Mantle formation, coagulation, and the origin of cloud/core shine

Abstract

Context. Many dense interstellar clouds are observable in emission in the near-IR (J, H, and K photometric bands), commonly referred to as "Cloudshine", and in the mid-IR (Spitzer IRAC 3.6 and 4.5 μm bands), the so-called "Coreshine". These C-shine observations have usually been explained in terms of grain growth but no model has yet been able to self-consistently explain the dust spectral energy distribution from the near-IR to the submm. Aims. Our new core/mantle evolutionary dust model, The Heterogeneous dust Evolution Model at the IaS (THEMIS), has been shown to be valid in the far-IR and submm. We want to demonstrate its ability to reproduce the C-shine observations. Methods. Our starting point is a physically motivated core/mantle dust model. It consists of three dust populations: small polyaromatic-rich carbon grains, bigger core/mantle grains with mantles of aromatic-rich carbon, and cores made of either amorphous aliphatic-rich carbon or amorphous silicate. Then, we assume an evolutionary path where these grains, when entering denser regions, may first form a second aliphatic-rich carbon mantle (coagulation of small grains, accretion of carbon from the gas phase), second coagulate together to form large aggregates, and third accrete gas phase molecules coating them with an ice mantle. To compute the corresponding dust emission and scattering, we use a 3D Monte Carlo radiative transfer code. Results. We show that our global evolutionary dust modelling approach THEMIS allows us to reproduce C-shine observations towards dense starless clouds. Dust scattering and emission is most sensitive to the cloud central density and to the steepness of the cloud density profile. Varying these two parameters leads to changes that are stronger in the near-IR, in both the C-shine intensity and profile. Conclusions. With a combination of aliphatic-rich mantle formation and low-level coagulation into aggregates, we can selfconsistently explain the observed C-shine and far-IR/submm emission towards dense starless clouds.

The global dust modelling framework THEMIS

“…The accretion of a-C:H mantles in denser regions is also consistent with both cloud-and core-shine (C-shine) observations Ysard et al 2016). Figure 1 shows a schematic view of the above-described dust evolution stages and the associated grain structures and compositions.…”

“…As per all dust models, variations in the grain size distribution can be used to explain the observed variations in dust emission and extinction. However, a major advantage of the core-mantle models is that they can, in addition and quite naturally, account for dust variations through environmentally-driven changes in the carbonaceous mantle composition and/or its depth (e.g., Jones et al 1987Jones et al , 1990Jones et al , 2013Jones et al , 2016Köhler et al 2014;Duley et al 1989;Li & Greenberg 1997;Cecchi-Pestellini et al 2010;Zonca et al 2011;Cecchi-Pestellini et al 2014a,b;Ysard et al 2015Ysard et al , 2016.…”

“…The result of this is the THEMIS dust modelling framework, which provides a coremantle model for dust in the diffuse ISM and the evolution of the dust properties in response to their local environment Köhler et al 2014;Bocchio et al 2014;Ysard et al 2015Ysard et al , 2016. The underlying principle of the THEMIS modelling framework is the supposition that interstellar dust is not the same everywhere but that it evolves within a given region of the ISM as it reacts to and interacts with its local environment.…”

“…All of these processes directly affect the dust optical properties (i.e., the particle absorption and scattering cross-sections) as the composition, structure and shape of the grains evolve as they transit from one region to another. For example, in the tenuous ISM the outer carbonaceous layers of the grains, be they carbon grains or the mantles on other grains, will be H-poor and aromatic rich due to UV photolysis by stellar FUV/EUV photons (e.g., Hecht 1986;Sorrell 1990Sorrell , 1991Jones 2012aJones ,b,c, 2016aJones et al 2013) but in denser regions accreted hydrocarbon mantles are likely H-rich and aromatic poor Ysard et al 2016). …”

Mantle formation, coagulation, and the origin of cloud/core shine

“…The combination of mantle formation and minimal coagulation lead to rather unusual optical properties. In the companion paper (Ysard et al 2016, Paper II) we use a radiative transfer model to explore the effects of this dust evolution scenario to explain the origin of C-shine.…”