Cosmic-ray-induced H₂ line emission

Context. It has been proposed that H2 near-infrared lines may be excited by cosmic rays and thus allow for a determination of the cosmic-ray ionization rate in dense gas. One-dimensional models show that measuring both the H2 gas column density and H2 line intensity enables a constraint on the cosmic-ray ionization rate as well as on the spectral slope of low-energy cosmic-ray protons in the interstellar medium. Aims. We aim to investigate the impact of certain assumptions regarding the H2 chemical models and interstellar medium density distributions on the emission of cosmic-ray-induced H2 emission lines. This is of particular importance for utilizing observations of these lines with the James Webb Space Telescope to constrain the cosmic-ray ionization rate. Methods. We compare the predicted emission from cosmic-ray-induced, rovibrationally excited H2 emission lines for different one-and three-dimensional models with varying assumptions regarding the gas chemistry and density distribution. Results. We find that the model predictions of the H2 line intensities for the (1-0)S(0), (1-0)Q(2), (1-0)O(2), and (1-0)O(4) transitions at 2.22, 2.41, 2.63, and 3.00 μm, respectively, are relatively independent of the astro-chemical model and the gas density distribution when compared against the H2 column density, making them robust tracers of the cosmic-ray ionization rate. Conclusions. We recommend the use of rovibrational H2 line emission in combination with estimations of the cloud’s H2 column density to constrain the ionization rate and the spectrum of low-energy cosmic rays.