2024PASJ...76..895Y

W3 is one of the most outstanding regions of high-mass star formation in the outer solar circle, and includes two active star-forming clouds: W3 Main and W3(OH). Based on a new analysis of the ${^{12}\text{CO}(J = 2-1)}$ data obtained at $38^{\prime \prime }$ resolution, we have found three clouds that have molecular masses from 2000 to $8000\, {M_\odot }$ at velocities $-50\:\rm{km\: s^{-1}}$, $-43\:\rm{km\:s^{-1}}$, and $-39\:\rm{km\:s^{-1}}$. The $-43\:\rm{km\:s^{-1}}$ cloud is the most massive one, overlapping with the $-39\:\rm{km\:s^{-1}}$ cloud and the $-50\:\rm{km\:s^{-1}}$ 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 $-39\:\rm{km\:s^{-1}}$ cloud and the $-43\:\rm{km\:s^{-1}}$ cloud. The collision likely produced a cavity in the $-43\:\rm{km\:s^{-1}}$ cloud that has a size similar to the $-39\:\rm{km\:s^{-1}}$ cloud and triggered the formation of young high-mass stars in IC 1795 $2\:$Myr ago. We suggest that the $-39\:\rm{km\:s^{-1}}$ cloud is still triggering the high-mass objects younger than $1\:$Myr currently embedded in W3 Main. On the other hand, another collision between the $-50\:\rm{km\:s^{-1}}$ cloud and the $-43\:\rm{km\:s^{-1}}$ 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).