O-star formation triggered by cloud-cloud collision; From super star clusters to Spitzer bubbles

Mon, July 06, 2015

We find the following sentence in Zinnecker and York “Toward Understanding Massive Star Formation” in Annual Review of Astronomy and Astrophysics 45, (2007),

“Rapid external shock compression (i.e., supersonic gas motions) generating high column densities in less than a local free-fall time rather than slow quasi-static build-up of massive cores may be the receipt to set up the initial conditions for local and global burst of massive star formation…. but such a discussion is beyond the scope of this review.”

We saw no observations of such supersonic motion until the discovery of triggered formation of a super star cluster by colliding clouds in Westerlund2 (Furukawa et al. 2009). Subsequent  observations until now strongly suggest that such supersonic gas motions are indeed triggering O star formation by cloud-cloud collision at velocity around 20 km s-1 in the super star clusters and Spitzer bubbles (NGC3603, RCW38, RCW120, M20, etc. Fukui et al. 2014, 2015a; Torii et al. 2011; 2015). Most recently, with ALMA we discovered a filamentary-cloud collision forming a 40-Mo star in the LMC (Fukui et al. 2015b), suggesting that collision is possibly a common mechanism of O-star formation in galaxies. Cloud-cloud collision efficiently collects mass into a small volume non-gravitationally in a timescale of 10^5 yrs, whereas soon later self-gravity plays a role in forming stellar cores on a much smaller scale. This scenario is supported by the magneto-hydro-dynamical simulations of the collision-compressed layer which shows formation of a top-heavy dense core mass function, leading to formation of O star(s) (Inoue and Fukui 2013). Finally, I will discuss possible implications of the scenario on the origin of IMF and starburst.