The Interstellar Oxygen Crisis, or Where Have All the Oxygen Atoms Gone?

TL;DR

This paper investigates whether micron-sized H2O ice grains can account for the missing interstellar oxygen, fitting observational data without conflicting with IR extinction and emission constraints.

Contribution

It demonstrates that micron-sized H2O ice grains can plausibly hide the missing oxygen in the diffuse ISM without violating observational constraints.

Findings

H2O ice grains of ~4 microns can account for ~160ppm of O/H

These grains explain the flat IR extinction between 3-8 microns

The grains do not produce excessive far-IR emission

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 ~160ppm of O/H. Recently, it has been hypothesized that the missing O could be hidden in micrometer-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 ~160ppm of O/H in micron-sized H2O ice grains without violating the observational constraints including the absence of the 3.1micron O-H absorption feature. More specifically, H2O ice grains of radii ~4micron and O/H = 160 ppm are capable of accounting for the observed flat extinction at ~ 3-8 micron 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.