Phase-Referenced VLBA Observations of OH Masers at 4765 MHz

We report VLBA observations of maser emission from the rotationally excited Π1/2, 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 ∼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 ∼4 mas (∼8 AU). We imaged DR21EX without phase-referencing and detected six ∼5 AU diameter emission regions along an approximately N-S arc with linear extent ∼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. Subject headings: ISM: individual(DR21EX, K3-50, W3(OH))—masers—radio lines: ISM—techniques: interferometric