EDP Sciences
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Volume 391, Number 1, August III 2002
Page(s) 275 - 285
Section Diffuse matter in space
DOI http://dx.doi.org/10.1051/0004-6361:20020786

A&A 391, 275-285 (2002)
DOI: 10.1051/0004-6361:20020786

C $\mathsf{^{18}}$O abundance in the nearby globule Barnard 68

S. Hotzel1, 2, J. Harju2, M. Juvela2, K. Mattila2 and L. K. Haikala3

1  Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
2  Observatory, PO Box 14, 00014 University of Helsinki, Finland
3  Swedish-ESO Submillimetre Telescope, European Southern Observatory, Casilla 19001, Santiago, Chile

(Received 23 January 2002 / Accepted 23 May 2002 )

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 , and . 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 fd can be well fitted with the law , 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 $\approx$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  $M_\odot$, considerably less than previously estimated. We find in B68 no clear deviance of the near-infrared reddening efficiency of dust grains per unit column density with respect to values derived in diffuse clouds.

Key words: ISM: individual objects: Barnard 68 -- ISM: abundances -- ISM: molecules -- ISM: dust, extinction

Offprint request: S. Hotzel, hotzel@astro.helsinki.fi

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