The physical and chemical structure of Sagittarius B2. III. Radiative
  transfer simulations of the hot core SgrB2(M) for methyl cyanide
  We model the emission of methyl cyanide (CH3CN) lines towards the massive hot
molecular core SgrB2(M). We aim at reconstructing the CH3CN abundance field and
investigating the gas temperature distribution as well as the velocity field.
SgrB2(M) was observed with the ALMA in a spectral line survey from 211 to 275
GHz. This frequency range includes several transitions of CH3CN (including
isotopologues and vibrationally excited states). We employ the
three-dimensional radiative transfer toolbox Pandora in order to retrieve the
velocity and abundance field by modeling different CH3CN lines. For this
purpose, we base our model on the results of a previous study that determined
the physical structure of SgrB2(M), i.e.\ the distribution of dust dense cores,
ionized regions and heating sources. The morphology of the CH3CN emission can
be reproduced by a molecular density field that consists of a superposition of
cores with modified Plummer-like density profiles. The averaged relative
abundance of CH3CN with respect to H2 ranges from 4x10^{-11} to 2x10^{-8} in
the northern part of SgrB2(M) and from 2x10^{-10} to 5x10^{-7} in the southern
part. In general, we find that the relative abundance of CH3CN is lower at the
center of the very dense and hot cores, causing the general morphology of the
CH3CN emission to be shifted with respect to the dust continuum emission. The
dust temperature calculated by the radiative transfer simulation based on the
available luminosity reaches values up to 900 K. However, in some regions
vibrationally excited transitions of CH3CN are underestimated by the model,
indicating that the predicted gas temperature, which is assumed to be equal to
the dust temperature, is partly underestimated. The determination of the
velocity component along the line of sight reveals that a velocity gradient
from the north to the south exists in SgrB2(M).
