Chemical evolution of a protoplanetary disk
  In this paper we review recent progress in our understanding of the chemical
evolution of protoplanetary disks. Current observational constraints and
theoretical modeling on the chemical composition of gas and dust in these
systems are presented. Strong variations of temperature, density, high-energy
radiation intensities in these disks, both radially and vertically, result in a
peculiar disk chemical structure, where a variety of processes are active. In
hot, dilute and heavily irradiated atmosphere only the most photostable simple
radicals and atoms and atomic ions exist, formed by gas-phase processes.
Beneath the atmosphere a partly UV-shielded, warm molecular layer is located,
where high-energy radiation drives rich ion-molecule and radical-radical
chemistry, both in the gas phase and on dust surfaces. In a cold, dense, dark
disk midplane many molecules are frozen out, forming thick icy mantles where
surface chemistry is active and where complex polyatomic (organic) species are
synthesized. Dynamical processes affect disk chemical composition by enriching
it in abundances of complex species produced via slow surface processes, which
will become detectable with ALMA.
