High-mass Star Formation through Filamentary Collapse and Clump-fed
  Accretion in G22
  How mass is accumulated from cloud-scale down to individual stars is a key
open question in understanding high-mass star formation. Here, we present the
mass accumulation process in a hub-filament cloud G22 which is composed of four
supercritical filaments. Velocity gradients detected along three filaments
indicate that they are collapsing with a total mass infall rate of about 440
$M_\odot$ Myr$^{-1}$, suggesting the hub mass would be doubled in six free-fall
times, adding up to $ \sim2 $ Myr. A fraction of the masses in the central
clumps C1 and C2 can be accounted for through large-scale filamentary collapse.
Ubiquitous blue profiles in HCO$^+$ $ (3-2) $ and $^{13}$CO $ (3-2) $ spectra
suggest a clump-scale collapse scenario in the most massive and densest clump
C1. The estimated infall velocity and mass infall rate are 0.31 km s$^{-1}$ and
$ 7.2 \times10^{-4} $ $M_\odot$ yr$^{-1}$, respectively. In clump C1, a hot
molecular core (SMA1) is revealed by the SMA observations and an
outflow-driving high-mass protostar is located at the center of SMA1. The mass
of the protostar is estimated to be $ 11-15 $ $M_\odot$ and it is still growing
with an accretion rate of $ 7\times10^{-5} $ $M_\odot$ yr$^{-1}$. The
coexistent infall in filaments, clump C1, and the central hot core in G22
suggests that pre-assembled mass reservoirs (i.e., high-mass starless cores)
may not be required to form high-mass stars. In the course of high-mass star
formation, the central protostar, the core, and the clump can simultaneously
grow in mass via core-fed/disk accretion, clump-fed accretion, and
filamentary/cloud collapse.
