SWAS Observations of Water in Molecular Outflows

TL;DR

This study uses SWAS and other observations to measure water vapor in molecular outflows, revealing that only a small fraction of gas has high water abundance due to shocks, impacting our understanding of outflow dynamics.

Contribution

First detection of water in multiple outflows with SWAS, showing water abundance varies with velocity and is lower than shock models predict, suggesting limited shock processing.

Findings

Water abundance increases with outflow velocity.

Most outflow gas has low water abundance, below shock model predictions.

High-velocity water-rich gas constitutes less than 1% of total outflow mass.

Abstract

We present SWAS detections of the ground-state 1(10)-1(01) transition of o-H2O at 557 GHz in 18 molecular outflows. These results are combined with ground-based observations of the J=1-0 transitions of 12CO and 13CO obtained at the FCRAO and, for a subset of the outflows, data from ISO. Assuming the SWAS water line emission originates from the same gas traced by CO emission, we find that the outflowing gas in most outflows has an o-H2O abundance relative to H2 of between 10(-7) and 10(-6). Analysis of the water abundance as a function of outflow velocity reveals a strong dependence. The water abundance increases with velocity, and at the highest outflow velocities some outflows have relative o-H2O abundances of order 10(-4). However the mass of gas with such elevated water abundances represents less that 1% of the total outflow gas mass. The ISO LWS observations of high-J rotational lines of CO and the 179.5 micron transition of o-H2O provide evidence for a warmer outflow component than required to produce either the SWAS or FCRAO lines. The mass associated with the ISO emission is similar to that responsible for the highest velocity water emission detected by SWAS. The bulk of the outflowing gas has an abundance of o-H2O well below what would be expected if the gas has passed through a C-shock with shock velocities greater than 10 km/s. Gas-phase water can be depleted in the post-shock gas due to freeze-out onto grain mantles, however the rate of freeze-out is too slow to explain our results. Therefore we believe that only a small fraction of the outflowing molecular gas has passed through shocks strong enough to fully convert the gas-phase oxygen to water. This result has implications for the acceleration mechanism of the molecular gas in these outflows.