High-resolution absorption spectroscopy of the OH 2Pi 3/2 ground state line

TL;DR

This study uses high-resolution absorption spectroscopy with SOFIA to measure OH in diffuse interstellar clouds, revealing its abundance and relation to water, thus enhancing understanding of interstellar chemistry.

Contribution

First direct measurement of ground-state OH in diffuse clouds using high-resolution absorption spectroscopy, providing new insights into interstellar chemical abundances.

Findings

OH column densities ~10^14 cm^-2 in spiral arm clouds

OH fractional abundance comparable to H2O (~10^-7 to 10^-8)

H2O/OH ratio ranges from 0.3 to 1.0

Abstract

The chemical composition of the interstellar medium is determined by gas phase chemistry, assisted by grain surface reactions, and by shock chemistry. The aim of this study is to measure the abundance of the hydroxyl radical (OH) in diffuse spiral arm clouds as a contribution to our understanding of the underlying network of chemical reactions. Owing to their high critical density, the ground states of light hydrides provide a tool to directly estimate column densities by means of absorption spectroscopy against bright background sources. We observed onboard the SOFIA observatory the 2Pi3/2, J = 5/2 3/2 2.5 THz line of ground-state OH in the diffuse clouds of the Carina-Sagittarius spiral arm. OH column densities in the spiral arm clouds along the sightlines to W49N, W51 and G34.26+0.15 were found to be of the order of 10^14 cm^-2, which corresponds to a fractional abundance of 10^-7 to 10^-8, which is comparable to that of H_2O. The absorption spectra of both species have similar velocity components, and the ratio of the derived H_2O to OH column densities ranges from 0.3 to 1.0. In W49N we also detected the corresponding line of ^18OH.