BISTABILITY AND DUST GAS CHEMICAL MODELING IN DARK INTERSTELLAR CLOUDS

We have investigated the conditions required for the existence of the Bistability Phenomenon (BP) in dark interstellar clouds; i.e, the phenomenon that the possibilty for chemical modelling produces two simultaneous stable solutions referred to as the high ionization phase (HIP) and the low ionization phase (LIP) leading to different molecular abundances in clouds with identical density and temperature. The results of pure gasphase and dust/gas chemical models are compared. Our study is the first to discuss the bistability phenomenon in models in which all the major dust/gas interaction processes have been taken into consideration. The dust/gas model includes accretion, surface reactions, photoproduction and desorption processes. In our models, two sets of initial fractional elemental abundances and two cosmic ray ionization rates have been used. The main results are that the use of higher initial gas phase elemental abundances and/or inclusion of dust/gas interactions prevent the appearance of the BP phenomenon.