Oxygen Depletion in the Interstellar Medium: Implications for Grain Models and the Distribution of Elemental Oxygen

TL;DR

This study investigates the unexplained depletion of atomic oxygen in the interstellar medium, suggesting that organic carbonaceous matter could be a major reservoir, impacting models of dust and the oxygen budget in space.

Contribution

It introduces a comprehensive analysis of oxygen depletion across different interstellar environments and proposes organic refractory material as a key carrier of missing oxygen.

Findings

Up to 160 ppm of oxygen is unaccounted for at the diffuse-dense interface.

Organic carbonaceous matter is a plausible carrier of the missing oxygen.

Further observations are needed to confirm the presence of organic refractory material.

Abstract

This paper assesses the implications of a recent discovery (Jenkins 2009) that atomic oxygen is being depleted from diffuse interstellar gas at a rate that cannot be accounted for by its presence in silicate and metallic oxide particles. To place this discovery in context, the uptake of elemental O into dust is considered over a wide range of environments, from the tenuous intercloud gas and diffuse clouds sampled by the depletion observations to dense clouds where ice mantles and gaseous CO become important reservoirs of O. The distribution of O in these contrasting regions is quantified in terms of a common parameter, the mean number density of hydrogen. At the interface between diffuse and dense phases (just before the onset of ice mantle growth) as much as 160 ppm of the O abundance is unaccounted for. If this reservoir of depleted oxygen persists to higher densities it has implications for the oxygen budget in molecular clouds, where a shortfall of the same order is observed. Of various potential carriers, the most plausible appears to be a form of O-bearing carbonaceous matter similar to the organics found in cometary particles returned by the Stardust mission. The "organic refractory" model for interstellar dust is re-examined in the light of these findings, and it is concluded that further observations and laboratory work are needed to determine whether this class of material is present in quantities sufficient to account for a significant fraction of the unidentified depleted oxygen.