HD and H2 formation in low-metallicity dusty gas clouds at high redshift

TL;DR

This study investigates how trace amounts of dust in low-metallicity gas clouds at high redshift significantly enhance the formation of H2 and HD molecules, impacting cooling processes in primordial environments.

Contribution

It provides detailed models of grain surface and gas phase formation of H2 and HD, highlighting the importance of dust even at very low metallicities and including realistic grain surface physics.

Findings

Dust as low as 10^{-5} solar boosts H2 formation significantly.

Gas phase HD formation is enhanced via increased H2 availability.

Grain surface reactions dominate H2 and HD formation above 10^{-5} solar metallicity.

Abstract

Context: The HD and H2 molecules play important roles in the cooling of primordial and very metal-poor gas at high redshift. Aims: Grain surface and gas phase formation of HD and H2 is investigated to assess the importance of trace amounts of dust, 10^{-5}-10^{-3} Zo, in the production of HD and H2. Methods: We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find, for a collapsing gas cloud environment with coupled chemical and thermal balance, that dust abundances as small as 10^{-5} solar lead to a strong boost in the H2 formation rate due to surface reactions. As a result of this enhancement in H2, HD is formed more efficiently in the gas phase through the D+ + H2 reaction. Direct formation of HD on dust grains cannot compete well with this gas phase process for dust temperatures below 150 K. We also derive up-to-date analytic fitting formulae for the grain surface formation of H2 and HD, including the different binding energies of H and D. Results: Grain surface reactions are crucial to the availability of H2 and HD in very metal-poor environments. Above metallicities of 10^{-5} solar, the grain surface route dominates the formation of H2, which in turn, drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modelling suggest that H2 formation on grains is suppressed while HD formation on grains is not.