Kinematics and star formation toward W33: a central hub as a
  hub--filament system
  We performed a large-scale mapping observation toward the W33 complex and its
surroundings, covering an area of $1.3^\circ \times 1.0^\circ$ , in $^{12}$CO
(1-0), $^{13}$CO (1-0), and C$^{18}$O (1-0) lines from the Purple Mountain
Observatory (PMO). We found a new hub--filament system ranging from 30 to 38.5
\kms 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 $\rm 10^{-3}\,M_\odot\,
yr^{-1}$. The central hub W33 has a total mass of $\rm\sim
1.8\times10^5\,M_\odot$, accounting for $\sim 60\%$ 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
(YSOs) 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. 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.
