8733(  1) - 12/30/99 12:59  - [  1]

    PROPOSAL FOR HUBBLE SPACE TELESCOPE OBSERVATIONS   ST ScI Use Only
                                                       ID:      8733
                                                       Version: 1
                                                       Check-in Date: 30-Dec-1999
                                                                      12:30:39

1.Proposal Title:
Confirming the Interstellar Abundance of Oxygen
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2. Proposal For  3. Cycle
AR               9
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4. Investigators
                                                                                     Contact?
    PI: Edward Fitzpatrick                Villanova University

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5. Abstract

The precise determination of interstellar column densities was a major scientific
goal --- and a major success --- of the GHRS. Particularly important results have
been obtained for the element oxygen in the diffuse ISM. High quality observations
and careful analyses suggest that the total (gas + dust) abundance of oxygen is only
2/3 that found in the Sun. This result has been interpreted to indicate that the
nearby ISM is generally underabundant in metals compared to the Sun. This has
critical bearing on our view of chemical evolution in the nearby regions of the
Galaxy, and also on the interpretation of other interstellar column density
measurements and on the assumed composition of interstellar dust. As good as the
oxygen results appear to be, they all depend on only one spectral feature, namely, O
I Lambda1355.5977. The generally adopted f-value (i.e., the intrinsic transition
probability) for this line is a theoretical estimate with a quoted uncertainty of
15\ weight of the oxygen results and interpretation rests on this single theoretical
f-value. I propose to perform an independent empirical determination of the
Lambda1355.5977 f-value using GHRS echelle observations along 6 sightlines. The
results of this program will place the oxygen analyses on a much firmer footing. The
success of this empirical approach has been demonstrated previously for the Mg II
doublet at LambdaLambda1239, 1240.
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                                                8733(  1) - 12/30/99 12:59  - [  2]

            Summary Form for Proposal  8733

  Item                         Used in this proposal
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  Proposal Category            AR
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