DdDm 1; thus H4-1 appears to belong to a more metal-poor population. This supports previoussuggestions that PNe arising from metal-poor progenitor stars can have elevatedoxygen abundances due to internal nucleosynthesis and convective dredge-up. Itis generally accepted that high abundances of carbon in many PNe results fromself-enrichment. To the extent that oxygen can also be affected, the use ofnebular O/H values to infer the overall metallicity of a parent stellarpopulation (for example, in external galaxies) may be suspect, particularly forlow metallicities.Comment: accepted by AJ; 27 pages, 2 postscript figures; typo in abstract correcte