Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies

TL;DR

This study reveals the reversal of infall in the massive star-forming region SgrB2(M), showing how feedback processes begin to disrupt accretion flows, based on Herschel/HIFI observations of HCN lines at THz frequencies.

Contribution

It demonstrates that HCN line profiles can trace the transition from infall to expansion in massive star formation, providing evidence of feedback onset.

Findings

HCN line asymmetries indicate infall in outer regions and expansion in inner regions.

Modeling shows velocity field change from infall to expansion explains observed line shapes.

Reveals feedback process starting to reverse accretion in SgrB2(M).

Abstract

To investigate the accretion and feedback processes in massive star formation, we analyze the shapes of emission lines from hot molecular cores, whose asymmetries trace infall and expansion motions. The high-mass star forming region SgrB2(M) was observed with Herschel/HIFI (HEXOS key project) in various lines of HCN and its isotopologues, complemented by APEX data. The observations are compared to spherically symmetric, centrally heated models with density power-law gradient and different velocity fields (infall or infall+expansion), using the radiative transfer code RATRAN. The HCN line profiles are asymmetric, with the emission peak shifting from blue to red with increasing J and decreasing line opacity (HCN to H$^{13}$CN). This is most evident in the HCN 12--11 line at 1062 GHz. These line shapes are reproduced by a model whose velocity field changes from infall in the outer part to expansion in the inner part. The qualitative reproduction of the HCN lines suggests that infall dominates in the colder, outer regions, but expansion dominates in the warmer, inner regions. We are thus witnessing the onset of feedback in massive star formation, starting to reverse the infall and finally disrupting the whole molecular cloud. To obtain our result, the THz lines uniquely covered by HIFI were critically important.