S316p.59 — Formation of the youngest super star cluster in the Galaxy RCW38; further evidence for triggered O star formation by a supersonic cloud-cloud collision

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

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

Institution(s): 1. Nagoya University

It is becoming more likely that cloud-cloud collisions paly an important role in triggering formation of super star clusters (for NGC3603, Fukui et al. 2014; for Westrelund 2 Furukawa et al. 2009). We have carried out new CO J=1-0 and J=3-2 observations toward the super star cluster RCW38 with NANTEN2, ASTE and Mopra mm/sub-mm telescopes. We discovered two molecular clouds having velocities of 2 km s-1 and 14 km s-1 toward RCW38. They 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, the bridging feature connecting the two clouds in velocity and the morphological correspondence with the infrared dust emission of RCW38. Since the total mass of the clouds and the cluster is too small to gravitationally bind the velocity, we suggest that a collision happened by chance between the two clouds. We set up a scenario that the collision triggered formation of the ~30 candidate O stars which are localized, within ~0.5 pc of the cluster center, just toward the northern tip of the 14 km s-1. The other member low-mass stars are likely preexistent prior to the collision. The formation timescale of the O stars is estimated to be ~3x104 yrs (~0.5 pc/16 km s-1), implying a mass accretion rate ~10-3 Mo/yr for a 20 Mo star. This is the third super star cluster along side Westrelund2 and NGC3603 where cloud-cloud collision triggered the cluster formation. By considering the recent increasing evidence for cloud-cloud collisions to form single O stars in M20, the two Spitzer bubbles, RCW120 and S116/117/118, and N159 West we argue that cloud-cloud collisions are a possible alternative for high-mass star formation to core accretion and competitive accretion scenarios.