Item

Abstract

We present numerical computations and analysis of atomic-to-molecular (H I-to-H₂) transitions in cool (∼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_LW/n, and the ratio of the cosmic-ray ionization rate to the gas density, ζ/n. We present sets of H I-to-H₂ abundance profiles for a wide range of ζ/n and I_LW/n for dust-free gas. We determine the conditions for which H₂ 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_LW = 1) with LW flux 2.07 × 10⁷ photons cm⁻² s⁻¹ and a uniform cosmic-ray ionization rate ζ = 10⁻¹⁶ s⁻¹, an H I-to-H₂ transition occurs at a total hydrogen gas column density of 4 × 10²¹ cm⁻², within 3 × 10⁷ yr, for a gas volume density of n = 10⁶ cm−3 at 100 K. For these parameters, the dust-free limit is reached for a dust-to-gas ratio Z´_d ≲ 10^-5, which may be reached for overall metallicities Z´≲ 0.01 relative to Galactic solar values.