FIRST TIME-DEPENDENT STUDY OF H₂AND H$_3^+$ORTHO-PARACHEMISTRY IN THE DIFFUSE INTERSTELLAR MEDIUM: OBSERVATIONS MEET THEORETICAL PREDICTIONS

Abstract

The chemistry in the diffuse interstellar medium 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 H 2 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 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 + 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 control 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 CR 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.

Water deuterium fractionation in the high-mass star-forming region G34.26+0.15 based on Herschel/HIFI data

“…They predict a decrease of its gas phase abundance by one order of magnitude at 10 6 yr. Under the same conditions, we predict a decrease in the gas phase HD abundance of only a factor ≈ 2, similar to the model of Albertsson et al (2013Albertsson et al ( , 2014. The inclusion in our model of photodesorption and reactive desorption may have some effect on HD depletion.…”

Chemistry in disks

“…5a. (e.g., Pagani et al 1992Pagani et al , 2009Pagani et al , 2013Crabtree et al 2011;Albertsson et al 2014a). However, the relatively high densities of the molecular layer in a disk ensure that this is a relative quick process taking ∼10 5 yrs.…”

Chemistry of TMC-1 with multiply deuterated species and spin chemistry of H 2 , H 2 + , H 3 + and their isotopologues

“…a TMC-1 like environment), we have benchmarked our model with other published works. The importance of spin-state chemistry is now widely accepted and has been tested in different stages of star formation process: diffuse clouds (Albertsson et al 2014b), starless/prestellar cores (Pagani et al (2009), Sipilä et al (2013)) and protostellar systems (Taquet et al (2013)). But in all cases spin-state chemistry was discussed to address particular issues by adopting various types of physical and chemical model making a direct comparison with our model very difficult.…”