HIGH-RESOLUTION CENTIMETER RADIO-CONTINUUM AND AMMONIA MASER OBSERVATIONS OF THE W51 REGION OF STAR-FORMATION

High angular resolution and high-sensitivity observations at wavelengt hs of 1.3 and 3.6 cm have detected three new ultracompact emission reg ions in the core of the W51 (regions d and e). In total, five ultracom pact continuum objects of diameter less than 300 to 3000 AU are locate d in W51 d and e. These sources may best be explained as photoionized stellar winds. This suggests that there may exist a quasi-stable point in early stellar evolution where observable H II regions are formed b y stellar winds around massive stars. These observations demonstrate t hat the taxonomy of H II region classification may be heavily affected by sensitivity, and a lack of complete spatial sampling by interferom etric mapping. For example, the W51 d region, previously classified as cometary, is shown by our higher resolution and sensitivity observati ons also to have the properties normally associated with shell-like re gions. These new observations suggest that the bow shock model, which is suggested to explain the morphology of cometary H II regions, but c annot explain shell sources, does not fully explain the W51 d H II reg ion. The J, K = (9, 8) NH3 maser associated with W51 d has been shown to have a minimum brightness temperature of 2.7 x 10(6) K. This maser is most probably saturated.