WATER MASERS EMBEDDED IN ULTRACOMPACT HII-REGIONS - THE W-75N CLOUD CORE

We present radio observations of the W 75N star forming region consist ing of. 1) high-resolution VLA radio continuum and H2O maser maps, 2) CSO molecular maps in the CS and CO lines, 3) results from a six-year monitoring campaign of the H2O Maser emission with the 32-m Medicina t elescope. Using the VLA in A-configuration, we find that the 22 GHz H2 O maser spots in W 75N spatially coincide with very weak, small diamet er (< 1'') radio continuum sources at the center of the dense molecula r cloud core. The very high optical obscuration (A(upsilon) greater-th an-or-equal-to 90 mag), the high total luminosity of the source and th e high electron density suggest that in W 75N we are witnessing the fi rst appearance of an ultracompact H II region (UCH II) produced by an early type star still deeply embedded in the progenitor molecular clou d. The H2O maser - UCH II association may last only for the short time (approximately 10(3) y) needed by the H II to expand out to the dista nce where masers are formed. Both the H2O and OH masers as well as the continuum sources extend a few seconds of arc along a roughly N-S lin e, matching the extension of the core evident in the CS J = 7 --> 6 ma p and nearly perpendicular to the axis of the molecular outflow. As tr aced by the CO J = 3 --> 2 transition, the origin of the outflow lies at the position of the UCH IIs, suggesting that they are associated wi th the powering source. The H2O spectrum extends over approximately 30 km s-1, with many distinct velocity components, part of which can be unambiguously identified with individual spatial components of the VLA H2O map. The variability of the H2O emission of each velocity compone nt is very high, with time scales of the order of a few years and dyna mic range almost up to two orders of magnitude. For the bluest velocit y component, well isolated in the velocity and space domains, there is evidence of a continuous velocity drift of approximately 4 km s-1 ove r 2 years. The energy required for this acceleration is easily compati ble with the mechanical energy input from the molecular outflow. We be lieve W 75N to be an important source for further study as it presents all the typical aspects of a region where star formation just occurre d and it may help unify the various aspects into a single evolutionary scheme.