First time-dependent study of H2 and H3+ ortho-para chemistry in the diffuse interstellar medium: Observations meet theoretical predictions

Abstract

The chemistry in the diffuse interstellarmedium (ISM) initiates the gradual increase ofmolecular 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+3 . Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H2 and H+3 in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H+3 , unlike H2, 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 H2 and H+3 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+3 and find that the destruction of the lowest rotational states of H+3 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+3 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+3 under interstellar conditions. © 2014. The American Astronomical Society. All rights reserved.