1 Introduction

It was recently discovered by Alves et al. (2001) that the circularly-averaged visual extinction profile of the globule Barnard 68 (B68) agrees remarkably well with the predictions for an isothermal sphere in hydrostatic equilibrium or a so-called Bonnor-Ebert sphere (BES). The suggested structural simplicity makes B68 particularly favourable for the determination of physical parameters and chemical abundances, and, since the cloud probably represents a state prior to protostellar collapse, for studying processes influencing star formation.

One important process is gas-phase depletion of molecules through freezing out onto dust grains. CO depletion changes the chemical and physical structure of globules by affecting the deuterium fractionation and the ionization degree (Caselli et al. 1998), the cooling efficiency and thermal balance, and the gas phase chemical composition. Additionally, depletion must be taken into account when using CO spectroscopy to trace molecular hydrogen. Observational evidence for molecular accretion onto dust grains comes from the detection of absorption features of molecular ices (e.g. Tielens et al. 1991), the observation of chemical fractionation between more and less volatile molecular species (e.g. Zhou et al. 1989) and the comparison of column densities of gaseous CO and dust (e.g. Caselli et al. 1999).

B68 was observed earlier in CO (Avery et al. 1987) and NH₃ (Bourke et al. 1995; Lemme et al. 1996), which yielded temperature and density estimates of the globule. Avery et al. (1987) used multi-level CO and $\ensuremath{\:\!} ^{13}\ensuremath{\!\;\!}$CO(J=1-0) observations to deduce an outward increasing kinetic temperature between 6 and 11 K. Bourke et al. (1995) observed B68 in the (J,K)=(1,1) and (2,2) inversion lines of ammonia and derived a kinetic temperature of 16 K and an H₂ number density of 9.1 $\ensuremath{\;\!}\times\ensuremath{\;\!}$10³  .

The distance D to B68 cited by most authors originates from Bok & McCarthy (1974), who allocated D=200 pc because of the globule's proximity to the Ophiuchus complex. However, the distance to the Ophiuchus dark clouds was redetermined by de Geus et al. (1989), who found the complex extending from $D=80\mbox{ to }170$ pc with a central value of 125 pc. Other determinations of the distance to the centre of the complex also fall in the D=125-200 pc range (Chini 1981; Straizys 1984). It should be emphasised that no distance estimate for B68 itself is available, and due to the lack of foreground stars would be difficult to obtain using classical methods.

Alves et al. (2001) used H and K imaging of B68 to determine the near-infrared (NIR) colours of thousands of background stars. These extinction measurements provided a high resolution column density profile and yielded a physical cloud model for B68. These achievements allow us to study CO depletion in more detail than done before in other objects, where both CO and H₂ density distributions are described by ad hoc models.

In the present study we determine the CO column density distribution and the degree of CO depletion in B68 by using isotopic CO line observations. Combining our observations with the extinction measurements of Alves et al. (2001), we are able to quantify the CO abundance as a function of density and compare the observed depletion with theoretical expectations. We also give an estimate for the cloud's kinetic temperature. Using this and the assumption of a BES we propose new values for the cloud's distance, mass and H₂-to-extinction ratio.