| In order to understand the chemical and dynamical conditions in low mass star forming cores, this dissertation focuses on (1) the development of an evolutionary chemical model combining a dynamical model of star formation and an evolutionary model of luminosity in the process of star formation, and (2) comparisons of observations with results of the chemical model or with simple empirical models. The evolutionary chemical model combines self-consistently a dynamical model (Shu's inside-out collapse model) with a chemical network, which includes interactions between gas and dust grains as well as gas-phase chemistry. Dust radiative transfer and gas-energetics codes are also combined with the evolutionary model to provide proper dust and gas temperatures. The evolutionary model shows that carbon- and sulfur-bearing molecules such as CO and CS are frozen onto grain surfaces in pre-protostellar cores, so nitrogen-bearing molecules such as N2H+ and NH3 are good tracers of cold and dense material before stars form. However, once a central star forms, and in turn, surrounding material is heated by the formation of the protostellar object, molecules start to evaporate from grain surfaces, making other molecules than N2H + and NH3 better tracers. The evolutionary model developed in this thesis has been compared with other, simpler models, such as empirical models and static models, to show the effects of the dynamical evolution on the chemical evolution. Observations toward three pre-protostellar cores (L1512, L1544, and L1689B) and a more evolved core (L1251B) support the results of the evolutionary chemical model. |
