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

Abstract

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 μm 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.

The global dust modelling framework THEMIS

“…The extended model is consistent with the dust scattering properties needed to explain observations of cloud-shine and core-shine (C-shine, Sect. 2.3 and Jones et al 2016;Ysard et al 2016). Thus, the THEMIS modelling approach encompasses, and indeed requires, rather wide variations in the composition and size distribution from region to region within the ISM, which is supported by observational studies in addition to those of Ysard et al (2015) and Fanciullo et al (2015) mentioned above.…”

“…Given that the dust evolution observed by Reach et al (2015) cannot be driven by silicate-forming elemental depletion variations, because these elements (e.g., Si, Mg and Fe) are maximally-depleted, this leaves the vestige carbon in the gas phase as the most likely accreting and dust emissivity-enhancing agent (e.g., Jones et al 2013Jones et al , 2014Jones et al , 2016Ysard et al 2015Ysard et al , 2016. That significant dust variations are related to variations in the carbon depletion in the ISM are supported by observational studies (Parvathi et al 2012;Mishra & Li 2015).…”

“…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.…”

“…Indeed, variations in the dust properties from one region to another have long been interpreted as due to dust evolution (e.g., Lefevre 1974;Jura 1980;Aannestad & Greenberg 1983;Whittet et al 1992Whittet et al , 2001Ossenkopf 1993;Stepnik et al 2003). For example, photon, ion and electron irradiation can induce changes in the dust chemical composition and structure (e.g., Demyk et al 2001Demyk et al , 2004, hydrogenation and accretion can drive changes in the grain chemical composition (e.g., Hecht 1986;Sorrell 1990Sorrell , 1991Mennella 2008Mennella , 2010Jones 2012a) and accretion/coagulation will change the grain structure (e.g., Köhler et al 2011Köhler et al , 2012Köhler et al , 2015Jones et al 2014Jones et al , 2016. 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.…”

The dust content of galaxies from z = 0 to z = 9

“…More complex dust models have been developed to account for the distribution of radiation field intensities as well as to propose a more physical treatment of the dust composition, using a mixture of amorphous silicate and carbonaceous grains. Such models include, for example, the Draine et al (2007), Galliano et al (2011), GRASIL (Silva et al 1998), CIGALE (Burgarella, Buat & Iglesias-Páramo 2005), and THEMIS (Jones et al 2016) models. In this paper, we will also make use of dust mass estimates derived using these more complex models, such as in Sandstrom et al (2013), Rémy-Ruyer et al (2014b), Ciesla et al (2014) and Santini et al (2014).…”