2000A&A...357L...9C

We model the H₂CO and H₂¹³CO line emission observed towards the solar-type protostar IRAS 16293-2422. Based upon previous analysis of the physical structure of the envelope surrounding IRAS 16293-2422, we develop a model in which the H₂CO lines are emitted by two components: a cold H₂CO-poor outer envelope and a warm H₂CO-rich core. We find that the model reproduces successfully all the available H₂CO and H₂¹³CO data for a H₂CO abundance equal to (1.1 ±0.3)x10^–9 in the outer and (1.1 ±0.4)x10^–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 the 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 about 3% of the H₂O-ice abundance. This is the first measurement of the H₂CO abundance in grain mantles around a low-mass protostar.