Spectrophotometric Results from the Copernicus Satellite.IV. Molecular Hydrogen in Interstellar Space

Strong H2 lines are measured in all 11 reddened stars [E(B - V) > 0.10] observed; the fraction I of hydrogen gas in molecular form exceeds 10-1. In eight out of nine unreddened stars [E(B - V) <0.051 there is no trace of H2 absorption, with I less than l0- . In two stars of intermediate reddening I is between l0- and 10-6 The relatively large column densities in higher rotational levels, up to J = S or 6, correspond to excitation temperatures mostly between 150 and 200 the ratios of ortho-hydrogen (J = 1) to para-hydrogen (J = 0) correspond to lower temperatures, averaging about 80 . Measures of two HD lines in nine stars indicate a ratio of HD to H2 equal to about 10-6; correction for the more rapid disruption of HD molecules, in the absence of effective optical shielding by many other such molecules, indicates that one HD molecule is formed and dissociated for about every 200 H2 molecules. Subject headings:abundances - interstellar matter - molecules, interstellar - spectra, ultraviolet - spectrophotometry