L183 (L134N) revisited^*,**

III. The gas depletion

¹ LERMA & UMR 8112 du CNRS, Observatoire de Paris, 61 Av. de l'Observatoire, 75014 Paris, France e-mail: laurent.pagani@obspm.fr
² Instituto de Estructura de la Materia, Dpto de Física Molecular CSIC, Madrid, Spain
³ University of Arizona, Steward Observatory, 933 N. Cherry Ave., Tucson, AZ 85721, USA
⁴ Observatoire de Bordeaux, BP 89, 33270 Floirac, France

Received: 6 April 2004
Accepted: 27 July 2004

Abstract

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 ~3 10⁴ cm^-3, while N₂H⁺and NH₃ seem to deplete at densities close to 10⁶ cm^-3. The latter result is consistent with the Bergin & Langer ([CITE], ApJ, 486, 316) polar (H₂O) ice case but not with the more recent models of Aikawa et al. ([CITE], ApJ, 593, 906). CS depletion occurs much below its (J:2–1)  critical density, (7 10⁵ cm^-3) and therefore makes this species unsuitable to study the density structure of many dark cloud cores.