Observational tests for grain chemistry: Posterior isotopic labelling

Abstract

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. If atom addition reactions to solid CO and HCO are the origin of the organic molecules detected in hot molecular cores, these molecules should still possess the same [12C/13C] ratio. The theory that methanol ice is produced by reduction of CO ice can be tested by radio-astronomical observations of several protostellar sources with known [12CO2/13CO2]ice ratios to see if the observed trend in [12CO2/ 13CO2]ice is reflected in [12CH 3OH/13CH3OH]. We also critically discuss observations designed to elucidate the origin of solid OCS, as well as of many other larger organic molecules.