Surface chemistry in the interstellar medium I. H2 formation by Langmuir-Hinshelwood and Eley-Rideal mechanisms

Context. It has been found from ISO, Spitzer, and Herschel observations that molecular hydrogen, H-2, can form on warm grains. Numerical models of interstellar chemistry have failed to reproduce the observed formation rates of H-2, which remains a difficulty when interpreting observations of photon-dominated regions (PDRs). Aims. We attempt to include as much experimental and theoretical information as possible to describe H-2 formation in astrophysical environments to solve this problem. Methods. We modified our "Meudon PDR code" to include a detailed treatment of H-2 formation mechanisms including: i) the Langmuir-Hinshelwood mechanism taking into account the contribution of the different sizes of dust grains in the diffusion processes; and ii) the Eley-Rideal mechanism. Results. We are able to form H-2 even in regions where the dust temperature is higher than 25 K. We also show that formation by the Eley-Rideal mechanism can be a significant source of gas heating. We derive line intensities for various astrophysical conditions. Conclusions. Our approach results in a higher H-2 formation rate than for the "standard" 3 x 10(-17) nH n(H) cm(3) s(-1) expression.