New Estimate for the Cosmic Ray-Induced H 2 Photodissociation Rate in the Interstellar Medium

High Resolution Image Download MS PowerPoint Slide In the interstellar medium, cosmic rays (CRs) generate a field of ultraviolet (UV) photons via excitation and subsequent radiative decay of H 2 molecules. This UV field is a major agent of ionization and dissociation in the inner regions of molecular clouds that are shielded from the effects of the interstellar radiation field. In particular, the dissociation of H 2, by far the most abundant molecule in interstellar clouds, leads to the production of atomic hydrogen, which then takes part in the production of a multitude of molecules, in particular complex organics on the surfaces of interstellar dust grains. Precise knowledge of the rates of CR-induced dissociation processes is thus crucial for constructing reliable chemical models. For the present paper, we have derived a new value of k diss,CR (H 2 ) = 0.831ζ for the rate of H 2 dissociation, where ζ is the CR ionization rate of H 2 . This prediction contrasts a previous value from the Leiden database which overestimated the rate due to an inconsistent treatment of the H 2 abundances and photodissociation cross sections. By running a series of chemical models, we show that the overestimated dissociation rate has a large effect on the results of chemical simulations, with the abundance of methanol being overestimated by over 1 order of magnitude. Hence, we strongly recommend the adoption of our new estimate, k diss,CR (H 2 ) = 0.831ζ in all chemical models that include this process. Our newly derived value corresponds to H 2 being purely in the para form ( J ″ = 0). However, in the interiors of molecular clouds, the H 2 ortho-to-para ratio is low and using the rate for para-H 2 is an adequate approximation.