Redshifted methanol absorption tracing infall motions of high-mass star
  formation regions
  Gravitational collapse is one of the most important processes in high-mass
star formation. Compared with the classic blue-skewed profiles, redshifted
absorption against continuum emission is a more reliable method to detect
inward motions within high-mass star formation regions. We aim to test if
methanol transitions can be used to trace infall motions within high-mass star
formation regions. Using the Effelsberg-100 m, IRAM-30 m, and APEX-12 m
telescopes, we carried out observations of 37 and 16 methanol transitions
towards two well-known collapsing dense clumps, W31C (G10.6-0.4) and W3(OH), to
search for redshifted absorption features or inverse P-Cygni profiles.
Redshifted absorption is observed in 14 and 11 methanol transitions towards
W31C and W3(OH), respectively. The infall velocities fitted from a simple
two-layer model agree with previously reported values derived from other
tracers, suggesting that redshifted methanol absorption is a reliable tracer of
infall motions within high-mass star formation regions. Our observations
indicate the presence of large-scale inward motions, and the mass infall rates
are roughly estimated to be $\gtrsim$10$^{-3}$ $M_{\odot}$yr$^{-1}$, which
supports the global hierarchical collapse and clump-fed scenario. With the aid
of bright continuum sources and the overcooling of methanol transitions leading
to enhanced absorption, redshifted methanol absorption can trace infall motions
within high-mass star formation regions hosting bright H{\scriptsize II}
regions.
