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

TL;DR

This study models the nuclear-spin chemistry of interstellar water, predicting ortho-to-para ratios that align with observations and depend on environmental conditions, advancing understanding of water's spin states in space.

Contribution

The paper introduces a comprehensive astrochemical network including nuclear-spin states and applies it to model water OPR in various interstellar environments, highlighting the influence of physical conditions.

Findings

Water OPR ranges between 1.5 and 2.6 in dark clouds.

OPR reaches 3 during prestellar core collapse with high depletion.

Water OPR is independent of temperature below ~30 K.

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 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 $\sim $30~K. The OPR of H$_2$O is shown to lie between 1.5 and 2.6, depending on the spin-state of H$_2$, in good agreement with values derived in translucent clouds with relatively high extinction. In the prestellar core collapse calculations, the OPR of H$_2$O is shown to reach the statistical value of 3 in regions with severe depletion ($n_{\rm H}> 10^7$~cm$^{-3}$). We conclude that a low water OPR ($\lesssim 2.5$) is consistent with gas-phase ion-neutral chemistry and reflects a gas with OPR(H$_2)\lesssim 1$. Available OPR measurements in protoplanetary disks and comets are finally discussed.