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

Kapteyn Astronomical Institute, PO box 800, 9700 AV Groningen, The Netherlands e-mail: cazaux@astro.rug.nl

Received: 6 November 2008
Accepted: 19 December 2008

Abstract

Context. 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 are investigated to assess the importance of trace amounts of dust, 10^-5-10^-3 , 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 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 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 an  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 modeling suggest that H₂ formation on grains is suppressed while HD formation on grains is not.