The Two Molecular Clouds in RCW 38: Evidence for the Formation of the Youngest Super Star Cluster in the Milky Way Triggered by Cloud–cloud Collision

Abstract

We present distributions of two molecular clouds having velocities of 2 and 14 km s −1 toward RCW 38, the youngest super star cluster in the Milky Way, in the 12 CO J = 1-0 and 3-2 and 13 CO J = 1-0 transitions. 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, the bridging feature connecting the two clouds in velocity, and their morphological correspondence with the infrared dust emission. The velocity difference is too large for the clouds to be gravitationally bound. We frame a hypothesis that the two clouds are colliding with each other by chance to trigger formation of the ∼20 O stars that are localized within ∼0.5 pc of the cluster center in the 2 km s −1 cloud. We suggest that the collision is currently continuing toward part of the 2 km s −1 cloud where the bridging feature is localized. This is the third super star cluster alongside Westerlund 2 and NGC 3603 where cloud-cloud collision has triggered the cluster formation. RCW 38 is the youngest super star cluster in the Milky Way, holding a possible sign of on-going O star formation, and is a promising site where we may be able to witness the moment of O star formation.

Triggered O Star Formation in M20 via Cloud–Cloud Collision: Comparisons between High-resolution CO Observations and Simulations

“…The bridge feature probes the turbulent motion of the gas enhanced by the collision. The bridge feature was observationally confirmed in the young massive star cluster RCW 38 by Fukui et al (2016). It was identified at a spot very nearby the O stars in RCW 38, suggesting that the cloud-cloud collision in RCW 38 is still continuing.…”

“…M 20 and RCW 120 are Hii regions dominated by a single O star, while Westerlund 2, NGC 3603, RCW 38, and the ONC are massive star clusters which harbor several or more than ten O stars. Fukui et al (2016) discussed that H 2 column density of the clouds is a critical parameter to determine the difference and that 10 23 cm −2 is required to form a massive star cluster. A pioneering study of the numerical calculations of cloud-cloud collision was performed by Habe & Ohta (1992), followed by Anathpindika (2010) and Takahira et al (2014).…”

FOREST Unbiased Galactic plane Imaging survey with the Nobeyama 45 m telescope (FUGIN): Molecular clouds toward W 33; possible evidence for a cloud–cloud collision triggering O star formation

“…Further evidence for the physical association of the two molecular clouds with the cluster reinforced the interpretation (Ohama et al 2010). Subsequently, three other super star clusters including 10-20 O stars are found to be associated with two molecular clouds with different velocities, and formation of O stars triggered by a cloud-cloud collision is likely to be a common process for clusters having more than 10 O stars (NGC 3603 by Fukui et al 2014; RCW38 by Fukui et al 2016;DBS[2003]179 by Kuwahara et al in preparation). Among the 8 superstar clusters listed in the review article of Portegies Zwart et al (2010), only three are known to be associated with localized nebulosities, indicating that the three are young and still associated with the remnant of natal molecular gas without heavy ionization.…”

Evolutionary Description of Giant Molecular Cloud Mass Functions on Galactic Disks

“…In addition, T * can be one order of magnitude shorter if CCC triggers rapid formation of massive stars (c.f. Fukui et al 2016) (see also Section 4.3). For simplicity, however, we do not consider any T * and T dest variation in the current article (see also Section 4.3).…”

“…There are claims that CCC possibly drives the most of massive star formation within the Galaxy (c.f. Tan 2000;Nakamura et al 2012;Fukui et al 2014;Torii et al 2015;Fukui et al 2015bFukui et al , 2016. Therefore, the GMCMF time evolution may also be modified by CCC.…”