The optical properties of dust: the effects of composition, size, and structure

Abstract

Context. Dust grains are determinant for setting the chemical, physical, dynamical, and radiative properties of all the media in which they are present. Their influence depends on the grain composition, size, and geometrical structure which vary throughout the lifecycle of dust. Particularly grain growth arises in dense molecular clouds and protoplanetary disks as traced by an enhancement of the dust far-IR emissivity and by the effects of cloudshine and coreshine. Aims. Our aim is to investigate the imprint of the grain characteristics on the dust unpolarised optical properties from the visible to the far-IR wavelengths for isolated grains as well as for aggregates. Methods. Using optical constants for both carbonaceous and silicate materials, we derive the absorption and scattering efficiencies, the asymmetry factor of the phase function, the single scattering albedo, and the mass opacity for isolated grains and aggregates, using either the Mie theory or the Discrete Dipole Approximation (DDA). We investigate the effects of the size, porosity and shape of the grains, and of the monomers constituting the aggregates, on the optical properties. Besides, for aggregates we study the influence of the number of monomers and of mixing monomer sizes. Results. Grain structure changes result in optical property variations at all wavelengths. Porosity, grain elongation, as well as aggregation all produce an increase in the far-IR opacity. The spectral dependence of this increase depends on the nature of the material composing the grain: it is independent of the wavelength for insulators but not for conductors. In the case of aggregates, the far-IR increase does not depend on the monomer size and saturates for aggregates containing 6 or more monomers. In the visible and near-IR, the aggregate behaviour is reminiscent of a compact sphere of the same mass whereas at longer wavelengths, it is closer to the effect of porosity. Finally, for silicates, the mid-IR spectral feature at 18 µm is more sensitive to the details of the grain structure than the 10 µm feature. Conclusions. Dust optical properties, from the visible to the far-IR, are highly dependent upon the grain composition, size, and structure. This study provides a basis for understanding the range of variations achievable as a result of varying the grain characteristics. It emphasises the importance of considering the detailed grain structure in determining the dust optical properties and of using exact methods because approximate methods are unable to reproduce the entire range of the observed variations at all wavelengths.

precision: a fast python pipeline for high-contrast imaging – application to SPHERE observations of the red supergiant VX Sagitariae

“…The asymmetry of the emission is suggestive of the in- ner region of a disc or torus seen nearly face on, consistent with the Northern sector of the outflow being closer to the observer and hence producing more flux through the difference in scattering phase function at different angles (e.g. Ysard et al 2018), although a spherical shell cannot be excluded on the basis of this data alone. Alternatively, the images may imply that there is significant clumpy or filamentary structure to the extended component, although additional, deeper observations would be required to determine whether these constitute discrete components in the ejecta or are artefacts.…”

The first maps of κd – the dust mass absorption coefficient – in nearby galaxies, with DustPedia

“…However, if all other parameters in Equation 3 are held fixed, higher metallicity (therefore higher fZ ) leads to lower values of κ500, meaning the trend for M 74 in Figure 16 is being driven by the data in spite of this. Greater ISM metallicity will lead to increased grain growth (Dwek 1998;Zhukovska 2014;Galliano et al 2018), and larger grains should give rise to larger values of κ d (Li 2005;Köhler et al 2015;Ysard et al 2018).…”

“…where κ λ is the value of κ d at a particular wavelength λ, κ0 is the value of κ d at a reference wavelength λ0, and β is the dust emissivity spectral index. Laboratory analysis of dust analogues and chemical modelling suggest that this relation is reliable in the wavelength range 150 λ 1000 µm; at wavelengths shorter than this the variation of κ d with wavelength becomes much more complex, whilst at longer wavelengths the behaviour of κ d is less clear, with some evidence of an upturn (Demyk et al 2017a,b;Ysard et al 2018). Figure 1 compiles a wide range of κ d values that have been reported in the literature (all have been converted to a reference wavelength of 500 µm as per Equation 2; we only plot values for which the original quoted reference wavelength was in the reliable 150-1000 µm range).…”

From grains to pebbles: the influence of size distribution and chemical composition on dust emission properties

“…Mishchenko et al (2016a,b) performed exhaustive studies of the applicability of the Maxwell Garnett mix-1 See http://www.ias.u-psud.fr/themis/. 2 For the full details of the model see: Jones (2012a,b,c); Jones et al (2013); Köhler et al (2014Köhler et al ( , 2015; Jones et al (2016); Ysard et al (2016Ysard et al ( , 2018 ing rule to heterogeneous particles. These latter authors showed that this rule can provide accurate estimates of the scattering matrix and absorption cross-section of heterogeneous grains at short wavelengths (typically up to the visible for a 0.1 µm grain and to the mid-infrared (MIR) for a 10 µm grain) if two criteria are met: both the size parameter of the inclusions and the refractive index contrast between the host material and the inclusions have to be small.…”

“…After exploring the effect of complex grain structures with a single chemical composition in Ysard et al (2018), the intent of this study is to investigate the effect of complex chemical compositions for isolated spherical grains up to very large sizes. This rather simple approach may not be too unrealistic in the case of very dense environments where grain-grain compaction and gas compression may be efficient processes (e.g.…”