HI in Molecular Clouds: Irradiation by FUV plus Cosmic Rays

Abstract

We extend the analytic theory presented by Sternberg et al. (2014) and Bialy & Sternberg (2016) for the production of atomic hydrogen (HI) via FUV photodissociation at the boundaries of dense interstellar molecular (H${}_2$) clouds, to also include the effects of penetrating (low-energy) cosmic-rays for the growth of the total HI column densities. We compute the steady-state abundances of the HI and H${}_2$ in one-dimensional gas slabs in which the FUV photodissociation rates are reduced by depth-dependent H${}_2$ self-shielding and dust absorption, and in which the cosmic-ray ionization rates are either constant or reduced by transport effects. The solutions for the HI and H${}_2$ density profiles and the integrated HI columns, depend primarily on the ratios $I_{\mathrm{UV}}/Rn$ and $\zeta /Rn$, where $I_{\mathrm{UV}}$ is the intensity of the photodissociating FUV field, $\zeta$ is the H${}_2$ cosmic-ray ionization rate, $n$ is the hydrogen gas density, and $R$ is the dust-surface H${}_2$ formation rate coefficient. We present computations for a wide range of FUV field strengths, cosmic-ray ionization rates, and dust-to-gas ratios. We develop analytic expressions for the growth of the HI column densities. For Galactic giant molecular clouds (GMCs) with multiphased (warm/cold) HI envelopes, the interior cosmic-ray zones will dominate the production of the HI only if $\zeta \gtrsim 4.5\times 10^{-16}\times (M_{\mathrm{GMC}}/10^6{M}_{\odot }{)}^{-1/2}$~s${}^{-1}$, where $M_{\mathrm{GMC}}$ is the GMC mass, and including attenuation of the cosmic-ray fluxes. For most Galactic GMCs and conditions, FUV photodissociation dominates over cosmic-ray ionization for the production of the HI column densities. Furthermore, the cosmic-rays do not affect the HI-to-H${}_2$ transition points.

Accepted for publication in ApJ