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

Context. HD and H-2 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 H-2 are investigated to assess the importance of trace amounts of dust, 10(-5)-10(-3) Z(circle dot), in the production of HD and H-2. Methods. We consider carbonaceous and silicate grains and include both physisorption and chemisorption, tunneling, and realistic grain surface barriers. We find that, for a collapsing gas cloud environment with coupled chemical and thermal balance, dust abundances as small as 10(-5) solar lead to a strong boost in the H-2 formation rate due to surface reactions. As a result of this enhancement in H-2, HD is formed more efficiently in the gas phase through the D+ + H-2 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 H-2 an HD, including the different binding energies of H and D. Results. Grain surface reactions are crucial to the availability of H-2 and HD in very metal-poor environments. Above metallicities of 10(-5) solar, the grain surface route dominates the formation of H-2, which in turn drives the formation of HD in the gas phase. At dust temperatures above 150 K, laboratory experiments and theoretical modeling suggest that H-2 formation on grains is suppressed while HD formation on grains is not.