Molecular gas in a compact H{\sc ii} region RCW 166; possible evidence for an early phase of cloud-cloud collision prior to the bubble formation (Draft 6/18)

Akio Ohama, Mikito Kono, Daichi Tsutsumi, Yusuke Hattori, Kazufumi Torii, Atsushi Nishimura, Hidetoshi Sano, Hiroaki Yamamoto, Kengo Tachihara, Yasuo Fukui

Published 2017-06-18Version 1

Young HII regions are an important site to study O star formation based on distributions of ionized and molecular gas. We revealed that two molecular clouds at 48 km s-1 and 53 km s-1 are associated with a compact HII region RCW 166 by using the JCMT High-Resolution Survey (COHRS) of the 12CO (J=3-2) emission (Dempsey et al. 2013). RCW 166 comprises a bright ring at 8 um and an elongated compact HII region inside the ring. The 48 km s-1 cloud delineates the ring, and the 53 km s-1 cloud is located within the ring, indicating a complementary distribution between the two molecular components. We propose a hypothesis that RCW 166 was formed by triggering in cloud-cloud collision at a projected velocity separation of 5 km s-1. The compact HII region is located toward the inner cloud which was impacted in the collision. We argue that RCW 166 is in an earliy evolutionary stage, ~ 0.1 Myr after the collision according to the scheme by Habe & Ohta (1992) which will be followed by a bubble formation stage like RCW 120. Inoue & Fukui (2013) showed that the interface gas becomes highly turbulent and realizes a high-mass accretion rate of 10-3 - 10-4 Mo/yr by magneto- hydrodynamical numerical simulations, which offers an explanation of the O star formation in RCW 166.