Observational tests for grain chemistry: posterior isotopic labelling

We propose a series of detailed observations that should allow current ideas concerning the important catalytic pathways to interstellar molecules on interstellar dust grains to be tested. The atoms and molecules that accrete on cold grains and take part in surface reactions will be selectively fractionated in ¹³C. Deeply embedded hot cores are surrounded by cold envelopes which still contain molecular ices that reflect the composition of the material evaporated into the hot gas. Recent Infrared Space Observatory observations of carbon dioxide ices towards embedded protostars have allowed the solid [¹²C/¹³C] ratio in CO₂ to be measured for the first time. We therefore now have a means to test theories of grain chemistry by comparing the isotopic signature of abundant ice molecules with that of species derived from them and which are only detectable in the gas - isotopic labelling a posteriori.