Molecular clouds associated with compact H II regions. II - The rapidly rotating condensation associated with ON1

Abstract

VLA observations of the ON1 region in the (J, K) = (1, 1) line of NH3 and the H76-alpha recombination line are reported. Dense and cool NH3 condensations with size scales of 0.2-0.3 pc are found near the ultracompact H II region, ON1. From the systematic shift in position with velocity, some of the condensations are interpreted as parts of a rapidly rotating structure with a large velocity gradient of 11 + or - 2 km/s/pc. This rapid rotation may be the spin-up motion expected from partial conservation of the angular momentum during the collapse process. This type of rapid rotation at the 0.1 pc scale may be a key in identifying a collapsing region. The H76-alpha recombination line has a peak velocity of 5.1 + or - 2.5 km/s and is blueshifted with respect to both the NH3 emission and the OH maser emission. From the morphology, it is likely that the H II region has a peculiar velocity with respect to the neutral molecular material, while the OH masers may still be falling in toward the exciting star. 29 references.