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          Institute: MPI für Astronomie     Collection: Publikationen_mpia     Display Documents

ID: 710222.0, MPI für Astronomie / Publikationen_mpia
First time-dependent study of H2 and H_3^+ ortho-para chemistry in the diffuse interstellar medium: Observations meet theoretical predictions
Authors:Albertsson, T.; Indriolo, N.; Kreckel, H.; Semenov, D.; Crabtree, K. N.; Henning, T.
Date of Publication (YYYY-MM-DD):2014
Title of Journal:The Astrophysical Journal
Issue / Number:1
Start Page:id. 44 (10 pp)
Audience:Not Specified
Abstract / Description: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_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. Partly based on observations collected at the European Organization for Astronomical Research in the Southern Hemisphere, Chile, as part of program 088.C-0351.
Free Keywords:astrochemistry; cosmic rays; ISM: clouds; ISM: molecules; methods: numerical; molecular processes
External Publication Status:published
Document Type:Article
Communicated by:N. N.
Affiliations:MPI für Astronomie
Identifiers:ISSN:0004-637X %R 10.1088/0004-637X/787/1/44
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