L183 (L134N) revisited - III. The gas depletion

We present a detailed study of the gas depletion in L183 (= L134N) for a set of important species, namely, CO, CS, SO, N2H+ and NH3. We show that all these species are depleted at some level. This level seems to depend mostly on a density threshold rather than on dust opacity. Therefore UV shielding would not be a main factor in the triggering of depletion. Our data suggest that CO, CS and SO depletion happen at densities of similar to3 x 10(4) cm(-3), while N2H+ and NH3 seem to deplete at densities close to 10(6) cm(-3). The latter result is consistent with the Bergin & Langer (1997, ApJ, 486, 316) polar (H2O) ice case but not with the more recent models of Aikawa et al. ( 2003, ApJ, 593, 906). CS depletion occurs much below its (J:2-1) critical density, (7 x 10(5) cm(-3)) and therefore makes this species unsuitable to study the density structure of many dark cloud cores.