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

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