First Time-Dependent Study of H₂and H₃⁺ortho-Parachemistry in the Diffuse Interstellar Medium: Observations Meet Theoretical Predictions

The chemistry in the diffuse interstellar medium (ISM) initiates the gradual increase of molecular complexity during the life cycle of matter. A key molecule that enables build-up of new molecular bonds and new molecules via proton donation is H . Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H 2 and H in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H , unlike H 2 , seems to be out of thermal equilibrium, contrary to the predictions of modern astrochemical models. We conduct the first time-dependent modeling of the para -fractions of H 2 and H in the diffuse ISM and compare our results to a set of line-of-sight observations, including new measurements presented in this study. We isolate a set of key reactions for H and find that the destruction of the lowest rotational states of H by dissociative recombination largely controls its ortho / para ratio. A plausible agreement with observations cannot be achieved unless a ratio larger than 1:5 for the destruction of (1, 1)- and (1, 0)-states of H is assumed. Additionally, an increased cosmic-ray ionization rate to 10 −15 s −1 further improves the fit whereas variations of other individual physical parameters, such as density and chemical age, have only a minor effect on the predicted ortho / para ratios. Thus, our study calls for new laboratory measurements of the dissociative recombination rate and branching ratio of the key ion H under interstellar conditions.