Redshifted methanol absorption tracing infall motions of high-mass star formation regions

<strong>Context:</strong> 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.<p></p> <p><strong>Aims:</strong> We aim to test if methanol transitions can be used to trace infall motions within high-mass star formation regions.</p> <p><strong>Methods:</strong> 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.</p> <p><strong>Results:</strong> 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 ≳10^-3 <i>M</i>_⊙ yr^-1, which supports the global hierarchical collapse and clump-fed scenario.</p> <p><strong>Conclusions:</strong> 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 II regions.</p>