The Ortho-to-para Ratio Of Water In Interstellar Clouds

By Keith Cowing
January 6, 2022
Filed under
The Ortho-to-para Ratio Of Water In Interstellar Clouds
OPR of H2O as measured in comets, protoplanetary disks and the ISM. References are: Faggi et al. (2018) for comets; Salinas et al. (2016) for the protoplanetary disk TW Hya (model Cm); Flagey et al. (2013) for W51, DR21(OH), G34.3+0.1, W28(A), W33(A) and W49(N); Lis et al. (2010) for Sgr B2(M) and W31C; Lis et al. (2013) for Sgr B2(N); Emprechtinger et al. (2013) for NGC 6334 I and Choi et al. (2015) for AFGL 2591. The black dashed-line gives the statistical value of 3. The two black dotted-lines correspond to our model at 10 K (OPR=1.48) and 100 K (OPR=2.64).

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 (H2O+ and H3O+) are computed under the steady-state conditions representative of a dark molecular cloud and during the early phase of gravitational collapse of a prestellar 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 H2O is shown to lie between 1.5 and 2.6, depending on the spin-state of H2, in good agreement with values derived in translucent clouds with relatively high extinction. In the prestellar core collapse calculations, the OPR of H2O is shown to reach the statistical value of 3 in regions with severe depletion (nH>107~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(H2)≲1. Available OPR measurements in protoplanetary disks and comets are finally discussed.

A. Faure, P. Hily-Blant, C. Rist, G. Pineau des Forêts, A. Matthews, D. R. Flower

Comments: 14 pages, 7 figures, accepted for publication in MNRAS on 29 May 2019
Subjects: Astrophysics of Galaxies (astro-ph.GA)
Journal reference: Monthly Notices of the Royal Astronomical Society, Volume 487, Issue 3, p.3392-3403 (2019)
DOI: 10.1093/mnras/stz1531
Cite as: arXiv:2201.02068 [astro-ph.GA] (or arXiv:2201.02068v1 [astro-ph.GA] for this version)
Submission history
From: Alexandre Faure
[v1] Thu, 6 Jan 2022 14:18:05 UTC (67 KB)
Astrobiology, Astrochemistry

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