Astronomy and Astrophysics – Astrophysics
Scientific paper
Dec 1997
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.8012s&link_type=abstract
HST Proposal ID #8012
Astronomy and Astrophysics
Astrophysics
Hst Proposal Id #8012 Stellar Astrophysics
Scientific paper
Massive stars are important constitutents of galaxies and are increasingly used as probes of galaxy evolution out to high redshifts. Yet, a very basic problem remains in understanding the distribution of massive stars across the Hertzsprung- Russell Diagram. This is known as the problem of the blue-to- red supergiant ratios in galaxies of different metallicities, a very sensitive indicator of the evolutionary paths that massive stars in different chemical environments appear to follow. Observations suggest a trend that the numbers of red supergiants increase with decreasing metallicity, but stellar- evolution models predict the opposite. We discuss various limitations of ground-based observations which have so far restricted accurate star counts to a few, nearby galaxies. We then argue that the HST archive contains a perfect set of photometric data to determine number counts of red supergiants in galaxies out to 5 Mpc. We propose to analyze WFPC2 observations in F555W {V} and F814W {I} filters to derive color-magnitude diagrams and complete luminosity functions of the red supergiant populations in 6 galaxies spanning a factor of 60 in metallicity. This systematic approach will put the functional form of the blue-to-red supergiant ratio with metallicity on firm observational footing.
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