2005A&A...435..177J

Radiative transfer models of multi-transition observations are used to determine molecular abundances as functions of position in pre- and protostellar cores. The data require a ``drop'' abundance profile with radius, with high abundances in the outermost regions probed by low excitation 3mm lines, and much lower abundances at intermediate zones probed by higher frequency lines. The results are illustrated by detailed analysis of CO and HCO⁺ lines for a subset of objects. We propose a scenario in which the molecules are frozen out in a region of the envelope where the temperature is low enough (≲40K) to prevent immediate desorption, but where the density is high enough (>10⁴-10⁵cm^–3) that the freeze-out timescales are shorter than the lifetime of the core. The size of the freeze-out zone is thereby a record of the thermal and dynamical evolution of the cores. Fits to CO data for a sample of 16 objects indicate that the size of the freeze-out zone decreases significantly between class 0 and I objects, explaining the variations in, for example, CO abundances with envelope masses. However, the corresponding timescales are 10^5±0.5-years, with no significant difference between class 0 and I objects. These timescales suggest that the dense pre-stellar phase with heavy depletions lasts only a short time, of the order of 10⁵yr, in agreement with recent chemical-dynamical models.