Star formation at the intermediate distances: Gravitational collapse in massive cores

Abstract

We present spectroscopic evidence of gravitational collapse in massive molecular cloud cores toward the W51 HII region complex. The NH3 (J,K)=(2,2) and (3,3) lines obtained at ∼1′′ angular resolution exhibit inverse P-Cygni or asymmetric profiles, suggesting inward motions of gas toward the central star. The infall motions appear to be localized to individual dense cores, contrary to the suggestion of overall collapse. The two collapsing cores are embedded in a more extended cloud of 1.6 pc. If both cores are formed out of this cloud through fragmentation, it seems that both cores are now under collapse again in making the final product: massive OB stars.