Abstract

We have studied the radial variation of the CO abundance in the nearby isolated globule Barnard 68 (B68). For this purpose, B68 was mapped in the three rotational lines (CO)-C-13(J = 1- 0), C-18 O(J = 1- 0) and C-18 O(J = 2- 1). Using the recent discovery of Alves et al. (2001) that the density structure of B68 agrees with the prediction for a pressure bound distribution of isothermal gas in hydrostatic equilibrium (Bonnor-Ebert sphere), we show that the flat CO column density distribution can be explained by molecular depletion. By combining the physical model with the observed CO column density profile, it was found that the density dependence of the CO depletion factor f(d) can be well fitted with the law f(d) = 1 + const. n(H-2), which is consistent with an equilibrium between the accretion and the desorption processes. In the cloud centre, between 0.5% and 5% of all CO molecules are in the gas phase. Our observations suggest a kinetic temperature of approximate to 8 K. In combination with the assumption that B68 is a Bonnor-Ebert sphere, this leads to a distance of 80 pc. The cloud mass consistent with these values is 0.7 M-., considerably less than previously estimated. We find in B68 no clear deviance of the near-infrared reddening efficiency of dust grains per unit H-2 column density with respect to values derived in diffuse clouds.