2020MNRAS.496.1278D

We present a multiwavelength investigation of a large-scale physical system containing the W33 complex. The extended system (∼50 pc x 37 pc) is selected based on the distribution of molecular gas at [29.6, 60.2] km s^–1 and of 88 ATLASGAL 870-µm dust clumps at d ∼2.6 kpc. The extended system/molecular cloud traced in the maps of ¹³CO and C¹⁸O emission contains several H II regions excited by OB stars (age ∼0.3-1.0 Myr) and a thermally supercritical filament (fs1, length ∼17 pc). The filament, which is devoid of ionized gas, shows a dust temperature (T_d) of ∼19 K, while the H II regions have a T_d of ∼21-29 K. It suggests the existence of two distinct environments in the cloud. The distribution of Class I young stellar objects (mean age ∼0.44 Myr) traces the early stage of star formation (SF) towards the cloud. At least three velocity components (around 35, 45 and 53 km s^–1) are investigated towards the system. The analysis of ¹³CO and C¹⁸O reveals spatial and velocity connections of cloud components at around 35 and 53 km s^–1. The observed positions of previously known sources, W33 Main, W33 A and O4-7I stars, are found towards a complementary distribution of these two cloud components. The filament fs1 and a previously known object W33 B are seen towards the overlapping areas of the clouds, where ongoing SF activity is evident. A scenario related to converging/colliding flows from two different velocity components appears to explain well the observed indications of SF activity in the system.