Ultraviolet photometry from the Orbiting Astronomical Observatory. XIV. An extension of the survey of Lyman-alpha absorption from interstellar hydrogen.
Data from 26 new stars scanned by the Wisconsin far-ultraviolet spectrometer aboard OAO-2 have been combined with an earlier survey of interstellar La absorption reported by Savage and Jenkins, thereby giving us the H I density to a total of 95 stars. We have also made small revisions in the column densities toward 15 of the stars in the original survey, owing to an improvement in our corrections for line blending from nearby stellar features. The new observations help fill in some of the vacant areas in our previous sky coverage and also confirm our earlier conclusions concerning the regional distribution of local interstellar H I. We have again studied the relation between H I column densities and B - V color excesses. The behavior of these quantities suggests H2 formation causes a noticeable depletion of H I when the extinction per unit distance is somewhat larger than average. We have derived NH( )I E(B - V) 7.5 x 1021 atoms mag-1 from those data for which H2 formation is unimportant, and this figure includes an approximate correction for ionization by the star observed. Using the predicted extinction of a recent grain model we find a density ratio of hydrogen to dust of 170. Except for stars closer than 140 pc, our survey is biased toward the selection of stars having low color excesses. As a result, our value of 0.62 atoms cm-2 for must be significantly lower than the true mean space density of interstellar hydrogen. An average reddening of 0.61 mag kpc-1 (Spitzer) combined with our value for NH(tot )/E(B - V) gives 1.5 cm -2 for hydrogen in all forms (atoms, molecules, and protons) within 1 kpc of the Sun. Column densities for Ca ii recently derived by Hobbs and our La results for 25 stars give Ca ll/H = 1.1 x 10- , but the individual values for this ratio show a large dispersion. On the basis of Ca i/Ca u observations by Hobbs and by White, together with an improved estimate for the ultraviolet radiation density by Witt and Johnson, we estimated the correction for ionization and calculated the abundance ratios Ca/H> = 1.7 x 10- and Na/H = 8.2 x 10-8. Subject headings: abundances - interstellar matter - spectra, ultraviolet