1995A&A...293L..13N

The 3₁₃->2₂₀ line of water at 183GHz has been observed from the ground at three different periods toward W49N, a key galactic source in the study of water masers. The line profiles consist of two strong velocity spikes superimposed on a broad plateau extending over a velocity range of about 400km/s. The first spike corresponds to the "ambient" cloud component (AC) which is observed in many other molecular lines, whereas the second spike (termed HVC, for high-velocity component) is blueshifted by about 36km/s with respect to the ambient emission, and does not have known counterparts in other tracers. Although probably of maser nature, the 183GHz water line emission is spatially extended along the cloud, and it is more stable in time than the 22 and 325GHz lines of water. We give plausible lower limits of the water column density and abundance. Furthermore, we report the discovery of a unusually fast C0 outflow -with velocities in excess of 150 km/s- toward the center of W49N. A comparison of the H20 and C0 data suggests that the HVC feature arises in a dense layer of hot material which has been shock-compressed by the action of the outflows.