AAS 201st Meeting, January, 2003
Session 77. The Interstellar Medium II
Poster, Wednesday, January 8, 2003, 9:20am-6:30pm, Exhibit Hall AB

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[77.10] VLBA and MERLIN Images of the 6cm Formaldehyde Masers in NGC7538 and G29.96-0.02

I. M. Hoffman (UNM/NRAO), W. M. Goss (NRAO), P. Palmer (Chicago), A. M. S. Richards (JBO)

The two formaldehyde (H2CO) 4.83GHz maser sources in the compact HII region NGC7538-IRS1 have been imaged at high resolution with both MERLIN (50mas beam) and the VLBA (8mas beam). The angular sizes of the masers are ~10mas (30AU) corresponding to brightness temperatures ~ 108K. Using the VLA, we find that both maser components show variability on 5-10 year timescales. A search for polarized emission provides 3-\sigma upper limits of 1% circular polarization and 10% linear polarization. A pronounced velocity gradient of 24 km/s/arcsec, or 1700 km/s/pc, is detected. The velocity gradients observed in other IRS1 masers (e.g., OH, CH3OH) and possible associations with the H2CO masers are discussed.

We also present VLBI images of the formaldehyde masers in G29.96-0.02 with 20mas resolution. Two maser velocity components are detected in G29.96-0.02 with flux densities 70mJy and 15mJy. The G29.96-0.02 detection represents the weakest Galactic maser imaged with VLBI techniques. The current images of NGC7538 and G29.96-0.02 represent the first time formaldehyde masers have been detected on VLBI baselines.

IMH is supprted by the NRAO pre-doctoral researcher program. The Very Large Array (VLA) and Very Long Baseline Array (VLBA) are part of The National Radio Astronomy Observatory (NRAO) which is a facility of the National Science Foundation operated under a cooperative agreement by Associated Universities, Inc. 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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