Copernicus observations of interstellar absorption at Lyman alpha

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Abstract
Atomic-hydrogen column densities are derived for 40 OB stars at distances between 60 and 1100 pc using Ly-alpha spectral scans obtained by the Copernicus satellite. It is found that the mean gas/color-excess for 27 stars without significant amounts of molecular hydrogen is 3.6 by 10 to the 21st power atoms/sq cm per mag and the mean column density is 0.12 atoms/cu cm. For ten stars where at least 5% of the hydrogen is molecular, the ratio is found to be 5.4 by 10 to the 21st power atoms/sq cm per mag and the total density is 0.7 atoms/cu cm. It is shown that for the present sample, the gas/color-excess ratio in clouds is one to three times greater than the mean outside the clouds. It is noted that the mean gas density for all 40 stars is much smaller than that obtained for the galaxy from 21-cm observations due to an observational selection bias.

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