2014MNRAS.443.3351L

HCl is supposed to be one of the main chlorine carriers in the interstellar medium (ISM). Then, accurate knowledge of chlorine chemistry requires accurate estimating the HCl abundance in molecular clouds which in turn requires the calculation of collisional excitation rate coefficients for the HCl molecule due to collisions with the most abundant collisional partner in the ISM. In this paper, we report theoretical calculations of the HCl-H2 rotationally inelastic rate coefficients. Using a recently developed potential energy surface, we have computed rate coefficients between the first 11 rotational levels of HCl for temperatures ranging from 5 to 300 K. These new HCl-H2 rate coefficients were compared to the available HCl-He rate coefficients currently used for astrophysical modelling. As one would expect, significant differences were found between new HCl-H2 and previous HCl-He rate coefficients. As a first application, we simulate the excitation of HCl in typical star-forming regions and in protostellar shocks. Electron-impact excitation is also included. It is found that the new H2 rate coefficients significantly increase the simulated line intensities. As a consequence, HCl abundance derived from the observations will be significantly reduced by the use of the present rate coefficients, confirming that HCl may not be the main chlorine carrier in the ISM.