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Paper: VLBA+Y27 Images of the Formaldehyde Masers in NGC 7538 and G29.96-0.02
Volume: 340, Future Directions in High Resolution Astronomy: The 10th Anniversary of the VLBA
Page: 347
Authors: Hoffman, I.M.; Goss, W.M.; Palmer, P.
Abstract: We present the first VLBI detections of formaldehyde (H2CO) masers. Using the VLBA+Y27, we have imaged the 6 cm H2CO masers in the HII regions NGC 7538 and G29.96-0.02 using phase-referencing. Our images include a 15 mJy source which is the weakest Galactic maser to be imaged with VLBI techniques. We find the angular sizes of the NGC 7538 masers to be ∼ 10 mas (30 AU) and the G29.96-0.02 masers to be ∼ 20 mas (130 AU). These sizes correspond to brightness temperatures ∼ 107—8 K, several orders of magnitude above theoretical expectations. We detect no formaldehyde emission in the 2 cm line, indicating the lack of a 2 cm maser and placing limits on the 6 cm excitation process. A pronounced velocity gradient of 28 kms−1 arcsecond−1 (1900 kms−1 pc−1) is detected in the NGC 7538 maser gas.
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