ISO far-infrared observations of the high-latitude cloud L1642 - II. Correlated variations of far-infrared emissivity and temperature of classical large dust particles

Aims. 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. Methods. The far-infrared properties of dust are based on large-scale 100 mu m and 200 mu m 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. Results. Dust emissivity, measured by the ratio of far-infrared optical depth to visual extinction, tau(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 19 K-14 K. 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 tau(app)(far-IR), derived from 100 mu m and 200 mu m intensities, is always lower than the true optical depth.