Computer Science
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8617b&link_type=abstract
HST Proposal ID #8617
Computer Science
Hst Proposal Id #8617 Hot Stars
Scientific paper
Blue horizontal-branch {BHB} stars in the metal-poor globular clusters M13, M15, NGC 288 and NGC 6752 have recently been found to exhibit remarkable metallicity enhancements and helium depletions relative to the canonical cluster composition. These abundance anomalies are most likely due to diffusion processes -- radiative levitation of the metals, and gravitational settling of helium -- in the stable radiative atmospheres of these hot stars. With available ground-based facilities, we have observed stars in M13 as hot as 19000 K, but beyond this point we are constrained by low V- and B-band flux and an insufficient number of visible-wavelength spectral lines. We therefore propose HST/STIS spectroscopic observations in the wavelength range 1700-3100 Angstrom to extend the abundance measurements in M13 to yet hotter stars. Such measurements will provide a new and important confrontation between theory and observations of the diffusion mechanisms, and will also offer insights into poorly-understood aspects of CMD morphology, including the BHB `gaps' and overluminous BHB stars observed in many clusters.
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