,. Subdwarf B (sdB) stars are thought to be core helium-burning stars which populate on the extreme blue end of horizontal branch in a Teff-logg diagram. Their atmospheres are generally helium deficient. However, almost 10% of the total subdwarf population comprise stars with helium-rich atmospheres. Helium-rich subdwarf B (He-sdB) stars represent a small group of low-mass hot stars with luminosities greater than those of conventional sdB stars and effective temperatures lower than those of sdO stars. The optical spectra of He- sdB stars are characterised by strong HeI and weak Hell lines. The most numerous He-sdB stars are both very H-poor and N-rich. Some show C-enhancement, and a few are moderately H-rich. We have carried out a detailed abundance analyses of He-sdB stars using high-resolution optical spectra. Line-blanketed LTE model atmospheres have been used to derive atmospheric properties and elemental abundances. By measuring their surface chemistry, we have explored the connection between He-sdB stars, He-sdO stars and sdB stars. We demonstrate a clear distinction between extreme He-sdB stars and intermediate He-sdB stars. The extreme He-sdB stars are mostly N-rich, a few show neon enrichments and might be best explained by the merger of two helium white dwarfs. Intermediate He-sdB stars mostly exhibit strong unusual elemental abundances. They are located roughly between the extreme He-sdBs and sdBs in Teff-logg diagram and are thought to be transition object from a helium-rich to a helium-poor phase. This work reports the first discovery of strontium, germanium, yttrium and zirconium in the optical spectrum of a hot subdwarf, LSIV-14 116. This is an intermediate He-sdB star which shows 4dex overabundance of zirconium, strontium and yttrium and 3dex overabundance of germanium. This work also reports 0.63dex overabundance of argon in another intermediate He-sdB star CPD-20 1123. It is argued that these abundance anomalies can be explained by radiatively-driven diffusion.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:580128 |
| Date | January 2013 |
| Creators | Neelamkodan, N. |
| Publisher | Queen's University Belfast |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
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