Context: The chemical composition of a molecular cloud changes dramatically
as it collapses to form a low-mass protostar and circumstellar disk.
Two-dimensional (2D) chemodynamical models are required to properly study this
process.
Aims: The goal of this work is to follow, for the first time, the chemical
evolution in two dimensions all the way from a pre-stellar core into a
circumstellar disk. Of special interest is the question whether the chemical
composition of the disk is a result of chemical processing during the collapse
phase, or whether it is determined by in situ processing after the disk has
formed.
Methods: Our model combines a semi-analytical method to get 2D axisymmetric
density and velocity structures with detailed radiative transfer calculations
to get temperature profiles and UV fluxes. Material is followed in from the
core to the disk and a full gas-phase chemistry network -- including freeze-out
onto and evaporation from cold dust grains -- is evolved along these
trajectories. The abundances thus obtained are compared to the results from a
static disk model and to observations of comets.
Results: The chemistry during the collapse phase is dominated by a few key
processes, such as the evaporation of CO or the photodissociation of H2O. At
the end of the collapse phase, the disk can be divided into zones with
different chemical histories. The disk is not in chemical equilibrium at the
end of the collapse, so care must be taken when choosing the initial abundances
for stand-alone disk chemistry models. Our model results imply that comets must
be formed from material with different chemical histories: some of it is
strongly processed, some of it remains pristine. Variations between individual
comets are possible if they formed at different positions or different times in
the solar nebula.Comment: 18 pages, 12 figures, accepted by A&
