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 mum and with SIMBA at 1.2 mm. The submillimetre and millimetre dust emission correlate well with the extinction map of Alves et al. (2001). The Av/850 mum 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 mum and 1.2 mm. We find kappa(850) (mum)/kappa(V) = 4.0 +/- 1.0 x 10(-5) and kappa(1.2 mm)/kappa(V) = 9.0 +/- 3.0 x 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.