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The Definitive Abundance of Interstellar Oxygen

Using the Goddard High Resolution Spectrograph (GHRS) onboard the Hubble Space Telescope, we have obtained high signal-to-noise (S/N) ratio echelle observations of the weak interstellar O I λ1356 absorption toward the stars γ Cas, ϵ Per, δ Ori, ϵ Ori, 15 Mon, τ CMa, and γ Ara. In combination with previous GHRS measurements in six other sight lines (ζ Per, ξ Per, λ Ori, < Ori, κ Ori, and ζ Oph), these new observations yield a mean interstellar gas-phase oxygen abundance (per 106 H atoms) of 106 O/H = 319 ± 14. The largest deviation from the mean is less than 18%, and there are no statistically significant variations in the measured O abundances from sight line to sight line and no evidence of density-dependent oxygen depletion from the gas phase. Assuming various mixtures of silicates and oxides, the abundance of interstellar oxygen tied up in dust grains is unlikely to surpass 106 O/H ≈ 180. Consequently, the GHRS observations imply that the total abundance of interstellar oxygen (gas plus grains) is homogeneous in the vicinity of the Sun and about two-thirds of the solar value of 106 O/H = 741 ± 130. This oxygen deficit is consistent with that observed in nearby B stars and similar to that recently found for interstellar krypton with GHRS. Possible explanations for this deficit include: (1) early solar system enrichment by a local supernova, (2) a recent infall of metal-poor gas in the local Milky Way, or (3) an outward diffusion of the Sun from a smaller Galactocentric distance.