A Kinematic Analysis of the Giant Molecular Complex W3; Possible Evidence for Cloud-Cloud Collisions that Triggered OB Star Clusters in W3 Main and W3(OH)

TL;DR

This study analyzes CO data of the W3 molecular complex, providing evidence that cloud-cloud collisions have likely triggered high-mass star formation in W3 Main and W3(OH), supporting the idea that such collisions are common in the outer galaxy.

Contribution

It presents new kinematic evidence of cloud-cloud collisions in W3, linking these events to the formation of OB star clusters, a novel insight into star formation mechanisms in the outer galaxy.

Findings

Identification of three molecular clouds with distinct velocities.

Detection of collision signatures such as complementary distribution and V-shapes.

Proposed timeline linking cloud collisions to star formation episodes.

Abstract

W3 is one of the most outstanding regions of high-mass star formation in the outer solar circle, including two active star-forming clouds, W3 Main and W3(OH). Based on a new analysis of the $^{12}$CO data obtained at 38$^{\prime\prime}$ resolution, we have found three clouds having molecular mass from 2000 to 8000~$M_\odot$ at velocities, $-50$~km s$^{-1}$, $-43$~km s$^{-1}$, and $-39$~km s$^{-1}$. The $-43$~km s$^{-1}$ cloud is the most massive one, overlapping with the $-39$~km s$^{-1}$ cloud and the $-50$~km s$^{-1}$ cloud toward W3 Main and W3(OH), respectively. In W3 Main and W3(OH), we have found typical signatures of a cloud-cloud collision, i.e., the complementary distribution with/without a displacement between the two clouds and/or a V-shape in the position-velocity diagram. We frame a hypothesis that a cloud-cloud collision triggered the high-mass star formation in each region. The collision in W3 Main involves the $-39$~km s$^{-1}$ cloud and the $-43$~km s$^{-1}$ cloud. The collision likely produced a cavity in the $-43$~km s$^{-1}$ cloud having a size similar to the $-39$~km s$^{-1}$ cloud and triggered the formation of young high-mass stars in IC~1795 2 Myr ago. We suggest that the $-39$~km s$^{-1}$ cloud is still triggering the high-mass objects younger than 1 Myr embedded in W3 Main currently. On the other hand, another collision between the $-50$~km s$^{-1}$ cloud and the $-43$~km s$^{-1}$ cloud likely formed the heavily embedded objects in W3(OH) within $\sim$0.5 Myr ago. The present results favour an idea that cloud-cloud collisions are common phenomena not only in the inner solar circle but also in the outer solar circle, where the number of reported cloud-cloud collisions is yet limited (Fukui et al. 2021, PASJ, 73, S1).