Chemical model including the gas, the dust grain surface and the icy mantle. The solid lines correspond to the gas-phase abundances versus time during warm-up stages in chemical models with gas maximum temperatures of 60, 100, and 150 K, and the points are the observed data. The abundances of H₂CS, HCS⁺ and H₂S relative to H₂ are presented in panels (a), (b), and (c), respectively. Since the samples are in the same evolutionary stage, two possible times from the form model are marked, with filled and transparent circles. Note that there are no intersections for H₂S at 60 K in model.