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 \fdg 2 , +78 \fdg 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 \pm 5 ) \% , with the warm ( T \sim 10 ^ { 4 } K ) and hot ( T \gtrsim 10 ^ { 5 } 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 \log N ( { S } ) / N ( { H } ) = -4.87 \pm 0.03 and \log N ( { Fe } ) / N ( { H } ) = -5.27 \pm 0.05 .
Our derived sulfur abundance is in excellent agreement with recent solar system determinations of Asplund , Grevesse , & Sauval .
However , it is -0.14 dex below the solar system abundance typically adopted in studies of the interstellar medium .
The iron abundance is \sim - 0.7 dex below the solar system abundance , consistent with the significant incorporation of iron into interstellar grains .
Abundance estimates derived by simply comparing S II and Fe II to H I are +0.17 and +0.11 dex higher , respectively , than the abundance estimates derived from our refined approach .
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 \lesssim + 0.03 dex ( +7 \% ) .
Uncertainties in the amount of very hot gas ( T \sim 10 ^ { 6 } K ) along the line of sight could also affect these determinations .