---
title: THEMIS Dust Evolution Framework
url: https://www.emergentmind.com/topics/themis-framework
type: topic
---

# THEMIS Dust Evolution Framework

THEMIS (The Heterogeneous dust Evolution Model for Interstellar Solids) is a comprehensive, physically-motivated framework for modeling the evolution, composition, and observational signatures of interstellar dust in a wide range of astrophysical environments. THEMIS supersedes empirical or ad hoc models by integrating laboratory-derived optical constants, well-constrained dust mineralogy, explicit treatment of evolutionary processes, and predictive calculations directly comparable with multiwavelength astronomical data. The framework is designed for extensibility and self-consistency, offering tools for modeling extinction, emission, and polarization from the diffuse interstellar medium (ISM) through dense molecular clouds into star-forming regions.

## 1. Structural Components of the THEMIS Dust Model

The THEMIS core is built on a physically justified inventory of dust populations distinguished by composition, structure, and grain size distribution:

- **Amorphous Silicates (a-SilFe,FeS):** Grains comprise a 1:1 mass mixture of olivine-type and pyroxene-type amorphous silicates with $7\%$ metallic Fe and $3\%$ FeS nano-inclusions (by volume). This composition is motivated by both laboratory data and depletion patterns in the ISM.
- **Hydrogenated Amorphous Carbons (a-C(:H)):** THEMIS features an ensemble of hydrogenated amorphous carbon materials spanning the full range from H-rich (aliphatic; wide band gap) to H-poor (aromatic; narrow band gap), with their optical constants derived from extended Random Covalent Network (eRCN/optEC(s)(a)) models constrained by laboratory measurements.
- **Core–Mantle Structures:** Large silicate grains are modeled as cores with an amorphous carbon mantle ($5$–$10\;\mathrm{nm}$), while large a-C(:H) grains carry an UV-processed outer a-C mantle ($7.5$–$20\;\mathrm{nm}$). A distinct nanoparticle population of a-C grains ($a \lesssim 20 \;\mathrm{nm}$) is also treated.

Grain size distributions are characterized by:
- **Nano-particles:** $n(a) \propto a^{-\alpha}$, $\alpha \approx 5$, for $a$ up to $20\;\mathrm{nm}$.
- **Large grains:** Log-normal forms, e.g., $n(a) = C \exp[-(\ln(a/a_0))^2 / (2\sigma^2)]$ with $a_0 \approx 140$–$160\;\mathrm{nm}$, $\sigma \approx 0.4$–$0.5$ dex.

Optical properties are determined using:
- **Complex refractive indices** $m(\phi, \lambda) = n(\lambda) + i k(\lambda)$, sourced from laboratory or eRCN models for each material type.
- **Cross-section computation:** $C_i(a, \lambda) = \pi a^2 Q_i(a, \lambda)$, with $Q_i$ (for extinction, absorption, scattering) from Mie theory (homogeneous/coated spheres) or DDSCAT (core–mantle/aggregates).

## 2. Physical Processes and Environmental Dependence

THEMIS incorporates essential dust processing mechanisms as a function of environmental parameters:

- **Diffuse ISM:** The default model reproduces observed extinction curves UV to millimeter, including the 217\,nm bump, FIR/submm emission, and low albedo. Small a-C grains dominate FUV extinction and mid-IR emission; large CM grains dominate in optical/NIR extinction and FIR emission.
- **Dense Regions:** As density increases:
  - *Accretion:* Gas-phase C (and H) forms new a-C:H mantles (CMM) on large grains; nano-particles aggregate onto larger grains, reducing small-grain numbers.
  - *Coagulation:* CMM grains form aggregates (AMM), gain ice mantles (AMMI) at $T \lesssim 20$ K and $A_V \gtrsim 3$. This boosts FIR emissivity by factors of $2$–$4$, decreases typical dust temperatures, steepens the sub-mm spectral index, and enhances scattered light (“cloud-shine”/“core-shine”).
- **PDRs and HII Regions:** UV photolysis aromatizes a-C(:H), eroding nanoparticles, altering optical properties, and releasing small hydrocarbons and H$_2$. The photoelectric effect on a-C(:H) nano-grains affects gas heating and charge states.

THEMIS accounts for the full lifecycle hysteresis: destruction and renewal routes in harsh environments are not the inverse of accretion/coagulation in shielded regions.

## 3. Model Implementation, Computational Methods, and Output

THEMIS is realized via the DustEM modeling tool, which orchestrates all subcomponents:

- **Materials library:** Manages complex refractive indices across wavelength grids.
- **Size-distribution/aggregation modules:** Parameterizes and evolves $n(a)$ for each population; supports accretion/coagulation prescriptions based on environmental input.
- **Optical property solvers:** Implements Mie theory for spherical/coated grain populations and DDSCAT for aggregates or non-spherical grains. Core–mantle structure, nano-inclusions, and shape/porosity variations are explicitly handled.
- **Radiative solver:** Computes the thermal balance and resulting spectral energy distributions (SEDs), with stochastic heating for small grains and equilibrium temperatures for large grains.

**Input requirements:**
- Interstellar radiation field (Mathis-MMP83 or user-specified).
- Gas density $n_H$, temperature, radiation field intensity scaling $\chi$.
- Elemental abundances: C, O, Si, Mg, Fe, S.

**Output diagnostics:**
- Extinction curves $A(\lambda)/N_H$, albedo, phase function.
- Emission SEDs ($I_\nu(\lambda)$) across UV-cm range.
- Dust temperature distributions $T(a)$, charge state distributions.

## 4. Model Validation, Observational Comparison, and Utility

THEMIS validation is based on a range of multiwavelength diagnostics:

- **Diffuse ISM:** Matches Planck-IRAS correlations, FIR/sub-mm SEDs [Ysard et al. 2015, 2016], and the mean Milky Way extinction curve ($R_V=3.1$), including the UV slope and 217 nm bump [Jones et al. 2013].
- **Dense clouds:** Accurately reproduces the transition in SEDs from $3\,\mu$m to $500\,\mu$m as well as scattered light phenomena (“coreshine”).
- **Comparative strengths:** Unlike MRN [Draine & Li] or PAH+silicate models (ZA11/C11), THEMIS uses laboratory-derived optical constants and realistic mixed chemistry rather than empirical “astronomical” values, and more faithfully tracks trends across extinction, emission, polarization, and depletion in varied environments.

| Environment         | Diagnostic              | THEMIS performance/claim                     |
|---------------------|------------------------|----------------------------------------------|
| Diffuse ISM         | $A(\lambda)/N_H$, SED  | Matches $<10\%$ up to mm; UV bump accurate  |
| Dense cloud         | SED, coreshine         | Accurately reproduces SED flow, scattered light |
| PDR/HII             | Aromatic features, H$_2$| Predicts aromatization, hydrocarbon release    |

## 5. Ongoing Developments and Future Directions

THEMIS continues to expand along several axes:

- **DustPedia integration:** THEMIS serves as the reference dust module in the DustPedia project, enhancing cross-comparison of dust modeling and galaxy SED databases.
- **Polarization and alignment:** Advances in treating grain alignment/polarization for core–mantle and aggregate populations are underway.
- **Improved long-wavelength optical constants:** Laboratory programs are providing improved $n,k$ data at $\lambda \gtrsim 60\,\mu$m.
- **Time-dependent and coupled chemistry:** Extension to dynamic accretion/coagulation and explicit coupling to gas-phase chemistry.
- **Expanding into PDRs, HII regions, and star-formation contexts:** Current developments target the aromatization pathways, destruction and renewal in harsh UV environments, and integration with star-formation models.

## 6. Resources, Adoption, and Community Use

- The THEMIS model, documentation, and DustEM input files are publicly accessible at [THEMIS](http://www.ias.u-psud.fr/themis/) and [DustEM](http://www.ias.u-psud.fr/DUSTEM/).
- Researchers may download the complete input suite, specify custom environmental parameters, and compute self-consistent extinction/emission properties over the UV–cm spectral domain.
- THEMIS is widely employed in ISM studies and has been foundational for Planck and Herschel dust analyses, with continued validation against new observations and laboratory constraints.

Key references: Jones et al. 2013, 2014, Köhler et al. 2015, Ysard et al. 2015, 2016, Jones 2012a, 2012b [1703.00775].

Source: https://www.emergentmind.com/topics/themis-framework