We have detected the [ S iv ] 10.5 \mu m and [ Ne ii ] 12.8 \mu m 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 Å – 1 \mu m. These nebulae have similar O/H abundances , ( O/H ) \sim 1 \times 10 ^ { -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 .