Dynamical Collapse in W51 Massive Cores: NH3Observations

We present high angular resolution (0 2-1'', ~1600-8000 AU) studies of the W51 H II region complex in the NH 3 ( J , K ) = (1, 1), (2, 2), and (3, 3) inversion transitions. This region has been identified previously with spectroscopic signatures of gravitational collapse. In one of the infalling cores, W51e2, we detect radial infall of ~3.5 km s -1 at scales of ~1'' (0.04 pc) from the star. The inward motion in the core is nearly constant. A velocity gradient of ~4 km s -1 exists at scales <0 4. The velocity gradient, which maximizes in the east-west direction, increases inward in radius as r -1.2 , which indicates rotation and spin-up of material during the course of collapse. The infall and rotational motions are comparable in the inner core close to the star. The molecular gas across both the e2 and e8 infall regions is hot (40-50 K) and dense (>10 6 cm -3 ). The NH 3 rotational temperature remains roughly constant throughout the collapsing region. The mass densities in the infall region increase inward and scale with radius as r -2 . This density distribution and the total mass in the infall zones are consistent with free falls of constant speed at different spatial scales.