First rate coefficients for an interstellar anion: application to the CN--H2 collisional system

Recently, several molecular anions have been detected in the interstellar medium. Accurate modelling of their microwave emission spectra requires the calculation of rates for rotational excitations by collisions with the most abundant species. We report the first collisional data for an interstellar anion, the rate coefficients for the rotational excitation of CN- by H2. State-to-state rate coefficients between the 11 lowest rotational levels of CN- were calculated for temperatures ranging from 5 to 100 K. Collisions with para- and ortho-H2 are taken into account. The CN- rate coefficients are compared with those of the neutral CO molecule, and, as expected, it is found that the rate coefficients for the anion are significantly larger than those of the neutral species. The new set of rate coefficients will help significantly in the interpretation of the CN- emission lines observed with current and future telescopes, and enable this molecule to become a powerful astrophysical tool.