Blue Horizontal Branch Stars in the Globular Clusters NGC 6752 and ω Cen

Details Blue Horizontal Branch Stars in the Globular Clusters NGC 6752 and ω Cen Blue Horizontal Branch Stars in the Globular Clusters NGC 6752 and ω Cen

Feb 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001noao.prop..383p&link_type=abstract

NOAO Proposal ID #2001A-0383

Computer Science

Scientific paper

The anticorrelation of N, Na, and Al versus O abundances seen in many metal-poor globular-cluster giants points to deep mixing. If accompanied by enhanced mass loss or helium abundance, such mixing could explain the presence of very blue horizontal branch (BHB) stars in globular clusters, possibly the extreme blue horizontal branch (EHB) stars of spectral type sdB or sdO believed to produce the unexpected upturn seen in the ultraviolet fluxes of luminous, metal-rich elliptical galaxies. However, high metallicity might do this by itself if mass loss is high. Mass transfer to a companion may also be involved, since two-thirds of field EHB stars are in binaries. To test the relative importance of these mechanisms in three different environments, we propose to measure Mg abundances in mid-BHB stars and the binary fraction of EHB stars in two globular clusters with an extremely blue BHB: NGC 6752, whose giants have feh=-1.5, and ocen, whose giants have -1.5 <=feh <=-0.8 (including some highly mixed stars), a candidate for a merger or a captive of the Galaxy. We will take multi-object 1angs resolution spectra for each cluster throughout each of two nights. Velocities good to 5km/s from He I lines should detect half the short-period EHB binaries. mgj diffusion corrections derived from NGC 6752 will be applied to ocen to establish its mid-BHB Mg abundances, showing whether its bluer BHB stars are more metal-rich as suggested by its color-magnitude diagram.

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