Water in Low-Mass Star-Forming Regions with Herschel: The Link Between Water Gas and Ice in Protostellar Envelopes

TL;DR

This study investigates water chemistry in low-mass protostars, revealing that only a fraction of oxygen is in water ice, influenced by short pre-collapse times and physical conditions, with water vapour profiles serving as key tracers.

Contribution

It introduces a simplified chemical model to analyze water gas and ice in protostellar envelopes, linking water chemistry to physical parameters and providing insights into oxygen budget distribution.

Findings

Water ice accounts for only 10-30% of volatile oxygen.

Short pre-collapse times (<0.1 Myr) explain low water ice abundance.

Water vapour profiles trace FUV flux and envelope kinematics.

Abstract

Aims: Our aim is to determine the critical parameters in water chemistry and the contribution of water to the oxygen budget by observing and modelling water gas and ice for a sample of eleven low-mass protostars, for which both forms of water have been observed. Methods: A simplified chemistry network, which is benchmarked against more sophisticated chemical networks, is developed that includes the necessary ingredients to determine the water vapour and ice abundance profiles in the cold, outer envelope in which the temperature increases towards the protostar. Comparing the results from this chemical network to observations of water emission lines and previously published water ice column densities, allows us to probe the influence of various agents (e.g., FUV field, initial abundances, timescales, and kinematics). Results: The observed water ice abundances with respect to hydrogen nuclei in our sample are 30-80ppm, and therefore contain only 10-30% of the volatile oxygen budget of 320ppm. The keys to reproduce this result are a low initial water ice abundance after the pre-collapse phase together with the fact that atomic oxygen cannot freeze-out and form water ice in regions with T(dust)>15 K. This requires short prestellar core lifetimes of less than about 0.1Myr. The water vapour profile is shaped through the interplay of FUV photodesorption, photodissociation, and freeze-out. The water vapour line profiles are an invaluable tracer for the FUV photon flux and envelope kinematics. Conclusions: The finding that only a fraction of the oxygen budget is locked in water ice can be explained either by a short pre-collapse time of less than 0.1 Myr at densities of n(H)~1e4 cm-3, or by some other process that resets the initial water ice abundance for the post-collapse phase. A key for the understanding of the water ice abundance is the binding energy of atomic oxygen on ice.