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

Y Gong,K M Menten,Y Xu,S P Ellingsen,F Wyrowski,W J Yang,X Chen,A Y Yang,C Henkel

doi:10.1051/0004-6361/202142811

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.

Highlights

Infall motions provide direct evidence of mass accretion and their observation plays a crucial role in star formation research
Our observations resulted in the detection of 37 methanol transitions in W31C and 16 methanol transitions in W3(OH)
We find only 10 out of 239 sources have flux densities of ≥1952.22 mJy and if we adopt that as the threshold criterion, it implies that redshifted methanol absorption may be detected in only 4% of the ultracompact Hii (UCHii) regions, if we neglect the collisional pumping effects

Summary

Introduction

Infall motions provide direct evidence of mass accretion and their observation plays a crucial role in star formation research. The interpretation of the blue-skewed profile as an infall signature can be impaired by kinematic peculiarities (e.g. outflow and rotation) and chemical abundance variations (Evans 2003) This situation becomes even more severe when analysing spectra obtained towards complex high-mass star formation regions by single-dish observations with a resolution 10 (corresponding to 0.24 pc at a distance of 5 kpc), because they incorporate emission which is statistically located at farther distances. Detecting redshifted absorption or inverse P-Cygni profiles towards background continuum sources turns out to be a more straightforward and reliable method to identify infall motions associated with high-mass young stellar objects Such redshifted absorption has already been detected in lines from a few molecules (e.g. NH3, Keto et al 1987a, 1988; Ho & Young 1996; Zhang & Ho 1997; HCO+, Welch et al 1987; CS, Zhang et al 1998; H2CO, Di Francesco et al 2001) against strong radio- and millimetre-wavelength continuum emission. Class II CH3OH masers are found in close proximity to infrared sources, OH masers, and ultracompact Hii (UCHii) regions, and radiative pumping is believed to

Objectives

Discussion

Conclusion