9191(  1) - 12/20/00 23:37  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      9191
                                                       Version: 1
                                                       Check-in Date: 20-Dec-2000
                                                                      23:27:56

1.Proposal Title:
Post-AGB Stars and the Population II Distance Ladder
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2. Proposal For  3. Cycle
AR               10
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4. Investigators
                                                                                     Contact?
    PI: Howard E. Bond                    Space Telescope Science Institute
   CoI: Dr. David R. Alves                Space Telescope Science Institute             N

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5. Abstract

The visually brightest members of Population II are post-AGB (PAGB) stars evolving
through spectral types F and ANull. They are easily recognized through their huge
Balmer jumps, if a filter at ~$3400 Angstrom\ is used, such as the F336W filter on
WFPC2. PAGB stars are superb candidates for building a Population II distance scale
because of their sharply peaked luminosity function, high luminosities, ease of
detection, and relative freedom from reddening, metallicity, and age issues. PAGB
stars are 4 visual magnitudes brighter than RR Lyrae variables and 1.5 mag brighter
than the red-giant tip. Unlike RR Lyrae stars (and Population I Cepheids), PAGB
stars only require a single epoch of observation and are thus extremely efficient in
terms of telescope usage. In this Archival program, we will use existing HST WFPC2
images to find PAGB A-F type stars in the Local Group dwarf ellipticals M32 and NGC
205, and in the more distant, massive elliptical Centaurus A (NGC 5128), all of
which of course lack Cepheid variables. We expect to establish distances with <10\
Moreover, we will demonstrate the feasibility of our technique with HST, laying the
foundation to find PAGB stars in more distant ellipticals as far away as the Virgo
Cluster.
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                                                9191(  1) - 12/20/00 23:37  - [  2]

            Summary Form for Proposal  9191

  Item                         Used in this proposal
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  Proposal Category            AR
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