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