7 Summary

We have mapped B68 in $\ensuremath{\:\!} ^{13}\ensuremath{\!\;\!}$CO $(J=1\mbox{--}0)$, C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=1\mbox{--}0)$ and C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O $(J=2\mbox{--}1)$. Combining our observations with the extinction profile of B68 (Alves et al. 2001), which closely follows the one predicted for a Bonnor-Ebert sphere, we have come to the following results:

1.

The kinetic temperature of the gas is $\approx$8 K.

2.

The density dependence of the C $\ensuremath{\:\!} ^{18}\ensuremath{\!\;\!}$O abundance distribution proves substantial molecular depletion in B68. The CO depletion factor $f_{\rm d}$ is well fitted with the law $f_{\rm d}=1+n(\mbox{H$_2$ })~A/B$. The agreement between the estimates for the ratio of the accretion constant and the depletion constant, A/B, based on previous model predictions and derived from our measurements, suggests that the degree of CO depletion can be understood in terms of accretion onto dust grains and cosmic ray induced desorption. In the centre of B68, between 0.5% and 5% of all CO molecules remain in the gas phase.  

Based on the assumption that B68 is indeed a BES, i.e. being isothermal and in hydrostatic equilibrium, we conclude futhermore:

3.

The most likely distance to B68 is $\approx$80 pc.

4.

The mass of the globule is $\approx$0.7  , which is considerably less than estimated previously on the basis of other distance and temperature values.

5.

The near-infrared reddening efficiency of dust grains per unit H₂ column density is close to the canonical value derived in diffuse clouds. A clearly higher reddening efficiency would disagree with our observations.

Acknowledgements

We thank Dr. João F. Alves for providing us with the data of the measured extinction profile of B68. The Second Palomar Observatory Sky Survey (POSS-II) was made by the California Institute of Technology with funds from the National Science Foundation, the National Geographic Society, the Sloan Foundation, the Samuel Oschin Foundation, and the Eastman Kodak Corporation. This project was supported by Deutsches Zentrum für Luft- und Raumfahrt e. V. (DLR) with funds of Bundesministerium für Bildung und Forschung, grant No. 50 QI 9801 3, and by the Academy of Finland, grant Nos. 173727 and 174854.