The Formaldehyde Masers in Sgr B2: Very Long Baseline Array and Very Large Array Observations

Observations of two of the formaldehyde (H2CO) masers (A and D) in Sgr B2using the VLBA+Y27 (resolution ~0.01") and the VLA (resolution ~9") arepresented. The VLBA observations show compact sources (<10 milliarcseconds, <80AU) with brightness temperatures >10^8 K. The maser sources are partiallyresolved in the VLBA observations. The flux densities in the VLBA observationsare about 1/2 those of the VLA; and, the linewidths are about 2/3 of the VLAvalues. The applicability of a core-halo model for the emission distribution isdemonstrated. Comparison with earlier H2CO absorption observations and withammonia (NH3) observations suggests that H2CO masers form in shocked gas.Comparison of the integrated flux densities in current VLA observations withthose in previous observations indicates that (1) most of the masers havevaried in the past 20 years, and (2) intensity variations are typically lessthan a factor of two compared to the 20-year mean. No significant linear orcircular polarization is detected with either instrument.Comment: 20 pages, 3 figures, 5 tables, accepted to Ap