Molecular gas in the H ii-region complex RCW 166: Possible evidence for an early phase of cloud–cloud collision prior to the bubble formation

Abstract

Abstract Young H ii regions are an important site for the study of O star formation based on distributions of ionized and molecular gas. We reveal that two molecular clouds at ∼48 km s−1 and ∼53 km s−1 are associated with the H ii regions G018.149−00.283 in RCW 166 by using the JCMT CO High-Resolution Survey (COHRS) of the 12CO(J = 3–2) emission. G018.149−00.283 comprises a bright ring at 8 μm and an extended H ii 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 high-mass stars within G018.149−00.283 were formed by triggering during cloud–cloud collision at a projected velocity separation of ∼5 km s−1. We argue that G018.149−00.283 is in an early evolutionary stage, ∼0.1 Myr after the collision according to the scheme detailed by Habe and Ohta (1992, PASJ, 44, 203), which will be followed by a bubble formation stage like RCW 120. We also suggest that nearby H ii regions N21 and N22 are candidates for bubbles possibly formed by cloud–cloud collision. Inoue and Fukui (2013, ApJ, 774, L31) showed that the interface gas becomes highly turbulent and realizes a high-mass accretion rate of 10−3–10−4 M⊙ yr−1 by magnetohydrodynamical numerical simulations, which offers an explanation of the O-star formation. The fairly high frequency of cloud–cloud collision in RCW 166 is probably due to the high cloud density in this part of the Scutum arm.