The ortho-to-para ratio of water in interstellar clouds

Abstract

The nuclear-spin chemistry of interstellar water is investigated using the University of Grenoble Alpes Astrochemical Network (UGAN). This network includes reactions involving the different nuclear-spin states of the hydrides of carbon, nitrogen, oxygen, and sulphur, as well as their deuterated forms. Nuclear-spin selection rules are implemented within the scrambling hypothesis for reactions involving up to seven protons. The abundances and ortho-to-para ratios (OPRs) of gas-phase water and water ions (H 2 O + and H 3 O +) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a pre-stellar core. The model incorporates the freezing of the molecules on to grains, simple grain surface chemistry, and cosmic ray induced and direct desorption of ices. The predicted OPRs are found to deviate significantly from both thermal and statistical values and to be independent of temperature below ∼30 K. The OPR of H 2 O is shown to lie between 1.5 and 2.6, depending on the spin state of H 2 , in good agreement with values derived in translucent clouds with relatively high extinction. In the pre-stellar corecollapse calculations, the OPR of H 2 O is shown to reach the statistical value of 3 in regions with severe depletion (n H > 10 7 cm −3). We conclude that a low water OPR (2.5) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H 2) 1. Available OPR measurements in protoplanetary discs and comets are finally discussed.

Efficient Methanol Production on the Dark Side of a Prestellar Core

“…By way of comparison, the direct cosmic-ray-induced desorption rate of water in conditions described above can be estimated to be~´--4.6 10 s 21 1 per H atom or ∼9.3× 10 −17 mol -s 1 1 . Here, we have used the sputtering flux from water ice derived by Faure et al (2019)…”

“…Boogert et al (2015; see also Equation(7)ofFaure et al 2019)., the desorption rate estimate ofFaure et al (2019) should also be adequate for methanol when corrected for the methanol fraction, ∼0.08 with respect to H O 2 . The resulting CH OH…”

Depletion and fractionation of nitrogen in collapsing cores

“…A&A 643, A76 2020Since Hily-Blant et al (2013b), new calculations of the rates of dissociation and ionization by cosmic-ray induced secondary photons have become available (Heays et al 2017), and the results of these calculations have been incorporated in the present model (see also Paper I). Furthermore, compared to Paper I, cosmic-ray induced desorption of water and ammonia has been revised by Faure et al (2019). As a result, desorption of ammonia (and water) via direct impact of cosmic-rays is found to be non-negligible even at 10 K owing to the prompt desorption mechanism (Bringa & Johnson 2004).…”

“…(1)The rate is computed as inFlower et al (2005; see their Sect. 3.3) andFaure et al (2019) (Eq. (7)), where the yield (noted γ inFlower et al 2005) is given by αζ H 2 , and the binding energy is E ads .…”

Cosmic ray sputtering yield of interstellar ice mantles

“…It leads to a proportionality between the ionisation and sputtering rates (as in, e.g. Dartois et al 2015;Faure et al 2019). By doing so, one assumes that the energy spectra of all CR species are identical.…”