S316p.69 — Molecular clouds toward Spitzer Bubble S145 (RCW79); evidence for triggered formation of a twelve-O-star cluster by a cloud-cloud collision

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Aug 10th at 6:00 PM until 7:30 PM

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Author(s): Keisuke Hasegawa1, Kazufumi Torii1, Akio Ohama1, Hiroaki Yamamoto1, Kengo Tachihara1, Yasuo Fukui1

Institution(s): 1. Nagoya University

S145 (RCW79) is one of the largest Spitzer bubbles ionized by 12 O stars clustered inside the bubble. We have carried out multi-J CO observations toward S145 with NANTEN2, ASTE and Mopra mm/sub-mm telescopes. We discovered two molecular clouds having velocities around -60 km s-1 and -45 km s-1. The two clouds are likely physically associated with the cluster as verified by the high intensity ratio of the J=3-2 emission to the J=1-0 emission and the bridging features connecting the two clouds in velocity. We present a scenario that cloud-cloud collision triggered formation of the 12 O stars, because the total mass of the clouds and the cluster is too small to gravitationally bind the velocity and any gravitational motion is excluded.
S145 is different from another Spitzer bubble RCW120 where cloud-cloud collision is also suggested by Torii et al. (2015); the number of O stars is more than ten and the inside of the S145 bubble is almost completely empty, whereas RCW120 has a single O star and has yet a large amount of molecular gas inside the bubble. We suggest that the difference reflects the low density of the smaller cloud by considering the numerical simulations of collision between a small cloud and a large cloud (Habe and Ohta 1992).
It is becoming probable that cloud-cloud collisions play an important role in triggering formation of super star clusters (for NGC3603, Fukui et al. 2014; for Westerlund 2 Furukawa et al. 2009), single O stars in M20 (Torii et al. 2011) in addition to the two Spitzer bubbles, RCW120 and S116/117/118, and N159 West in the LMC.