THE CONTRACTING MOLECULAR CORES E1 AND E2 IN W51

We report on observations of the star-forming region W51 in the NH3 (J , K)=(1, 1) and (2, 2) transitions at 1.3 cm. These observations have much greater sensitivity than our previous observations and they allow a kinematic analysis of the motions of the gas in two small star-form ing cores. We present evidence for radial contraction of the gas in W5 1 e2 onto a young star and possibly also for similar contraction near W51 el. The H II region/molecular core W51 e2 seems to be in a stage w here the central star has already arrived on the main sequence, wherea s the cooler gas on scales of 0.1-0.2 pc is still. contracting onto th e mass concentration at the center. In contrast to the interpretation of Rudolph et al., we propose that the contraction extends only over a few tenths of a parsec near e2 instead of over the entire W51 molecul ar cloud complex.