Physics
Scientific paper
Jul 2004
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2004hst..prop10261b&link_type=abstract
HST Proposal ID #10261
Physics
Hst Proposal Id #10261 Stellar Populations
Scientific paper
We propose far-UV spectroscopy of NGC 2808, in order to better understand the formation mechanisms for extreme horizontal branch {EHB} stars. Our recent analysis of STIS UV imaging has revealed an unusual population of subluminous EHB stars in this cluster, an anomaly that is unexplained by canonical stellar evolution theory. Using new theoretical models of evolution, atmospheres, and spectra, we have demonstrated that these subluminous stars could be the product of a late helium core flash, which occurs while the stars are on the white dwarf cooling curve. The convection zone produced by such a flash will penetrate the hydrogen envelope, mixing hydrogen into the hot helium-burning interior, where it is rapidly consumed. The resulting EHB star would exhibit greatly enhanced helium and carbon at its surface, and would appear subluminous in the UV. Furthermore, the dichotomy between the normal EHB stars and the hotter flash-mixed EHB stars would explain the hottest gap observed in the HB distribution of NGC 2808. Far-UV spectroscopy of normal and subluminous EHB stars in this cluster will unambiguously test this new formation mechanism. The results will also advance our understanding of abundance anomalies in subdwarf B and O stars, the UV emission from elliptical galaxies, and the "born again" scenario for producing hydrogen-deficient R CrB stars.
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