Paper detail

Coagulation and Fragmentation in molecular clouds. II. The opacity of the dust aggregate size distribution

The dust size distribution in molecular clouds can be strongly affected by ice-mantle formation and (subsequent) grain coagulation. Following previous work where the dust size distribution has been calculated from a state-of-the art collision model for dust aggregates that involves both coagulation and fragmentation (Paper I), the corresponding opacities are presented in this study. The opacities are calculated by applying the effective medium theory assuming that the dust aggregates are a mix of 0.1μm silicate and graphite grains and vacuum. In particular, we explore how the coagulation affects the near-IR opacities and the opacity in the 9.7μm silicate feature. We find that as dust aggregates grow to μm-sizes both the near-IR color excess and the opacity in the 9.7 μm feature increases. Despite their coagulation, porous aggregates help to prolong the presence of the 9.7μm feature. We find that the ratio between the opacity in the silicate feature and the near-IR color excess becomes lower with respect to the ISM, in accordance with many observations of dark clouds. However, this trend is primarily a result of ice mantle formation and the mixed material composition of the aggregates, rather than being driven by coagulation. With stronger growth, when most of the dust mass resides in particles of size 10μm or larger, both the near-IR color excess and the 9.7μm silicate feature significantly diminish. Observations at additional wavelengths, in particular in the sub-mm range, are essential to provide quantitative constraints on the dust size distribution within dense cores. Our results indicate that the sub-mm index β will increase appreciably, if aggregates grow to ~100μm in size.