Kinematics And Star Formation Toward W33: A Central Hub As A Hub-Filament System

Aims. We investigate the gas kinematics and physical properties toward the W33 complex and its surrounding filaments. We study clump formation and star formation in a hub-filament system.Methods. We performed a large-scale mapping observation toward the W33 complex and its surroundings, covering an area of 1.3 degrees x 1.0 degrees, in (CO)-C-12 (1-0), (CO)-C-13 (1-0), and (CO)-O-18 (1-0) lines from the Purple Mountain Observatory (PMO). Infrared archival data were obtained from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE), the Multi-band Imaging Photometer Survey of the Galaxy (MIPSGAL), and the Herschel Infrared Galactic Plane Survey (Hi-GAL). We distinguished the dense clumps from the ATLASGAL survey. We used the GLIMPSE I catalogue to extract young stellar objects.Results. We found a new hub-filament system ranging from 30 to 38.5 km s(-1) located at the W33 complex. Three supercritical filaments are directly converging into the central hub W33. Velocity gradients are detected along the filaments and the accretion rates are in order of 10(-3)M(circle dot) yr(-1). The central hub W33 has a total mass of similar to 1.8 x 10(5)M(circle dot), accounting for similar to 60% of the mass of the hub-filament system. This indicates that the central hub is the mass reservoir of the hub-filament system. Furthermore, 49 ATLASGAL clumps are associated with the hub-filament system. We find 57% of the clumps to be situated in the central hub W33 and clustered at the intersections between the filaments and the W33 complex. Moreover, the distribution of Class I young stellar objects forms a structure resembling the hub-filament system and peaks at where the clumps group; it seems to suggest that the mechanisms of clump formation and star formation in this region are correlated.Conclusions. Gas flows along the filaments are likely to feed the materials into the intersections and lead to the clustering and formation of the clumps in the hub-filament system W33. The star formation in the intersections between the filaments and the W33 complex might be triggered by the motion of gas converging into the intersections.