HI-to-H${}_2$ Transitions in Dust-Free Interstellar Gas

Abstract

We present numerical computations and analysis of atomic to molecular (HI-to-H${}_2$) transitions in cool ($\sim$100 K) low-metallicity dust-free (primordial) gas, in which molecule formation occurs via cosmic-ray driven negative ion chemistry, and removal is by a combination of far-UV photodissociation and cosmic-ray ionization and dissociation. For any gas temperature, the behavior depends on the ratio of the Lyman-Werner (LW) band FUV intensity to gas density, $I_{\mathrm{LW}}/n$, and the ratio of the cosmic-ray ionization rate to the gas density, $\zeta /n$. We present sets of HI-to-H${}_2$ abundance profiles for a wide range of $\zeta /n$ and $I_{\mathrm{LW}}/n$, for dust-free gas. We determine the conditions for which H${}_2$ absorption line self-shielding in optically thick clouds enables a transition from atomic to molecular form for ionization-driven chemistry. We also examine the effects of cosmic-ray energy losses on the atomic and molecular density profiles and transition points. For a unit Galactic interstellar FUV field intensity ($I_{\mathrm{LW}}=1$) with LW flux $2.07\times 10^7$ photons cm${}^{-2}$ s${}^{-1}$, and a uniform cosmic-ray ionization rate $\zeta =10^{-16}$ s${}^{-1}$, an HI-to-H${}_2$ transition occurs at a total hydrogen gas column density of $4\times 10^{21}$ cm${}^{-2}$, within $3\times 10^7$ yr, for a gas volume density of $n=10^6$ cm${}^{-3}$ at 100 K. For these parameters, the dust-free limit obtains for a dust-to-gas ratio Z${}_d^{\prime }\lesssim 10^{-5}$, which may be reached for overall metallicities $Z^{\prime }\lesssim 0.01$ relative to Galactic solar values.

Accepted for publication in ApJ

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