Observations of [S IV] 10.5 μm and [Ne II] 12.8 μm in Two Halo Planetary Nebulae: Implications for Chemical Self-Enrichment

We have detected the [S IV] 10.5 μm and [Ne II] 12.8 μm fine-structure lines in the halo population planetary nebula (PN) DdDm 1 and set upper limits on their intensities in the halo PN H4-1. We also present new measurements 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 Å to 1 μm. These nebulae have similar O/H abundances, (O/H) ∼ 1 × 10-4, but S/H and Ne/H are about half an order of magnitude lower in H4-1 than in DdDm 1; thus H4-1 appears to belong to a more metal-poor population. This supports previous suggestions that PNe arising from metal-poor progenitor stars can have elevated oxygen abundances due to internal nucleosynthesis and convective dredge-up. It is generally accepted that high abundances of carbon in many PNe result from self-enrichment. To the extent that oxygen can also be affected, the use of nebular O/H values to infer the overall metallicity of a parent stellar population (for example, in external galaxies) may be suspect, particularly for low metallicities.