Mantle formation, coagulation, and the origin of cloud/core shine - I. Modelling dust scattering and absorption in the infrared

摘要

Context. The observed cloudshine and coreshine (C-shine) have been explained in terms of grain growth leading to enhanced scattering from clouds in the J , H , and K photometric bands and the Spitzer IRAC 3.6 and 4.5? μ m bands. Aims. Using our global dust-modelling approach THEMIS (The Heterogeneous dust Evolution Model at the IaS), we explore the effects of dust evolution in dense clouds, through aliphatic-rich carbonaceous mantle formation and grain-grain coagulation. Methods. We model the effects of wide band gap a-C:H mantle formation and the low-level aggregation of diffuse interstellar medium dust in the moderately-extinguished outer regions of molecular clouds. Results. The formation of wide band gap a-C:H mantles on amorphous silicate and amorphous carbon (a-C) grains leads to a decrease in their absorption cross-sections but no change in their scattering cross-sections at near-infrared wavelengths, resulting in higher albedos. Conclusions. The evolution of dust, with increasing density and extinction in the diffuse-to-dense molecular cloud transition, through mantle formation and grain aggregation, appears to be a likely explanation for the observed C-shine.