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

We describe a new method for accurately determining total gas-phase abundances for the Galactic halo interstellar medium with minimal ionization uncertainties. For sight lines toward globular clusters containing both ultraviolet-bright stars and radio pulsars, it is possible to measure column densities of H I and several ionization states of selected metals using ultraviolet absorption line measurements and of H II using radio dispersion measurements. By measuring the ionized hydrogen column, we minimize ionization uncertainties that plague abundance measurements of Galactic halo gas. We apply this method for the first time to the sight line toward the globular cluster Messier 3 [(l, b) = (42. 2,+78. 7); d = 10.2 kpc, z = 10.0 kpc] using Far Ultraviolet Spectroscopic Explorer and Hubble Space Telescope ultraviolet spectroscopy of the post-asymptotic giant branch star von Zeipel 1128 and radio observations by Ransom et al. of recently-discovered millisecond pulsars. The fraction of hydrogen associated with ionized gas along this sight line is (45 ± 5)%, with the warm (T ∼ 10 K) and hot (T & 10 K) ionized phases present in roughly a 5:1 ratio. This is the highest measured fraction of ionized hydrogen along a high-latitude pulsar sight line. We derive total gas-phase abundances logN(S)/N(H) = −4.87 ± 0.03 and logN(Fe)/N(H) = −5.27 ± 0.05. Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund, Grevesse, & Sauval. 1Based on observations with the NASA/ESA Hubble Space Telescope obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Incorporated, under NASA contract NAS5-26555. 2Center for Astrophysics and Space Sciences, University of California, San Diego, C-0424, La Jolla, CA, 92093 3Current address: Department of Physics, University of Notre Dame, Notre Dame, IN 46556; [email protected] 4Space Telescope Science Institute, Baltimore, MD, 21218; [email protected] 5Astronomy Department, University of Wisconsin-Madison, Madison, WI, 53711; [email protected]