Cold Water Vapor in the Barnard 5 Molecular Cloud

TL;DR

This study detects cold water vapor in Barnard 5, supporting the idea that transient desorption processes disrupt only the outer ice layers in cold molecular clouds, advancing understanding of gas-grain interactions.

Contribution

It provides new detections of water vapor in Barnard 5, expanding knowledge of ice desorption mechanisms in cold clouds.

Findings

Detection of water vapor in Barnard 5 at 556.9360 GHz.

Water and methanol abundances suggest outer ice mantle disruption.

Supports transient desorption as a key process in cold clouds.

Abstract

After more than 30 years of investigations, the nature of gas-grain interactions at low temperatures remains an unresolved issue in astrochemistry. Water ice is the dominant ice found in cold molecular clouds, however, there is only one region where cold (~10 K) water vapor has been detected - L1544. This study aims to shed light on ice desorption mechanisms under cold cloud conditions by expanding the sample. The clumpy distribution of methanol in dark clouds testifies to transient desorption processes at work -- likely to also disrupt water ice mantles. Therefore, the Herschel HIFI instrument was used to search for cold water in a small sample of prominent methanol emission peaks. We report detections of the ground-state transition of o-H2O (J = 1_10 - 1_01) at 556.9360 GHz toward two positions in the cold molecular cloud Barnard 5. The relative abundances of methanol and water gas support a desorption mechanism which disrupts the outer ice mantle layers, rather than causing complete mantle removal.