Dust emissivity in the submm/mm. SCUBA and SIMBA observations of Barnard 68

We have observed the dark cloud Barnard 68 with SCUBA at 850 mu m and with SIMBA at 1.2 mm. The submillimetre and millimetre dust emission correlate well with the extinction map of Alves et al. (\cite{AlvesNature2001}). The AV/850 mu m correlation is clearly not linear and suggests lower temperatures for the dust in the inner core of the cloud. Assuming a model for the temperature gradient, we derive the cloud-averaged dust emissivities (normalised to the V-band extinction efficiency) at 850 mu m and 1.2 mm. We find kappa850 mum/ kappaV =4.0+/- 1.0x 10-5 and kappa1.2 mm/ kappaV = 9.0+/- 3.0x 10-6. These values are compared with other determinations in this wavelength regime and with expectations for models of diffuse dust and grain growth in dense clouds.