Constraints on the nature of dust particles by infrared observations

The far-infrared (FIR) emissivity of dust is an important parameter characterizing the physical properties of the grains. With the availability of stellar data bases and FIR data from Infrared Space Observatory (ISO), it is possible to compare the optical and infrared properties of dust, and derive the FIR emissivity with respect to the optical extinction. In this paper, we present the results of a systematic analysis of the FIR emissivity of interstellar clouds observed with ISOPHOT (the photometer onboard ISO) at least at two infrared wavelengths, one close to similar to 100 mu m and the other at 200 mu m. We constructed FIR emission maps, determined dust temperatures, created extinction maps using Two-Micron All-Sky Survey data and calculated FIR emissivity for each of these clouds. We present the largest homogeneously reduced data base constructed so far for this purpose. During the data analysis, special care was taken on possible systematic errors. We find that FIR emissivity has a clear dependence on temperature. The emissivity is enhanced by a factor of usually less than 2 in the low dust temperature regime of 12 K <= T-d <= 14 K. This result suggests larger grain sizes in those regions. However, the emissivity increase of typically below 2 restricts the possible grain growth processes to ice mantle formation and coagulation of silicate grains, and excludes the coagulation of carbonaceous particles on the scales of the regions we investigated. In the temperature range 14K <= T-d <= 16 K, a systematic decrease of emissivity is observed with respect to the values of the diffuse interstellar matter. Possible scenarios for this behaviour are discussed in this paper.