New Estimate for the Cosmic Ray-Induced $\rm H_2$ Photodissociation Rate in the Interstellar Medium

TL;DR

This work tackles the uncertainty in the cosmic-ray–induced H$_2$ photodissociation rate in the interstellar medium, which shapes H atom supply and surface chemistry in dense clouds. It recalculates the rate by applying the CR-induced H$_2$ UV spectrum to para-H$_2$ cross sections, yielding $k_{ m diss, CR}({ m H_2}) = 0.831\,\zeta$ and highlighting para-H$_2$ dominance in cloud interiors. Through four L1544-core chemical simulations with the pyRate code, the study shows that using the previously tabulated rate of $24.7\,\zeta$ dramatically inflates methanol production and alters nitrogen chemistry, while the new rate has a more modest impact. The results support adopting $k_{ m diss, CR}({ m H_2}) = 0.831\,\zeta$ in astrochemical models and clarify that the older Leiden database value is inappropriate for para-rich molecular-cloud conditions, with future work needed to address ortho-H$_2$ contributions.

Abstract

In the interstellar medium, cosmic rays (CRs) generate a field of ultraviolet (UV) photons via the excitation and subsequent radiative decay of $\rm H_2$ 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 $\rm H_2$, 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 $k_{\rm diss, CR}(\mbox{$\rm H_2$})=0.831ζ$ for the rate of $\rm H_2$ dissociation, where $ζ$ is the CR ionization rate of $\rm H_2$. This prediction contrasts a previous value from the Leiden database which overestimated the rate due to an inconsistent treatment of the $\rm H_2$ 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 one order of magnitude. Hence, we strongly recommend the adoption of our new estimate $k_{\rm diss, CR}(\mbox{$\rm H_2$})=0.831ζ$ in all chemical models that include this process. Our newly derived value corresponds to $\rm H_2$ being purely in the para form ($J^{\prime\prime} = 0$). However, in the interiors of molecular clouds the $\rm H_2$ ortho-to-para ratio is low and using the rate for para-$\rm H_2$ is an adequate approximation.

Figures (4)

• Figure 1: Gas and dust temperatures (scale on left-hand $y$-axis) and $\zeta$ (right-hand $y$-axis) as a function of gas density (bottom $x$-axis) expressed as the total hydrogen nuclei number density $n_{\rm H} = 2n({\rm H_2}) + n({\rm H})$ or distance from the center of the core in astronomical units (top $x$-axis) in the L1544 model.
• Figure 2: Abundances of selected species, labeled on top of each panel, as a function of gas density, expressed as the total hydrogen nuclei number density $n_{\rm H} = 2n({\rm H_2}) + n({\rm H})$. The four curves correspond to the four different simulation cases, as indicated in the legend in the upper left-hand panel. The denomination "X ice" refers to species X on grain surfaces.
• Figure 3: Abundances of selected species, labeled on top of each panel, as a function of time in the model cell corresponding to $n({\rm H}) \sim 3 \times 10^4 \, \rm cm^{-3}$. The four curves correspond to the four different simulation cases, as indicated in the legend in the upper left-hand panel. The denomination "X ice" refers to species X on grain surfaces.
• Figure 4: $\rm H_2$ ortho/para (o/p) ratio as a function of gas density, expressed as the total hydrogen nuclei number density $n_{\rm H} = 2n({\rm H_2}) + n({\rm H})$. The four curves correspond to the four different simulation cases, as indicated in the legend.