1994A&A...284..215H

We present radio observations of the W 75N star forming region consisting 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 telescope. Using the VLA in A-configuration, we find that the 22 GHz H₂O maser spots in W 75N spatially coincide with very weak, small diameter (<1") radio continuum sources at the center of the dense molecular cloud core. The very high optical obscuration (A_v ≥ 90 mag), the high total luminosity of the source and the high electron density suggest that in W 75N we are witnessing the first appearance of an ultracompact HII region (UCHII) produced by an early type star still deeply embedded in the progenitor molecular cloud. The H₂O maser - UCHII association may last only for the short time (∼ 10³ y) needed by the HII to expand out to the distance 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 line, matching the extension of the core evident in the CS (J=7-6) map and nearly perpendicular to the axis of the molecular outflow. As traced by the CO (J=3-2) transition, the origin of the outflow lies at the position of the UCHIIs, suggesting that they are associated with the powering source. The H₂O spectrum extends over ∼30 km/s, 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 component is very high, with time scales of the order of a few years and dynamic range almost up to two orders of magnitude. For the bluest velocity component, well isolated in the velocity and space domains, there is vidence of a continuous velocity drift of ∼4 km/s over 2 years. The energy required for this acceleration is easily compatible with the mechanical energy input from the molecular outflow. We believe W 75N to be an important source for further study as it presents all the typical aspects of a region where star formation just occurred and it may help unify the various aspects into a single evolutionary scheme.