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

TL;DR

This study models HI-to-H2 transitions in dust-free, low-metallicity interstellar gas, revealing how cosmic-ray ionization and UV radiation influence molecular formation and the conditions needed for self-shielding in primordial environments.

Contribution

It provides detailed numerical profiles and conditions for HI-to-H2 transitions in dust-free gas, incorporating cosmic-ray chemistry and self-shielding effects, which were not extensively modeled before.

Findings

Transition occurs at a column density of 4x10^21 cm^-2 for specified parameters.

HI-to-H2 transition timescale is about 30 million years at certain densities.

Dust-free limit corresponds to metallicities below 1% of solar values.

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, $I_{\rm LW}/n$, 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 $I_{\rm LW}/n$, 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 ($I_{\rm LW}=1$) 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$^\prime_d \lesssim 10^{-5}$, which may be reached for overall metallicities $Z^\prime\lesssim 0.01$ relative to Galactic solar values.