H i in Molecular Clouds: Irradiation by FUV Plus Cosmic Rays

Abstract

We extend the analytic theory presented by Sternberg et al. and Bialy & Sternberg for the production of atomic hydrogen (H i ) via far-ultraviolet (FUV) photodissociation at the boundaries of dense interstellar molecular (H _2 ) clouds to also include the effects of penetrating (low-energy) cosmic rays (CRs) for the growth of the total H i column densities. We compute the steady-state abundances of the H i 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 the CR ionization rates are either constant or reduced by transport effects. The solutions for the H i and H _2 density profiles and the integrated H i columns depend primarily on the ratios I _UV / Rn and ζ / Rn , where I _UV is the intensity of the photodissociating FUV field, ζ is the H _2 CR 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, CR ionization rates, and dust-to-gas ratios. We develop analytic expressions for the growth of the H i column densities. For Galactic giant molecular clouds (GMCs) with multiphased (warm/cold) H i envelopes, the interior CR zones will dominate the production of the H i only if $\zeta \gtrsim 4.5\times {10}^{-16}\times {({M}_{\mathrm{GMC}}/{10}^{6}\ {M}_{\odot })}^{-1/2}$ s ^−1 , where M _GMC is the GMC mass, and including attenuation of the CR fluxes. For most Galactic GMCs and conditions, FUV photodissociation dominates over CR ionization for the production of the H i column densities. Furthermore, the CRs do not affect the H i -to-H _2 transition points.

Keywords

• Interstellar atomic gas
• Molecular clouds
• Galactic cosmic rays
• Interstellar dust extinction
• Radiative transfer
• Neutral hydrogen clouds