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

H2 formation remains a major issue for the understanding of interstellar physics. We investigate H2 formation in the interstellar medium at the light of the most recent experimental and theoretical data. We implemented detailed H2 formation mechanisms on grains surface in the Meudon PDR code : i) 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. We show that, thanks to these processes, H2 can be formed even in regions where dust temperature is larger than 25 K. We also show that formation by Eley-Rideal mechanism can be a significant source of heating of the gas. We derive line intensities for various astrophysical conditions. Such an approach results in an enhanced H2 formation rate compared to the standard formation determined by observations in absorption in the UV. We derive some H2 line intensities for isobaric and isochoric models.