A Method for Deriving Accurate Gas-Phase Abundances for the Multiphase Interstellar Galactic Halo

Abstract

We describe a new method for determining total gas-phase abundances for the Galactic ISM with minimal ionization uncertainties. For sight lines toward globular clusters containing both UV-bright stars and radio pulsars, one can measure column densities of HI and several metal ions using UV absorption measurements and of H II using radio dispersion measurements, thereby minimizing ionization uncertainties. We apply this method to the globular cluster Messier 3 sight line using FUSE and HST ultraviolet spectroscopy of the post-asymptotic giant branch star von Zeipel 1128 and radio observations by Ransom et al. of millisecond pulsars. Ionized hydrogen is 45+/-5% of the total along this sight line, the highest measured fraction along a high-latitude pulsar sight line. We derive total gas-phase abundances log N(S)/N(H) = -4.87+/-0.03 and log N(Fe)/N(H) = -5.27+/-0.05. Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund, Grevesse, & Sauval, but -0.14 dex below the solar system abundance typically adopted in studies of the ISM. The iron abundance is ~-0.7 dex below the solar system abundance, consistent with significant depletion. Abundance estimates derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex higher, respectively, than our measurements. Ionization corrections to the gas-phase abundances measured in the standard way are, therefore, significant compared with the measurement uncertainties along this sight line. The systematic uncertainties associated with the uncertain contribution to the electron column density from ionized helium could raise these abundances by <+0.03 dex (+7%). [Abridged]