Dust properties of the dark cloud IC5146 - Submillimeter and NIR imaging

We present the results of a submillimeter dust continuum study of a molecular ridge in IC 5146 carried out at 850 mum and 450 mum with SCUBA on the James Clerk Maxwell Telescope (JCMT). The mapped region is similar to14'x2'.5 in size (similar to2 pc x 0.3 pc) and consists of at least four dense cores which are likely to be prestellar in nature. To study the dust properties of the ridge and its embedded cores, we combined the dust emission data with dust extinction data obtained by Lada et al. (1999) from the NIR colors of background giant stars. The ridge shows dust extinctions above similar to10 mag, rising up to 35 mag in the cores. A map of dust temperatures, constructed from the continuum flux ratios, shows significant temperature gradients: we find temperatures of up to similar to20 K in the outskirts and between the cores, and down to similar to10 K in the cores themselves. The cores themselves are almost isothermal, although their average temperatures vary between 10 K and 18 K. We used the extinction data to derive in addition a map of the dust emissivity parametrized by kappa' = kappa(850)/kappa(V). The average value of kappa' agrees well with the canonical value of Mathis (1990). We find that kappa' increases by a factor of similar to4 from similar to1.3x10(-5) to similar to5x10(-5) when the dust temperature decreases from similar to20 K to similar to12 K. A Monte Carlo simulation shows that this change is significant with regard to the estimated calibration uncertainties. This is consistent with models of dust evolution in prestellar cores by Ossenkopf & Henning (1994) which predict that grain coagulation and the formation of ices on grain surfaces in the cold, dense cloud interiors lead to a significant increase of the 850 mum dust opacity. This interpretation is furthermore supported by the previous detection of gas-phase depletion of CO in one of the IC 5146 cores (Kramer et al. 1999). Observations of dust fluxes at short wavelengths are however needed to verify this result.