SIMBAD references

Observations of two of the formaldehyde (H₂CO) masers (A and D) in Sgr B2 using the VLBA+Y27 (resolution ~0.01") and the VLA (resolution ~9") are presented. The VLBA observations show compact sources (≲10 mas, ≲80 AU) with brightness temperatures >10⁸ K. The maser sources are partially resolved in the VLBA observations. The flux densities in the VLBA observations are about 1/2 those of the VLA, and the line widths are about 2/3 of the VLA values. The applicability of a core-halo model for the emission distribution is demonstrated. Comparison with earlier H₂CO absorption observations and with ammonia (NH₃) observations suggests that H₂CO masers form in shocked gas. Comparison of the integrated flux densities in current VLA observations with those in previous observations indicates that (1) most of the masers have varied in the past 20 years, and (2) intensity variations are typically less than a factor of 2 compared to the 20 yr mean. No significant linear or circular polarization is detected with either instrument.