The interstellar oxygen crisis, or where have all the oxygen atoms gone?

Abstract

The interstellar medium (ISM) seems to have a significant surplus of oxygen which was dubbed as the `O crisis': independent of the adopted interstellar reference abundance, the total number of O atoms depleted from the gas phase far exceeds that tied up in solids by as much as ∼160 ppm of O/H. Recently, it has been hypothesized that the missing O could be hidden in μm-sized H₂O ice grains. We examine this hypothesis by comparing the infrared (IR) extinction and far-IR emission arising from these grains with that observed in the Galactic diffuse ISM. We find that it is possible for the diffuse ISM to accommodate ∼160 ppm of O/H in μm-sized H₂O ice grains without violating the observational constraints including the absence of the 3.1 μm O-H absorption feature. More specifically, H₂O ice grains of radii ∼4 μm and O/H = 160 ppm are capable of accounting for the observed flat extinction at ∼3-8 μm and produce no excessive emission in the far-IR. These grains could be present in the diffuse ISM through rapid exchange of material between dense molecular clouds where they form and diffuse clouds where they are destroyed by photosputtering.