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 (n_H) 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 n_H 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 (n_H) are the same as abundances through low columns, suggesting that the composition of denser clouds is similar to that of the more diffuse ISM.