The Abundance of M[CLC]g[/CLC] in the Interstellar Medium

An empirical determination of the f-values of the far-UV Mg II lines at 1239A and 1240 A is reported. The strong near-UV Mg II lines at 2796 A and 2803 Aare generally highly saturated along most interstellar sightlines outside thelocal ISM and usually yield extremely uncertain estimates of Mg II columndensities in interstellar gas. Since Mg II is the dominant form of Mg in theneutral ISM and since Mg is expected to be a significant constituent ofinterstellar dust grains, the far-UV lines are critical for assessing the roleof this important element in the ISM. This study consists of complete componentanalyses of the absorption along the lines of sight toward HD 93521 in theGalactic halo and Xi Per and Zeta Oph in the Galactic disk, including all fourUV Mg II lines and numerous other transitions. The three analyses yieldconsistent 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 afactor of about 2.4 larger than a commonly used theoretical estimate, and afactor of about 2 smaller than a recently suggested empirical revision. Theeffects of this result on gas- and dust-phase abundance measurements of Mg arediscussed.Comment: 11 pages, LaTeX (uses aaspp4.sty), 2 Postscript figures, accepted for publication in ApJ Letter