The hot core of the solar-type protostar IRAS 16293-2422: H2CO emission

We model the H2CO and (H2CO)-C-13 line emission observed towards the solar- type protostar IRAS16293-2422. Based upon previous analysis of the physical structure of the envelope surrounding IRAS16293-2422, we develop a model i n which the H2CO lines are emitted by two components: a cold H2CO-poor oute r envelope and a warm H2CO-rich core. We find that the model reproduces suc cessfully all the available H2CO and (H2CO)-C-13 data for a H2CO abundance equal to (1.1 +/- 0.3) x 10(-9) in the outer and (1.1 +/- 0.4) x 10(-7) in the inner regions of the envelope respectively. We interpret this increase of the H2CO abundance as due to the evaporation of the grain mantles when t he dust temperature exceeds 100 K at about 150 AU from the center, forming a hot core like region. Assuming that all mantle constituents evaporate and are detected in the gas phase, we derive that the H2CO-ice abundance is ab out 3% of the H2O-ice abundance. This is the first measurement of the H2CO abundance in grain mantles around a low-mass protostar.