Phase-Referenced VLBA Observations of OH Masers at 4765 MHz

Abstract

We report VLBA observations of maser emission from the rotationally excited doublet Pi 1/2, J=1/2 state of OH at 4765 MHz. We made phase-referenced observations of W3(OH) at both 4765 MHz and 1720 MHz and found emission in three fields within a about 2000 AU diameter region and verified that in two of the three fields, 4765 MHz and 1720 MHz emission arise from the same position to within about 4 mas (about 5 AU diameter emission regions along an approximately N-S arc with linear extent about 500 AU. In addition, we carried out phase-referenced observations of 4765 MHz emission from K3-50. We searched for the 4765 MHz line in W49 (without phase referencing) and W75N (phase-referenced to the strongest 4765 MHz maser feature in DR21EX); we were unable to detect these sources with the VLBA. For 2 1/2 years (including the dates of the VLBA observations), we carried out monitoring observations of 4765 MHz emission with the VLA. Constraints on models for maser emission at 1720 MHz and 4765 MHz are derived from the observations. These observations are then briefly compared with existing models.

arXiv comment30 pages, 7 figures, to be published in Ap. J. (Dec 10, 2003)

References in corpus (2)

• OH 1720 MHz Masers in Supernova Remnants --- C-Shock Indicators
• A search for 4750- and 4765-MHz OH masers in Southern Star Forming Regions

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• Structure of W3(OH) from Very High Spectral Resolution Observations of 5 Centimeter OH Masers
• Excited-state OH Masers and Supernova Remnants
• The Formaldehyde Masers in Sgr B2: Very Long Baseline Array and Very Large Array Observations
• Magnetic Field Structures in Star Forming Regions: Mid-Infrared Imaging Polarimetry of K3-50
• Masers and star formation
• VLA observations of broad 6-cm excited state OH lines in W49A