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

TL;DR

This paper models dust evolution in molecular clouds, showing that mantle formation and grain coagulation increase dust albedo, providing a plausible explanation for observed cloudshine and coreshine phenomena.

Contribution

It introduces a detailed dust evolution model that links mantle formation and grain coagulation to IR scattering properties in dense clouds.

Findings

Formation of wide band gap a-C:H mantles increases dust albedo.

Mantle formation decreases absorption cross-sections without affecting scattering.

Dust evolution explains the observed cloudshine and coreshine phenomena.

Abstract

Context. The observed cloudshine and coreshine (C-shine) have been explained in terms of grain growth leading to enhanced scatter- ing from clouds in the J, H and K photometric bands and the Spitzer IRAC 3.6 and 4.5 {\mu}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-IR 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.