Physics
Scientific paper
Jul 2008
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2008hst..prop11665b&link_type=abstract
HST Proposal ID #11665. Cycle 17
Physics
Scientific paper
Blue hook stars are a class of hot { 35,000 K} subluminous extreme horizontal branch {EHB} stars that have been recently discovered using HST ultraviolet images of the massive globular clusters omega Cen and NGC 2808. These stars occupy a region of the HR diagram that is unexplained by canonical stellar evolution theory. Using new theoretical evolutionary and atmospheric models, we have shown that the blue hook stars are very likely the progeny of stars that undergo extensive internal mixing during a late helium core flash on the white dwarf cooling curve. This "flash mixing" produces an enormous enhancement of the surface helium and carbon abundances {relative to the abundance pattern that existed on the main sequence}, which suppresses the observed flux in the far-UV. Because stars born with a high helium abundance are more likely to evolve into hot horizontal branch stars, flash mixing is more likely to occur in those massive clusters capable of helium self-enrichment. However, a high initial helium abundance, by itself, is not sufficient to explain the presence of a blue hook population - flash mixing of the envelope is also required. We propose far-UV spectroscopy of normal and subluminous EHB stars in NGC 2808 that will unambiguously test this new formation mechanism. These observations will easily detect the helium and carbon enhancements predicted by flash mixing and will therefore determine if flash mixing represents a new evolutionary channel for populating the hot end of the EHB. More generally, our observations will help to clarify the role of helium self-enrichment in producing blue horizontal branch morphologies and multiple main sequences in massive globular clusters. Finally, these results will provide new insight into the origin and abundance anomalies of the hot helium-rich subdwarf B and O stars in the Galactic field.;
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