SWAS Observations of Water in Molecular Outflows

We present detections of the ground-state 110→ 101 transition of ortho-H2O at 557 GHz in 18 molecular outflows based on data from the Submillimeter Wave Astronomy Satellite (SWAS). These results are combined with ground-based observations of the J = 1–0 transitions of 12CO and 13CO obtained at the Five College Radio Astronomy Observatory (FCRAO). Data from Infrared Space Observatory (ISO) for a subset of the outflows are also discussed. Assuming that 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 ortho-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 abundance of ortho-H2O increases with velocity, and at the highest outflow velocities some of the outflows have relative ortho-H2O abundances of order 10−4. However, the mass of very high velocity gas with such elevated H2O abundances represents less than 1% of the total outflow gas mass. The ISO LWS observations of high-J rotational lines of CO and the 179.5 μm transition of ortho-H2O provide evidence for a warmer outflow component than required to produce either the SWAS or FCRAO lines. The ISO line-flux ratios can be reproduced with C-shock models with shock velocities of order 25 km s−1 and preshock densities of order 105 cm−3; these C-shocks have postshock relative water abundances greater than 10−4. The mass associated with the ISO emission is also quite small compared with the total outflow mass and is similar to that responsible for the highest velocity water emission detected by SWAS. Although the gas responsible for the ISO emission has elevated levels of water, the bulk of the outflowing gas has an abundance of ortho-H2O well below what would be expected if the gas has passed through a C-shock with shock velocities greater than 10 km s−1. Gas-phase water can be depleted in the postshock gas due to freezeout onto grain mantles; however, the rate of freezeout 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.

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