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Title: First time-dependent study of H{sub 2} and H{sub 3}{sup +} ortho-para chemistry 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{sub 3}{sup +}. Its evolution is tightly related to molecular hydrogen and thought to be well understood. However, recent observations of ortho and para lines of H{sub 2} and H{sub 3}{sup +} in the diffuse ISM showed a puzzling discrepancy in nuclear spin excitation temperatures and populations between these two key species. H{sub 3}{sup +}, unlike H{sub 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{sub 2} and H{sub 3}{sup +} 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{sub 3}{sup +} and find that the destruction of the lowest rotational states of H{sub 3}{sup +} 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,more » 1)- and (1, 0)-states of H{sub 3}{sup +} is assumed. Additionally, an increased cosmic-ray ionization rate to 10{sup –15} s{sup –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{sub 3}{sup +} under interstellar conditions.« less
Authors:
; ;  [1] ;  [2] ;  [3] ;  [4]
  1. Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg (Germany)
  2. Department of Physics and Astronomy, Johns Hopkins University, Baltimore, MD 21218 (United States)
  3. Max-Planck-Institut für Kernphysik, D-69117 Heidelberg (Germany)
  4. Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 (United States)
Publication Date:
OSTI Identifier:
22356844
Resource Type:
Journal Article
Resource Relation:
Journal Name: Astrophysical Journal; Journal Volume: 787; Journal Issue: 1; Other Information: Country of input: International Atomic Energy Agency (IAEA)
Country of Publication:
United States
Language:
English
Subject:
79 ASTROPHYSICS, COSMOLOGY AND ASTRONOMY; COMPARATIVE EVALUATIONS; COSMIC RADIATION; DENSITY; EVOLUTION; EXCITATION; HYDROGEN; INTERSTELLAR SPACE; IONIZATION; LIFE CYCLE; MOLECULAR IONS; MOLECULES; PROTONS; RECOMBINATION; ROTATIONAL STATES; SIMULATION; SPIN; THERMAL EQUILIBRIUM; TIME DEPENDENCE; VARIATIONS