The Influence of Deuteration and Turbulent Diffusion on the Observed D/H Ratio

TL;DR

This paper investigates how turbulent diffusion influences deuterium chemistry in molecular clouds, revealing it reduces deuteration efficiency and affects atomic D/H ratios, potentially explaining observed variations.

Contribution

It introduces a new model incorporating turbulent mixing effects on deuterium chemistry and the D/H ratio in molecular clouds.

Findings

Turbulent diffusion decreases deuterated species abundances.

It shifts the atomic-to-molecular transition deeper into clouds.

It may explain the observed scatter in galactic D/H ratios.

Abstract

The influence of turbulent mixing on the chemistry of the interstellar medium has so far received little attention. Previous studies of this effect have suggested that it might play an important role in mixing the various phases of the interstellar medium. In this paper we examine the potential effects of turbulent diffusion on the deuterium chemistry within molecular clouds. We find that such mixing acts to reduce the efficiency of deuteration in these clouds by increasing the ionization fraction and reducing freeze-out of heavy molecules. This leads to lower abundances for many deuterated species. We also examine the influence of turbulent mixing on the transition from atomic hydrogen to H2 and from atomic deuterium to HD near the cloud edge. We find that including turbulent diffusion in our models serves to push these transitions deeper into the cloud and helps maintain a higher atomic fraction throughout the cloud envelope. Based on these findings, we propose a new process to account for the significant scatter in the observed atomic D/H ratio for galactic sightlines extending beyond the Local Bubble. Although several mechanisms have been put forward to explain this scatter, they are unable to fully account for the range in D/H values. We suggest a scenario in which turbulent mixing of atomic and molecular gas at the edges of molecular clouds causes the observed atomic D/H ratio to vary by a factor of ~2.