SUMMARY

1) Overview

2) Solid state physics of carbon

3) Diffuse-ISM dust (core/mantle grains, CM)

4) Molecular cloud dust
  a) Core/mantle/mantle grains: CMM
  b) Aggregate/mantle/mantle(/ice): AMM(I)

5) Applications
  a) Dust scattering: coreshine/cloudshine
  b) Shocks
  c) Grain charging


1) Overview


THEMIS is a global approach to interstellar dust modelling anchored to the laboratory-measured properties of interstellar dust analogue materials: namely, amorphous hydrocarbons and amorphous silicates. A key element of the THEMIS framework is a self-consistent treatment of the evolution of the dust material properties (size distribution, chemical composition and structure) as they react to and adjust to the local radiation field intensity and hardness and to the gas density and dynamics. The principal tenet of the THEMIS approach is that interstellar dust is heterogeneous as a result of grain surface photo-processing, mantle accretion and coagulation. The general consequences of the THEMIS approach are: that interstellar dust is not the same everywhere, that interstellar gas and dust chemistry are intertwined and that understanding dust can only be advanced through an understanding of dust evolution.

TABLE - Observational constraints on interstellar dust.
ObservationTHEMIS
Extinction (EUV-IR)Coherent with the observations
Albedo & scatteringLarge CM grains (a ∼ 150 nm)
UV extinction shapea-C and CM grains
UV bump width & positiona-C nano-particles (peak & width size-dependent)
Extinction correlationsCoherent with the observations
Polarisation-to-extinctionIn progress
Absorption & emission bandsCoherent with the observations
Red & blue luminescenceConsistent with a-C(:H) materials
Full FUV-mm SEDCoherent with the observations
Dust evolutionConsistent with diffuse to dense cloud transition
Elemental depletionsModel satisfies abundances & depletions
X-ray absorption dataCoherent with silicates & Fe,FeS in dust
Known dust sourcesConsistent with star dust & ISM-formed dust
Dust re-formation in the ISMConsistent with carbon (re-)accretion in ISM
Physical reasonablenessModel based on real dust analogue materials
Material optical propertiesBased on laboratory data where possible

TABLE - Dust material constraints, Y ( O ) [ x ] = major ( minor ) [ no ] contribution.
GrainRadius (nm)Eg(bulk) [eV]Eg(eff) [eV]FUVUV bumpvis./NIREmission bands3.4 μmIR-mm
a-C<10.1>0.8YYxYxx
a-C1-50.10.1-0.8YYOOxx
a-C5-200.10.1OOOxxO
a-C:H/a-C1602.5/0.12.5/0.1xxOxYY
a-Sil/a-C140∼ 8/0.1∼ 8/0.1xxYxxY


2) Solid state physics of carbon


At the core of any model for interstellar dust are the optical constants (complex refractive indices) for the dust analogue materials. These are used to derive the required dust extinction, absorption, scattering and polarisation cross-sections, which are a function of the grain radius, material, structure and the incident wavelength. Amorphous and crystalline silicate material optical constants are well-determined and available for a wide range of interstellar silicate analogues. However, finding suitably-definitive laboratory data sets for physically-reasonable analogues of interstellar carbonaceous materials is difficult because of the much wider range in the physical and optical properties of these complex, semiconducting materials. In fact, hydrogenated amorphous carbons, a-C(:H), span the entire compositional range between the diamond-like solids and graphite-like solids (CHn, where n ∼ 0) and organic polymers (CHn, where n ∼ 2). However, it is the intervening amorphous hydrocarbon materials that are the most interesting (CHn, where 0 < n < 2).

There are numerous laboratory-measured and post-processed data for a-C(:H) materials but they are far from homogeneous in terms of their composition, structure, synthesis methods and wavelength coverage. Hence, we currently rely upon a recently-developed set of optical properties for a-C(:H). The crucial parameters that define the short-range order of a-C(:H) materials are the hydrogen content, the carbon atom bonding (sp3/sp2 ratio) and, in particular, the way in which the carbon atoms cluster into sp3 aliphatic and sp2 olefinic and aromatic domains (Jones 2012a). These sub-structures are connected into a contiguous network that exhibits no long-range order but where short-range order is imposed by the carbon bonding within the sub-structures.

The a-C(:H) optical properties used within THEMIS were built ground-up using extended random covalent network (eRCN) and Defective Graphite (DG) models (Jones 2012a). These were used to construct a solid-state framework method to derive their composition and structure and thence their complex indices of refraction as a function of the material Tauc band gap, Eg, and particle size (Jones 2012b; Jones 2012c). The effects of particle size on the derived complex indices of refraction are absolutely critical and impose two crucial constraints: 1) a limitation the maximum size of the band gap-determining aromatic domains and 2) the necessary passivation of the particle surface with hydrogen (Jones 2012c).

The adopted protocol for determining the optEC(s)(a) data is to first determine the material structure and from this the imaginary part of the complex refractive index for a-C(:H) materials from 50 eV to cm wavelengths. This is achieved using the well-determined solid state physics of a-C(:H) and the measured cross-sections for the constituent carbon-carbon and carbon-hydrogen bonds within the structure. The real part of the complex index of refraction is then calculated using the Kramers-Kronig relations (Jones 2012b). The resulting optEC(s)(a) data provide continuous coverage over the band gap range ∼ 0.1 to 2.7 eV and size range from sub-nm to bulk materials (Jones 2012c). With these optEC(s)(a) data it is then possible to predict how the optical properties of an a-C(:H) particle, of a given size and band gap, will behave in the interstellar medium and how these properties will evolve in response to, principally, the local interstellar radiation field (Jones 2012c; Jones et al. 2013).



3) Diffuse-ISM dust (CM)


In the ISM it is hard to see how the silicate and carbonaceous dust populations could be completely segregated because mixing, even at some minor (contaminant) level, must occur. Thus, the amorphous silicates must be mixed, to some degree, with a carbonaceous dust component. Our assumed dust properties for the Galactic diffuse ISM are the following (Jones et al. 2013):

  • a-C(:H) grains with size-dependent properties: This population represents a fundamental continuity in composition and size distribution, which is qualitatively consistent with a-C:H dust that has been exposed to the "equilibriating effects" of the local ISRF for at least 106 yr, i.e., long enough for any sub-nm particles to be aromatised to a-C (Jones 2012b, Jones et al. 2014). Any particles larger than a few tens of nm in radius will be incompletely aromatised or will consist of an aliphatic-rich core surrounded by a more absorbing, aromatised mantle layer 20 nm thick. The optical properties are computed using the optEC(s)(a) data described in the previous section (Jones 2012a, Jones 2012b, Jones 2012c).
  • a-Sil grainsħa-C(:H) mantles: The interpretation of observations made by the Planck satellite, coupled with Herschel and IRAS data, shows that the observed dust SED in the diffuse ISM can be empirically, and extremely well, fit with a single temperature blackbody. This strongly suggests that the dust emission at long wavelengths is dominated by emission from a single dust population that mixes, predominantly, amorphous silicate (a-Sil) materials with a carbonaceous (a-C or a-C:H) component in the form of mantles or accreted small grains (e.g., Koehler et al. 2011, 2012). The surface a-C(:H), whether accreted as a mantle or formed by the coagulation of small a-C particles, must be ∼ 20 nm thick (the depth at which the optical depth for the FUV processing photons is unity) otherwise it will be incompletely photolysedto a-C (see Jones 2012b). An a-C:H mantle would be traceable through the polarisation of the aliphatic C-H 3.4 μm absorption band, which will follow that of the host a-Sil. The presence of an a-C:H mantle on a-Sil dust is inconsistent with observations. In our model we assume 5 nm thick a-C mantles (Eg=0.1 eV) on the a-Sil grains. For the amorphous silicate optical properties we use those for a mixture of amorphous silicates with the normative chemical compositions of olivine and pyroxene (with half of the total mass of silicate grains in each type). Iron and sulphur are incorporated into the silicate cores in the form of metallic nano-inclusions of Fe and FeS. They represent 10% of the total core volume, of which 30% is FeS and 70% is pure Fe (see Koehler et al. 2014 for the silicate optical properties).



4) Molecular cloud dust: accretion and coagulation


Variations in the dust spectral energy distribution (SED) from the diffuse ISM to dense molecular clouds have clearly been observed in the Milky Way. These variations encompass variations in the dust temperature, opacity, and spectral index as measured from dust thermal emission in the far-IR and submm, as well as variations in the mid- to far-IR intensity ratio. Apart from variations in the IR to submm thermal emission, dust SED variations were also shown long ago in scattered light at shorter wavelengths. First observed in the visible dust scattering was then measured in the near-IR and in the mid-IR towards dense clouds. All these observations are usually interpreted in terms of grain evolution through grain growth and ice mantle formation with increasing local density.

The ability of the THEMIS approach to reproduce the variations in the dust spectral energy distribution in terms of temperature, far-IR/submm opacity and spectral index, observed in both diffuse (Ysard et al. 2015; Fanciullo et al. 2015) and dense ISM (Koehler et al. 2015), was demonstrated together with an overall agreement with extinction measurements.


a) Core/mantle/mantle grains: CMM

In a first step of dust evolution, gas species can accrete on the surface of the dust grains where they form mantles. We assume that these mantles are likely to have amorphous carbon structure (see Jones et al. 2014). In diffuse regions, where radiation is very strong, H atoms can be kicked out of the carbonaceous material structure, so that the amorphous carbon is rather aromatic-rich (H-poor). In denser regions, however, where the radiation is weaker, H atoms may remain in the amorphous carbon structure and the mantles are rather aliphatic-rich (H-rich). Due to the formation of these "second" mantles, the material composition of the dust grains changes (CM → CMM). This has an influence on their optical properties, i.e. how the particles absorb, emit and scatter light. In denser regions of the ISM, where we assume the formation of H-rich amorphous carbon mantles, a steepening of the slope at long wavelength (or an increase in the spectral index β) is found, which agrees with the observations. These changes also lead to higher albedo dust in agreement with the observational results for dark clouds.


b) Aggregate/mantle/mantle(/ice): AMM(I)

Due to coagulation processes, or simply hit and stick processes, aggregates can form in denser regions of the ISM. When density increases, it is likely that CMM grains collide and if their relative velocities are low, these colliding grains stick together and form aggregates. Following coagulation processes, not only do the physical properties of the dust grains change, but also their optical properties (CMM → AMM). Our calculations show that the grain temperature decreases and that the far-IR/submm emissivity increases for the aggregates compared to the single grains (Koehler et al. 2015). These aggregates are also able to scatter light efficiently from the visible to the mid-IR (Ysard et al. 2016).

In these denser regions, where the radiation is attenuated and thus weaker than in the diffuse ISM, ice mantles may also form on the aggregate surface (AMM → AMMI). Due to the ice-mantle formation, grains grow further in size which results in a further increase in emissivity and decrease in temperature. Although the material composition of the grains changes, it has no influence on the spectral index β.



5) Applications


a) Dust scattering: coreshine/cloudshine

Dust scattering is a strong function of the grain size distribution but it is also a function of the particle optical properties, which are determined by its chemical composition and structure (core/mantle, core/mantle/mantle, aggregation, ... ). For example, the observed enhanced scattering from clouds in the J, H, and K photometric bands and the Spitzer IRAC 3.6 and 4.5 μm bands, known as cloudshine and coreshine, respectively, or collectively as C-shine, are thought to arise from grain growth effects. The THEMIS approach self-consistently explains C-shine in terms of wide band gap a-C:H mantle formation and grain-grain coagulation in the moderately-extinguished outer regions of molecular clouds. The formation of a-C:H mantles on all grains decreases their NIR absorption but does not change their scattering properties, which leads to decreased dust cross-sections and higher albedos (Jones et al. 2016; Ysard et al. 2016).


b) Shocks

There is a long-standing and large discrepancy between the timescale for dust formation around evolved stars and the rapid dust destruction timescale in interstellar shocks. Using the THEMIS dust model, Bocchio et al. (2014) explore the dust destruction efficiency in supernova triggered shock waves, estimate the dust lifetime, and calculate the emission and extinction from shocked dust. The obtained destruction lifetime is short compared to the dust injection timescale, therefore implying the re-formation of dust in the dense regions.

Dust destruction (percentage) for carbonaceous grains (blue line) and silicate grains (orange line) as a function of the shock velocity. Dotted and dashed lines, respectively, represent the sputtering and vapourisation contributions to the destruction.

c) Grain charging

Grain charging results from the competition between photon absorption, proton and electron sticking. A sufficiently high charge may result in significant fragmentation due to Coulomb explosions, limiting the lifetime of small grains. Also, these processes regulate the heating and ionisation state of the surrounding gas. Within the THEMIS framework, Bocchio et al. (2016) explore nanoparticle grain charging and the associated gas heating and cooling. Small a-C(:H) grains are quickly processed in Hii regions, while in the diffuse ISM they show evolution, requiring a replenishment cascade process from the fragmentation of larger grains. Also, the THEMIS model, without any tweaking can reproduce within a factor of 2 the net gas heating rate in the diffuse ISM. Small adjustments to the size distribution can account for this discrepancy.


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