8012(  1) - 04/16/98 14:26  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      8012
                                                       Version: 1
                                                       Check-in Date: 16-Apr-1998
                                                                      14:26:04

1.Proposal Title:
Observational constraints on massive-star evolution
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2. Proposal For  3. Cycle
AR               7
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4. Investigators
                                                                                     Contact?
    PI: Prof. Regina Schulte-Ladbeck      University of Pittsburgh
   CoI: Dr. Ulrich  Hopp                  Universitddota tssternwarte Mddotu nchen      N

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5. Abstract

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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  Data Distribution

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  Blocking Factor: 10
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                                                8012(  1) - 04/16/98 14:26  - [  2]

            Summary Form for Proposal  8012

  Item                         Used in this proposal
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  Proposal Category            AR
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