The physical and chemical structure of Sagittarius B2

Context. We model the emission of methyl cyanide (CH3CN) lines towards the massive hot molecular core Sgr B2(M). Aims. We aim to reconstruct the CH3CN abundance field, and investigate the gas temperature distribution as well as the velocity field. Methods. Sgr B2(M) was observed with the Atacama Large Millimeter/submillimeter Array (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 employed 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 based our model on the results of a previous study that determined the physical structure of Sgr B2(M), i.e., the distribution of dust dense cores, ionized regions, and heating sources. Results. 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 4 × 10−11 to 2 × 10−8 in the northern part of Sgr B2(M) and from 2 × 10−10 to 5 × 10−7 in the southern part. In general, we find that the relative abundance of CH3CN is lower at the center of the very dense, 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 Sgr B2(M).