Atomic oxygen abundance toward Sagittarius B2

TL;DR

This study measures atomic oxygen in foreground clouds toward Sagittarius B2, finding an abundance consistent with previous observations and no evidence for significant unidentified depleted oxygen, refining our understanding of oxygen distribution in the interstellar medium.

Contribution

First velocity-resolved measurement of atomic oxygen toward Sagittarius B2, confirming previous ISO results and constraining the amount of unidentified depleted oxygen in the region.

Findings

Atomic oxygen abundance is approximately 2.5 x 10^-4 relative to hydrogen.

No significant presence of unidentified depleted oxygen detected.

Atomic oxygen abundance aligns with earlier ISO measurements.

Abstract

A substantial fraction of oxygen in diffuse clouds is unaccounted for by observations and is postulated to be in an unknown refractory form, referred to as unidentified depleted oxygen (UDO), which, depending on the local gas density, may contribute up to 50% of the total oxygen content. Previous Infrared Space Observatory (ISO) observations suggest that a significant fraction of oxygen in even denser, translucent clouds may be in atomic form. We have analyzed velocity-resolved archival SOFIA observations of the 63 $\mu$m fine-structure [O I] transition toward the high-mass star-forming region Sgr B2(M) in the Central Molecular Zone. The foreground spiral-arm clouds as well as the extended Sgr B2 envelope between the Sun and the background dust continuum source produce multiple [O i] absorption components, spectrally separated in velocity space. The gas-phase atomic oxygen column density in foreground clouds toward Sgr B2 is well correlated with the total hydrogen column density, with an average atomic oxygen abundance of $(2.51 \pm 0.69) \times 10^{-4}$ with respect to hydrogen nuclei. This value is in good agreement with the earlier ISO measurements on the same line of sight, and is about 35% lower than the total interstellar medium oxygen abundance in the low-density warm gas, as measured in the UV. We find no evidence that a significant fraction of the oxygen on the line of sight toward Sagittarius B2 is in the form of UDO.