The photospheric iron abundance of UV bright stars in globular clusters

Details The photospheric iron abundance of UV bright stars in globular clusters The photospheric iron abundance of UV bright stars in globular clusters

Jul 1996

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6456n&link_type=abstract

HST Proposal ID #6456

Physics

Hst Proposal Id #6456 Stellar Populations

Scientific paper

UV bright stars can be found in colour-magnitude diagrams of globular clusters, lying above the horizontal branch and to the blue of the giant branch. They are mostly post-AGB stars evolving to white dwarfs. Recent analyses of field pop. II post-AGB stars raised the question if the chemical abundances in their atmospheres are the product of dredge-ups or of a gas -dust-separation. The knowledge of the original metallicities {= metallicity of the globular cluster} of the cluster post- AGB stars will allow to overcome this ambiguity. From the measurement of the iron abundance in these stars it can directly be judged if dust depletion is at work. Existing high -S/N, high resolution optical spectra allow the abundances of lighter elements {C,N,O,Si} to be measured, but no iron lines are present. In the UV, however, lines of iron {Fe III, Fe IV and Fe V} can be used. Highly sophisticated NLTE model atmospheres and accurate atomic data are at hand in order to determine precise iron abundances. Thus a crucial test of the predictions of stellar evolutionary theory for low-mass stars can be performed. We therefore propose high-resolution spectroscopic observations of iron lines {in carefully selected wavelengths regions} in the UV for five post-AGB stars spread out over a large range in T{eff} {20000 Kldots51000 K}. Parallel observations with WFPC2 in the outer regions of the clusters shall collect images in FUV {1600Angstrom}, U and V to search for faint blue stars.

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