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I. M. Hoffman (Cleriq Sciences), A. M. S. Richards (Jodrell Bank), M. Hoare (Leeds)
We present MERLIN images of the 22-GHz water masers in the HII region NGC 7538. We have detected approximately 20 masers toward the infrared sources IRS1, 2, 3, and 11. The images have a 1-sigma noise level of 30 mJy/beam and a beam of 9 x 8 milliarcseconds.
The absolute positions of the masers are known to within 1 milliarcsecond, phase-referenced to the calibrator J2302+6405. This positional accuracy is a significant improvement from the uncertainty of previous observations, which was 500 milliarcseconds. Based on the new position registration, the relationship of the water masers to other masers in NGC 7538 is discussed.
Several of the masers have increased in intensity by factors of about two to three over approximately 10 years, consistent with other suggestions of long-term variability in the NGC 7538 water masers. The increased output from the masers has made possible the detection of Zeeman circular-polarization profiles which were too weak to be detected in previous observations. We observe circular polarization in 3 masers, indicating a magnetic field strength of a few tens of milliGauss.
The Multi-element Radio Linked Interferometry Network (MERLIN) is operated as a National Facility by the University of Manchester, Jodrell Bank Observatory, on behalf of Particle Physics and Astronomy Research Council (PPARC).
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Bulletin of the American Astronomical Society, 37 #2
© 2005. The American Astronomical Soceity.