The Abundance of Mg in the Interstellar Medium

Abstract

An empirical determination of the f-values of the far-UV Mg II lines at 1239 A and 1240 A is reported. The strong near-UV Mg II lines at 2796 A and 2803 A are generally highly saturated along most interstellar sightlines outside the local ISM and usually yield extremely uncertain estimates of Mg II column densities in interstellar gas. Since Mg II is the dominant form of Mg in the neutral ISM and since Mg is expected to be a significant constituent of interstellar dust grains, the far-UV lines are critical for assessing the role of this important element in the ISM. This study consists of complete component analyses of the absorption along the lines of sight toward HD 93521 in the Galactic halo and Xi Per and Zeta Oph in the Galactic disk, including all four UV Mg II lines and numerous other transitions. The three analyses yield consistent determinations of the 1239,1240 f-values, with weighted means of 6.4 +/- 0.4 x 10^{-4} and 3.2 +/- 0.2 x 10^{-4}, respectively. These results are a factor of about 2.4 larger than a commonly used theoretical estimate, and a factor of about 2 smaller than a recently suggested empirical revision. The effects of this result on gas- and dust-phase abundance measurements of Mg are discussed.