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

In the interstellar medium, cosmic rays (CRs) generate a field of ultraviolet (UV) photons via excitation and subsequent radiative decay of H2 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 H2, 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 kdiss,CR(H2) = 0.831ζ for the rate of H2 dissociation, where ζ is the CR ionization rate of H2. This prediction contrasts a previous value from the Leiden database which overestimated the rate due to an inconsistent treatment of the H2 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, kdiss,CR(H2) = 0.831ζ in all chemical models that include this process. Our newly derived value corresponds to H2 being purely in the para form (J″ = 0). However, in the interiors of molecular clouds, the H2 ortho-to-para ratio is low and using the rate for para-H2 is an adequate approximation.