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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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Bulletin of the American Astronomical Society, 34, #4
© 2002. The American Astronomical Soceity.