Chemistry in low-mass star forming regions

We review molecular evolution in low-mass star-forming regions and discuss what we can observe with ALMA. Recent observations have revealed chemical fractionation, i.e. spatial variation of molecular abundances, in dense prestellar cores. In the central regions of cold prestellar cores, CO is heavily depleted, while the depletion of N-bearing species are rare. Models show that CO is frozen onto grains, while N-bearing species survive because of the CO depletion and slow formation of N₂ in the gas phase. CO depletion also enhances the molecular D/H ratio. Chemical fractionation and its variation among cores can be an indicator of evolutionary stage and/or accumulation process of cores. As the core contracts, central region of the core is eventually heated by compressional heating and a new-born protostar. CO is sublimated back to the gas phase, if the temperature reaches 20 K. Warm temperature enhances the endothermic reactions which were negligible in the prestellar core stage, and also enhances grain-surface reactions among heavy-element species to form large organic molecules, which sublimate when the temperature reaches ～100 K. Warm regions with high abundances of the gaseous organic species are called hot corinos or low-mass hot cores. Adopting a theoretical model of core contraction, we present the temporal variation of the radius inside which CO and large organic species are sublimated. We also investigate the molecular evolution in infalling shells to derive molecular distribution in a protostellar core.