Chemistry of Planet Formation
The dust of protoplanetary discs is made of surviving grains from the interstellar medium (ISM, mainly SiC grains) and condensates formed during the cooling of the stellar nebula. Consequently, the chemical composition of the disc is a mixture of very refractory grains that did not sublimate during stellar formation, and gaseous material that condensed. The contribution of pre-stellar grains is currently unknown, and it is often assumed that they play no role in the resulting disc chemistry.There are different way to account for chemistry in protoplanetary discs. Kinetics are the most interesting as they enable us to determine and follow with time all processes that play a role, but need the knowledge of chemical networks that are difficult to achieve. If it is possible to realize almost complete networks
for simple molecules such as volatile molecules in general in protoplanetary discs (including non equilibrium effects, see, e.g., Walsh et al. 2010; Heinzeller et al. 2011; Walsh et al. 2012), long and complex molecules, such as refractory materials, are difficult to account for. Due to lack of data concerning reaction rates and network, it is usually assumed for such species that they form in thermodynamical equilibrium, following the so called "condensation sequence" which tells us at which pressures and temperatures a specie is stable in solid state, and traces the slow cooling of the stellar nebula (Lodders 2003, Ebel 2006). This assumes that the stellar nebula was initially hot (>3000K) so that every solid matter of the molecular cloud has sublimated, "resetting" the chemistry, and that the subsequent cooling has been slow enough to be able to reach equilibrium.
Such assumption is used for the computation of molecular abundances of refractory species, since only thermodynamical data are usually available (see, e.g., the NIST Chemistry WebBook). In a search for consistency, volatile molecules are also computed in equilibrium, although not quite in the same fashion.