Oxygen Abundances in the Milky Way Using X-Ray Absorption Measurements toward Galaxy Clusters

We present measurements of the oxygen abundance of the Milky Way's ISM by observing the K-shell X-ray photoionization edge toward galaxy clusters. This effect is most easily observed toward objects with Galactic columns (nH) of a few times 1021 cm-2. We measure X-ray column densities toward 11 clusters and find that at high Galactic columns above approximately 1021 cm-2 the X-ray columns are generally 1.5-3.0 times greater than the 21 cm H I columns, indicating that molecular clouds become an important contributor to nH at higher columns. We find the average ISM oxygen abundance to be (O/H) = (4.85 ± 0.06) × 10-4, or 0.99 solar when using the most recent solar photospheric values. Since X-ray observations are sensitive to the total amount of oxygen present (gas+dust), these results indicate a high gas to dust ratio. Also, the oxygen abundances along lines of sight through high Galactic columns (nH) are the same as abundances through low columns, suggesting that the composition of denser clouds is similar to that of the more diffuse ISM.