Chemical history of molecules in circumstellar disks
  The chemical composition of a protoplanetary disk is determined not only by
in situ chemical processes during the disk phase, but also by the history of
the gas and dust before it accreted from the natal envelope. In order to
understand the disk's chemical composition at the time of planet formation,
especially in the midplane, one has to go back in time and retrace the
chemistry to the molecular cloud that collapsed to form the disk and the
central star. Here we present a new astrochemical model that aims to do just
that. The model follows the core collapse and disk formation in two dimensions,
which turns out to be a critical upgrade over older collapse models. We predict
chemical stratification in the disk due to different physical conditions
encountered along different streamlines. We argue that the disk-envelope
accretion shock does not play a significant role for the material in the disk
at the end of the collapse phase. Finally, our model suggests that complex
organic species are formed on the grain surfaces at temperatures of 20 to 40 K,
rather than in the gas phase in the T>100 K hot corino.
