HD and H$_{2}$ formation in low-metallicity dusty gas clouds at high redshift

摘要

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, ,in the production of HD and H2. Results. 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 10-5solar lead to a strong boost in the H2formation 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 ofH andD. Conclusions. Grain surface reactions are crucial to the availability of H2 andHD in very metal-poor environments. Above metallicities of 10-5solar, 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 150K, laboratory experiments and theoretical modeling suggest thatH2 formation on grains is suppressed while HDformation on grains is not. Key words: ISM: dust, extinction - ISM: molecules - galaxies: high-redshift