Molecular Evolution and Star Formation: From Prestellar Cores to Protostellar Cores

We investigate molecular evolution in a star-forming core that is initially a hydrostatic starless core and collapses to form a low-mass protostar. The results of a one-dimensional radiation-hydrodynamics calculation are adopted as a physical model of the core. We first derive radii at which CO and large organic species sublimate. CO sublimation in the central region starts shortly before the formation of the first hydrostatic core. When the protostar is born, the CO sublimation radius extends to 100 AU, and the region inside $\lesssim 10$ AU is hotter than 100 K, at which some large organic species evaporate. We calculate the temporal variation of physical parameters in infalling shells, in which the molecular evolution is solved using an updated gas-grain chemical model to derive the spatial distribution of molecules in a protostellar core. The shells pass through the warm region of $10 -100$ K in several $\times$ $10^4$ yr, and fall into the central star $\sim 100$ yr after they enter the region where $T \gtrsim 100$ K. We find that large organic species are formed mainly via grain-surface reactions at temperatures of $20 -40$ K and then desorbed into the gas-phase at their sublimation temperatures. Carbon-chain species can be formed by a combination of gas-phase reactions and grain-surface reactions following the sublimation of CH$_4$. Our model also predicts that CO$_2$ is more abundant in isolated cores, while gas-phase large organic species are more abundant in cores embedded in ambient clouds.