The Influence of Deuteration and Turbulent Diffusion on the Observed D/H Ratio
The influence of turbulent mixing on the chemistry of the interstellar medium (ISM) 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 ISM. 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 H_2 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.
Additional Information© 2011 American Astronomical Society. Received 2010 May 16; accepted 2011 February 11; published 2011 March 22. We thank W. D. Langer for helpful discussions and the referee for constructive comments which helped to improve an earlier draft of this paper. This research has been supported by the National Science Foundation grant AST-0838261 to the Caltech Submillimeter Observatory. Part of this research was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration.
Published - Bell2011p13759Astrophys_J.pdf