Thermal reactions in interstellar ice: A step towards molecular complexity in the interstellar medium

Complex organic molecules are widely observed in star-forming regions, although their formation mechanisms are not well understood. Solid-state chemistry is thought to play an important role, but the solid-state reaction network is poorly known. We provide a list of purely thermal reactions involving electronically stable reactants to complement existing grain chemistry networks. The kinetic parameters of the reactions are given when available. These reactions lead to the formation of complex organic molecules, which were not considered previously. Eventually, these complex molecules are either released into the gas phase or incorporated into the organic residue found in meteorites. Thermal reactions are important because they are not limited by the UV flux or the slow diffusion of the radicals, and because they involve both surface and mantle molecules. Thermal reactions represent an important step in the formation of complex organic molecules that constitute the primitive material of comets and asteroids.