2001A&A...372..998C

We present new observations of the H₂CO and D₂CO emission around IRAS16293-2422, a low mass protostar in the ρ Ophiuchus complex. Bright H₂CO and D₂CO emission is detected up to 40'' from the center, corresponding to a linear distance of ∼5000AU. The derived H₂CO abundance profile has two jumps at r≤150AU and r≤700AU, where the dust temperature reaches 100K and 50K respectively. The measured [D₂CO]/[H₂CO] abundance ratio in the envelope is between 0.03 and 0.16, an extremely high value. We demonstrate that the present new observations can only be explained if the D₂CO (and H₂CO) are formed during the previous cold pre-collapse phase, stored in the grain mantles, and released in the gas phase during the pre-collapse phase. We consider the two main competing theories for mantle formation, i.e. pure accretion against grain surface chemistry, and we conclude that the former theory cannot explain the present observations, whereas grain active chemistry very naturally does. We found that the mantles are evaporated because of the thermal heating of the grains by the central source and that in the outer cold envelope H₂CO and D₂CO molecules are embedded in CO-rich mantles which sublimate when the dust is warmer than 25K. Finally, the present day H₂CO and D₂CO abundances very probably reflect the mantle composition. We argue that mantles have likely formed in an onion-like structure with the innermost ice layers more enriched in H₂CO molecules and we give estimates of the CO hydrogenation efficiency across the envelope and/or in different ices.