title: From Prestellar to Protostellar Cores: Time Dependence, Deuterium Fractionation, and Disk Formation

authors: Y. Aikawa, V. Wakelam, F. Hersant, R.T. Garrod, E. Herbst

abstract: We investigate the molecular evolution and D/H abundance ratios that develop as star formation proceeds from dense cloud cores to protostellar cores by solving a gas-grain reaction network using a 1-D radiative hydrodynamic model with infalling fluid parcels. We find that the abundances of large organic species in the central region increase with time. The duration of the warm-up phase, in which large organic species are efficiently formed, is longer in infalling fluid parcels at later stages. The formation of unsaturated carbon chains in the CH$_4$ sublimation zone, known as warm carbon chain chemistry, is more effective in later stage. The carbon ion, which reacts with CH$_4$ to form carbon chains, increases in abundance as the envelope density decreases. The large organic molecules and carbon chains are both heavily deuterated, mainly because their mother molecules have high D/H ratios, which are determined in the cold phase.  The observed CH$_2$DOH/CH$_3$OH ratio is reproduced if we assume that the grain-surface exchange and abstraction reactions of CH$_3$OH + D occur efficiently. In our 1-D collapse model, the fluid parcels directly fall into the protostar, and the warm-up phase in the fluid parcels is rather short. But, in reality, a circumstellar disk can be formed. We investigate the molecular evolution in such a disk by assuming that a fluid parcel stays at a constant temperature (i.e.,  a fixed disk radius) after the infall. The species CH$_3$OCH$_3$ and HCOOCH$_3$ become more abundant in the disk than in the envelope. Both have high D/H abundance ratios as well.