Skip to main content

Research Repository

Advanced Search

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

Faure, A; Hily-Blant, P; Rist, C; Pineau des Forêts, G; Matthews, A; Flower, DR

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


A Faure

P Hily-Blant

C Rist

G Pineau des Forêts

A Matthews

DR Flower


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 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 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 pre-stellar 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 discs and comets are finally discussed.


Faure, A., Hily-Blant, P., Rist, C., Pineau des Forêts, G., Matthews, A., & Flower, D. (2019). The ortho-to-para ratio of water in interstellar clouds. Monthly Notices of the Royal Astronomical Society, 487(3), 3392-3403.

Journal Article Type Article
Acceptance Date May 29, 2019
Online Publication Date Jun 4, 2019
Publication Date Jun 4, 2019
Deposit Date Aug 15, 2019
Publicly Available Date Aug 15, 2019
Journal Monthly Notices of the Royal Astronomical Society
Print ISSN 0035-8711
Electronic ISSN 1365-2966
Publisher Royal Astronomical Society
Peer Reviewed Peer Reviewed
Volume 487
Issue 3
Pages 3392-3403


Published Journal Article (670 Kb)

Copyright Statement
This article has been accepted for publication in Monthly Notices of the Royal Astronomical Society ©: 2019The Royal Astronomical Society Published by Oxford University Press on behalf of the Royal Astronomical Society. All rights reserved.

You might also like

Downloadable Citations