The Composition of Interstellar Grains Toward Zeta Ophiuchi: Constraining the Elemental Budget Near the Diffuse-Dense Cloud Transition

TL;DR

This study analyzes the composition of interstellar dust toward Zeta Ophiuchi, revealing silicate grains, potential water ice mantles, and a significant amount of missing oxygen, providing insights into elemental distribution near cloud transitions.

Contribution

It provides detailed spectral analysis of interstellar grains, identifying their composition and revealing the missing oxygen reservoir in large, opaque grains.

Findings

Silicate grains are dominant in the line of sight.

No convincing evidence of water ice mantles on small grains.

A large fraction of oxygen appears unaccounted for in known solids.

Abstract

We investigate the composition of interstellar grains along the line of sight toward Zeta Ophiuchi, a well-studied environment near the diffuse-dense cloud transition. A spectral decomposition analysis of the solid-state absorbers is performed using archival spectroscopic observations from the Spitzer Space Telescope and Infrared Space Observatory. We find strong evidence for the presence of sub-micron-sized amorphous silicate grains, principally comprised of olivine-like composition, with no convincing evidence of H2O ice mantles. However, tentative evidence for thick H2O ice mantles on large (a ~ 2.8 microns) grains is presented. Solid-state abundances of elemental Mg, Si, Fe, and O are inferred from our analysis and compared to standard reference abundances. We find that nearly all of elemental Mg and Si along the line of sight are present in amorphous silicate grains, while a substantial fraction of elemental Fe resides in compounds other than silicates. Moreover, we find that the total abundance of elemental O is largely inconsistent with the adopted reference abundances, indicating that as much as ~156 ppm of interstellar O is missing along the line of sight. After taking into account additional limits on the abundance of elemental O in other O-bearing solids, we conclude that any missing reservoir of elemental O must reside on large grains that are nearly opaque to infrared radiation.