Mathematics – Probability
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8733f&link_type=abstract
HST Proposal ID #8733
Mathematics
Probability
Hst Proposal Id #8733 Ism And Circumstellar Matter
Scientific paper
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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