English (United Kingdom)

https://curated-unify.zendy.io/wp-json/zendy-region/v1/featured_content/oa?rat=en

https://curated-unify.zendy.io/wp-json/zendy-region/v1/highlighted_journal/

Global: Zendy Plus annual

Presents the access of premium content as premium feature

Premium Content

Presents the keyphrase highlighting as premium feature

Keyphrase Highlighting

Presents the summarisation as premium feature

Summarisation

Insights

Presents the pdf analysis as premium feature

PDF Analysis

Presents the zaia usage as premium feature

ZAIA

Global: Zendy Plus monthly

Global: Zendy Tools annual

Global: Zendy Tools monthly

Global: Zendy Free Plan

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

the astronomical journal

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