Neon in WC and WO stars - testing nucleosynthesis and and massive star evolutionary predictions

Details Neon in WC and WO stars - testing nucleosynthesis and and massive star evolutionary predictions Neon in WC and WO stars - testing nucleosynthesis and and massive star evolutionary predictions

Jun 2005

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005sptz.prop20154c&link_type=abstract

Spitzer Proposal ID #20154

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Scientific paper

During the final stages of evolution of the most massive stars, identified with the Wolf-Rayet WC and WO subtypes, the abundance of Neon is predicted to undergo rapid, and substantial enhancements via nuclear processing in Helium-burning and alpha-capture reactions. At the onset of He-burning, 22Ne is predicted to increase by a factor of 100, and, later on, that of 20Ne by a further factor of 100 - yielding, respectively overall Ne/He enhancements of x10 (WC phase) and x100 (WO phase). Measurements of the Neon abundance at differing evolution stages provides the most definitive tests of such nucleosynthesis and of massive star evolution models. For WC and WO stars the only available lines for quantitative analysis occur in the mid-IR, via the fine-structure lines of [NeII]12.8 and [NeIII]15.5micron. We propose to obtain Spitzer IRS spectra of these lines in a sample of early-type WC stars (C/He = 0.1-0.5) and WO stars (C/He =1) to determine their Neon abundances and provide such tests of theory. Results to date for late-type WC stars suggest a discrepancy between observations and predictions. Spitzer is the only facility that can address this issue. Datasets will also cover other fine-structure transitions, notably [SIV]10.5 and [SIII]18.7, which will provide checks on the outer wind ionization balance, while [OIV] 25.9 will allow oxygen abundance determinations in a small sample of transition WN/C stars, which have C/He approx 0.001. O and Ne determinations for such stars will test abundance predictions from recent evolutionary models incorporating rotational mixing. He and C transitions formed in the inner, dense stellar wind, to allow a complete chemical analysis of the stars. Confirmation, or otherwise, of massive star evolution theory has implications for our understanding of young starbursts and the feedback of chemical, ionizing and kinetic enrichment into the ISM environments in galaxies.

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