1995A&A...294..815F

Metastable levels of ammonia up to (J,K)=(14,14) have been observed in absorption towards SgrB2, a giant molecular cloud close to the galactic centre. We investigate the physical conditions that can explain these and related observations and conclude that hot (500K) and low density (10³cm^–3) gas is responsible. We find that C-shock models are able to reproduce these conditions and hence the metastable column densities, providing that the formation of NH₃ on grain surfaces and its sputtering by the shock wave are taken into account. The column density of NH₂ predicted by the model of the hot ammonia regions is consistent with recent observations of SgrB2-M, but the H₂O column density appears to be too high.