  Using the high angular resolution provided by the ALMA interferometre we want
to resolve the COM emission in the hot molecular core Sagittarius B2(N1) and
thereby shed light on the desorption process of Complex Organic Molecules
(COMs) in hot cores. We use data taken as part of the 3 mm spectral line survey
Re-exploring Molecular Complexity with ALMA (ReMoCA) to investigate the
morphology of COM emission in Sagittarius B2(N1). Spectra of ten COMs are
modelled under the assumption of LTE and population diagrams are derived for
positions at various distances to the south and west from the continuum peak.
Based on this analysis, resolved COM rotation temperature and COM abundance
profiles are derived. Based on the morphology, a rough separation into O- and
N-bearing COMs can be done. Temperature profiles are in agreement with
expectations of protostellar heating of an envelope with optically thick dust.
Abundance profiles reflect a similar trend as seen in the morphology and, to a
great extent, agree with results of astrochemical models that, besides the
co-desorption with water, predict that O-bearing COMs are mainly formed on dust
grain surfaces at low temperatures while at least some N-bearing COMs and
CH$_3$CHO are substantially formed in the gas phase at higher temperatures. Our
observational results, in comparison with model predictions, suggest that COMs
that are exclusively or to a great extent formed on dust grains desorb
thermally at ~100 K from the grain surface likely alongside water. Non-zero
abundance values below ~100 K suggest that another desorption process is at
work at these low temperatures: either non-thermal desorption or partial
thermal desorption related to lower binding energies experienced by COMs in the
outer, water-poor ice layers. In either case, this is the first time that the
transition between two regimes of COM desorption has been resolved in a hot
core.
