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

¹ Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53121 Bonn, Germany
e-mail: wjyang@mpifr-bonn.mpg.de
² School of Natural Sciences, University of Tasmania, Private Bag 37, Hobart, Tasmania 7001, Australia
³ Astronomy Department, Faculty of Science, King Abdulaziz University, PO Box 80203, Jeddah, 21589, Saudi Arabia
⁴ Xinjiang Astronomical Observatory, Chinese Academy of Sciences, Urumqi 830011, PR China
⁵ Center for Astrophysics, GuangZhou University, Guangzhou 510006, PR China
⁶ Purple Mountain Observatory, Chinese Academy of Science, Nanjing 210023, PR China

Received: 2 December 2021
Accepted: 30 December 2021

Abstract

Context. 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.

Aims. We aim to test if methanol transitions can be used to trace infall motions within high-mass star formation regions.

Methods. 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.

Results. 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 M_⊙ yr⁻¹, which supports the global hierarchical collapse and clump-fed scenario.

Conclusions. 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.