The essential elements of dust evolution: A viable solution to the interstellar oxygen depletion problem?

TL;DR

This paper proposes that organic carbonates in interstellar dust could account for the missing oxygen in dense clouds, offering a new explanation for oxygen depletion and its role in CO2 formation.

Contribution

It introduces the idea that organic carbonates sequester oxygen in dust, providing a novel solution to the interstellar oxygen depletion problem based on recent organic analyses.

Findings

Oxygen may be incorporated as organic carbonate in dust

Organic carbonate could explain 170-255 ppm oxygen depletion

Decomposition of carbonate may influence CO2 formation in ISM

Abstract

There remain many open questions relating to the depletion of elements into dust, e.g., exactly how are C and O incorporated into dust in dense clouds and, in particular, what drives the disappearance of oxygen in the denser interstellar medium? This work is, in part, an attempt to explain the apparently anomalous incorporation of O atoms into dust in dense clouds. We re-visit the question of the depletion of the elements incorporated into the carbonaceous component of interstellar dust, i.e., C, H, O, N and S, in the light of recent analyses of the organics in comets, meteorites and interplanetary dust particles. We find that oxygen could be combined with ~ 10 to 20 % of the carbon in the dust in dense regions in the form of a difficult to observe, organic carbonate, which could explain the unaccounted for 170-255 ppm oxygen depletion. We conclude that, while C, O and N atoms are depleted into an amorphous a-C:H:O:N phase, we posit that a significant fraction of C and O atoms could be sequestered into an organic carbonate, which provides a viable solution to the oxygen depletion problem. Further, the thermal or photolytic decomposition of this carbonate may have a bearing on the formation of CO_2 in the ISM.