The generation of low-energy cosmic rays in molecular clouds

It is argued that if cosmic rays penetrate into molecular clouds, the total energy they lose can exceed the energy from galactic supernovae shocks. It is shown that most likely galactic cosmic rays interacting with the surface layers of molecular clouds are efficiently reflected and do not penetrate into the cloud interior. Low-energy cosmic rays ($E<1$ GeV) that provide the primary ionization of the molecular cloud gas can be generated inside such clouds by multiple shocks arising due to supersonic turbulence.Comment: 11 pages, no figure

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 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.Comment: 14 pages, 14 figures, accepted for publication in Ap

Rate coefficients for the endothermic reactions C+(^2P)+H2(D2)→CH^+(CD^+)+H(D) as functions of temperature from 400–1300 K

This is the publisher's version, also available electronically from http://scitation.aip.org/content/aip/journal/jcp/106/24/10.1063/1.474093.We have measured the bimolecular rate coefficients for the reactions of C+(2P) with H2 and D2 as functions of temperature from 400 to 1300 K using a high temperatureflowing afterglow apparatus. The temperature dependences of these rate coefficients are accurately fit by the Arrhenius equation, with activation energies equal within experimental uncertainty to the reaction endothermicities. Internal energy dependences have been deduced by combining the present data with previous drift tube and ion beammeasurements. We found that reactant rotational energy and translational energy are equally effective in surmounting the energy barrier to reaction, and that vibrational excitation of the neutral reactant to the v=1 state enhances the rate coefficients by a factor of ∼1000 for the reaction with H2 and by ∼6000 for the reaction with D2 at temperatures of 800 and 500 K, respectively. This vibrational enhancement is larger than the enhancement that would be produced if the same amount of energy were put into translational and/or rotational modes of the reactants. In addition, rate coefficients have been derived for the three-body association reaction of C+(2P) with H2 in a helium buffer over the temperature range 300–600 K