2004ApJS..155..577M

Using the Very Large Array (VLA) of the National Radio Astronomy Observatory in the A and B configurations, we have obtained simultaneous high-resolution observations of both the 22 GHz H₂O maser lines as well as the 22 GHz continuum for the H II regions W49N and Sagittarius B2. The angular resolution of both observations is ∼ 0".1, which at the distance of W49N (11.4 kpc) and Sgr B2 (8.5 kpc) corresponds to a physical size of less than 1000 AU in both sources. The velocity coverage for W49N is ±435 km/s; positions for 316 H₂O maser components were obtained. The velocity coverage for Sgr B2 is -40 to +120 km/s; positions for 68 maser components were determined in Sgr B2 Main, 79 in Sgr B2 North, 14 in Sgr B2 Mid-North, and 17 in Sgr B2 South, for a total of 178 H₂O maser positions in Sgr B2.

The cross calibration scheme of Reid & Menten was used. Using this procedure, high dynamic range continuum images were obtained with accurate registration (σ∼0".01) of the continuum and maser positions. A detailed comparison between H II components and maser positions for both Sgr B2 and W49N is presented. In Sgr B2 Main, the H₂O masers are predominantly located at the outside edge of the high-frequency continuum, lending support to the proposal that entrainment by stellar winds may play an important role in H₂O maser emission.