New Clues to the Origin of the Extreme Helium Stars

Details New Clues to the Origin of the Extreme Helium Stars New Clues to the Origin of the Extreme Helium Stars

Jul 2002

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2002hst..prop.9417l&link_type=abstract

HST Proposal ID #9417

Computer Science

Hst Proposal Id #9417

Scientific paper

The extreme helium stars {EHes} are H-poor supergiants whose origins are not yet understood despite thorough analyses of optical spectra. This proposal seeks STIS echelle spectra for 7 stars from which novel data on their chemical compositions will be obtained to pin down key abundances. First, even the EHe's initial metallicity is uncertain; certain abundance ratios - e.g., Ca/S, Ti/S, and Fe/S - imply alterations of surface abundances among elements from Na to Ni resulting from fractionation or diffusive separation, possibly the result of winnowing of dust grains from gas. The zinc abundance measurable only from UV spectra will be a powerful clue to the true metallicity because it is known not to be removed by such winnowing. Second, elements affected by the s-process, the last of the major nucleosynthetic processes for which surface abundances are unknown for EHes, will be studied. The new abundances will be used to probe the evolutionary origins of these peculiar stars by comparisons with theoretical scenarios involving a merger of white dwarfs or a final He-shell flash in a low mass white dwarf, and with observed abundances for R Coronae Borealis stars that would seem to be close relatives of the EHes. Spectrophotometric observations of EHes obtained with GO 8603 will give accurate estimates of effective temperature and surface gravity that will be used in our abundance determinations.

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