Observations of [S IV] 10.5 micron and [Ne II] 12.8 micron in Two Halo   Planetary Nebulae: Implications for Chemical Self-Enrichment
  We have detected the [S IV] 10.5 micron and [Ne II] 12.8 micron fine-structure lines in the halo population planetary nebula (PN) DdDm 1, and set upper limits on their intensities in the halo PN H 4-1. 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 A to 1 micron. These nebulae have similar O/H abundances, (O/H) = 1e-4, but S/H and Ne/H are about half an order of magnitude lower in H 4-1 than in DdDm 1; thus H 4-1 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 results 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. 
Harriet L. Dinerstein (1), Matthew J. Richter (2), John H. Lacy (1),   and K. Sellgren (3) ((1) University of Texas at Austi...