CHEMICAL EVOLUTION OF FREE-FALL COLLAPSING INTERSTELLAR CLOUDS - PSEUDO AND REAL-TIME DEPENDENT MODELS

We have investigated the effect of using real and pseudo time dependen t models on the chemical evolution of collapsing interstellar clouds c onsidering, as a first step, the early free-fall collapse phase. Two c hemical models have been presented in all cases: 1) pure gas phase che mistry; 2) combined dust/gas chemistry in which solid grain chemistry is considered simultaneously with the gas. Accretion, explosive desorp tion, surface reactions and photochemical processing are the main addi tional processes involved in the model of dust/gas interactions relati ve to pure gas phase processes. The differences in this early stage of a collapsing interstellar cloud resulting from real and pseudo time d ependent models in both pure gas phase and dust/gas models are small f or low mass (1 M.). For a high mass (10 M.) cloud in which its free-fa ll collapse starts from lower density and evolves to higher density wi th longer time of chemical evolution the use of real time dependence b ecomes important. We found, as expected, a clear difference between us ing pure gas phase and combined dust/gas models both in pseudo and rea l time dependent models. This result confirms the important role of du st/gas interactions not only in chemical models of quiescent clouds bu t also for collapsing interstellar clouds. Our dust/gas models which i nclude explicitly the gas dust interactions and start with normal depl eted atomic abundances can give an alternative scenario to the so-call ed ''cyclic-dynamical chemical model'' which appears to be required by another chemical evolution model of collapsing clouds. The justificat ion and relevance of initial elemental and molecular abundances are di scussed in relation to the collapsing cloud models.