Water masers embedded in ultracompact H II regions: The W 75N cloud core

Details Water masers embedded in ultracompact H II regions: The W 75N cloud core Water masers embedded in ultracompact H II regions: The W 75N cloud core

Apr 1994

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1994a%26a...284..215h&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 284, no. 1, p. 215-226

Astronomy and Astrophysics

Astronomy

73

Astronomical Maps, H Ii Regions, Line Spectra, Molecular Clouds, Radio Astronomy, Star Formation, Water Masers, Early Stars, Interstellar Masers, Radio Jets (Astronomy), Stellar Evolution, Very Large Array (Vla)

Scientific paper

We present radio observations of the W 75N star forming region consisting of:(1) high-resolution Very Large Array (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 (less than 1 sec) radio continuum sources at the center of the dense molecular cloud core. The very high optical obscuration (Ar greater than or equal 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 H II region (UCH II) produced by an early type star still deeply embedded in the progenitor molecular cloud. The H2O maser - UCH II association may last only for the short time (approximately 103 y) needed by the H II 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 goes to 6 map and nearly perpendicular to the axis of the molecular outflow. As traced by the CO J = 3 goes to 2 transition, the origin of the outflow lies at the position of the UCH IIs, suggesting that they are associated with the powering source. The H2O spectrum extends over approximately 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 evidence of a continuous velocity drift of approximately 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.

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