2007A&A...466..969L

Our aim is to compare the infrared properties of big, ``classical'' dust grains with visual extinction in the cloud L1642. In particular, we study the differences in grain emissivity between diffuse and dense regions in the cloud. The far-infrared properties of dust are based on large-scale 100µm and 200µm maps. Extinction through the cloud was derived by using the star count method in the B- and I-bands, and colour excess method in the J, H, and K_ s_ bands. Radiative transfer calculations were used to study the effects of increasing absorption cross-section on the far-infrared emission and dust temperature. Dust emissivity, measured by the ratio of far-infrared optical depth to visual extinction, τ(far-IR)/A_V, increases with decreasing dust temperature in L1642. There is about a two-fold increase in emissivity over the dust temperature range of 19K-14K. Radiative transfer calculations show that, in order to explain the observed decrease of dust temperature towards the centre of L1642, an increase of absorption cross-section of dust at far-IR is necessary. This temperature decrease cannot be explained solely by the attenuation of interstellar radiation field. Increased absorption cross-section also manifests itself as an increased emissivity. We find that, due to temperature effects, the apparent value of optical depth τ_app(far-IR), derived from 100µm and 200µm intensities, is always lower than the true optical depth.