7546(  1) - 12/14/96 01:00  - [  0]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      7546
                                                       Version: 1
                                                       Check-in Date: 13-Dec-1996
                                                                      18:21:56

1.Proposal Title:
Massive Stars as Ionization Sources for Diffuse Ionized Gas in Spiral Galaxies
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2. Scientific Category   3. Proposal For  4. Cycle
INTERSTELLAR MATTER         AR               7
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5. Investigators
                                                                                     Contact?
    PI: Rene Walterbos                    New Mexico State University
   CoI: Charles Hoopes                    New Mexico State University                   N

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6. Abstract

We propose to analyze archival WFPC2 exposures in far-UV and visible filters of star
forming regions in M31 and M33 to test photoionization models of the diffuse ionized
gas. Diffuse Ionized Gas (DIG, also referred to as the Warm Ionized Medium) is the
dominant component of the interstellar medium in ionized form, contributing as much
as 30\ of the total atomic hydrogen column density in galaxies. Sensitive H Alpha\
observations of nearby galaxies have shown that the DIG contributes 40+/-10\
galaxies, which implies that 40\ emitted by OB stars apparently goes into ionizing
the DIG rather than discrete HII regions. Where these photons come from is an
unsolved issue. Can this large fraction be provided by field OB stars, i.e. OB stars
outside traditional HII regions, or are Lyman continuum photons leaking out of HII
regions? By studying the massive stellar populations in- and outside of HII regions
we will answer this question. Specifically, we will combine our ground-based data on
the DIG in nearby galaxies with the HST photometry to study the detailed association
between the massive stars and the DIG, determine the average spectral types of
massive stars in- and outside HII regions, and determine if field stars can produce
enough Lyman continuum photons to ionize the DIG. A large HST database of 21 fields
imaged in four filters, including the extremely important FUV filter, is available
for this analysis.
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  Data Distribution

  Media:           ELECTRONIC
  Blocking Factor: 10
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  Email:                                                              ,
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                                                7546(  1) - 12/14/96 01:00  - [  1]

            Summary Form for Proposal  7546

  Item                         Used in this proposal
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  Proposal Category            AR
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  Scientific Category          INTERSTELLAR MATTER
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