Millimeter continuum observations of a massive protostellar core in Sagittarius B2

The Hat Creek interferometer has been used to map 3.4 mm continuum emission from Sgr B2 with 4.5 x 2 arcsec resolution. The fluxes of two of the detected sources are substantially higher at 3.4 mm than at 1.3 cm; both sources are coincident with centroids of water maser clusters to less than 1 arcsec. In one, the flux can be attributed to free emission with a turnover frequency greater than 23 GHz; in the other, 3.4 mm emission comes mainly from hot dust grains in a dense protostellar core containing two embedded ultracompact H II regions. The amount of molecular material within 0.1 pc of the embedded protostars is at least one order of magnitude greater than within 0.1 pc of any of the other dozen ultracompact H II regions in Sgr B2. The observations indicate that ultracompact H II regions are much more common than the dense cores from which they presumably form, and that they survive long after most of the surrounding gas has been dispersed by outflow. 21 references