Astronomy and Astrophysics, volume 284, 215-226 (1994/4-1)
Water masers embedded in ultracompact HII regions : the W 75N cloud core.
HUNTER T.R., TAYLOR G.B., FELLI M. and TOFANI G.
Abstract (from CDS):
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 (Av ≥ 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.
masers - stars: formation - HII regions - ISM: jets and outflows - radio lines: ISM - ISM: individual objects: W 75N
Table 1: [HTF94] W75N N (Nos 1-5). Table 2, Fig.1: [HRB81] W75N(Ba) (Nos Ba-Bd).
View the reference in ADS
To bookmark this query, right click on this link: simbad:1994A&A...284..215H and select 'bookmark this link' or equivalent in the popup menu