THEMIS framework

THEMIS is a cosmic dust modelling framework within which the dust structure, composition and evolution are determined by the local physical conditions, i.e., density, radiation field, etc. (Jones et al. 2013; Koehler et al. 2014; Jones et al. 2014; Koehler et al. 2015; Ysard et al. 2015). The adopted approach is a significant departure from previous dust models because it takes a coherent and self-consistent view of the evolution of the dust constituents and how they interact with each other and with the gas.

THEMIS is built upon the foundations of the laboratory-measured properties of physically-reasonable interstellar dust analogue materials, namely, the family of hydrogenated amorphous carbon materials, from H-poor ( a-C ) to H-rich ( a-C:H ), collectively a-C(:H), and also amorphous olivine-type and pyroxene-type silicates with iron and iron sulphide nano-inclusions, a-Sil(Fe,FeS).

The THEMIS dust model for the diffuse ISM (Jones et al. 2013; Koehler et al. 2014; Ysard et al. 2015) comprises the following grain and core/mantle (CM) grain structures and compositions:

This diffuse ISM dust model uses ~ 200 ppm of carbon (~ 140 ppm in the a-C nano-particles, ~ 50 nm in the a-C:H/a-C CM grains and ~ 10 ppm in the a-C mantles on the a-Sil(Fe,FeS) grains), ~ 110 ppm of oxygen, ~ 45 ppm of magnesium, ~ 32 ppm of silicon, ~ 19 ppm of iron and ~ 3 ppm of sulphur in FeS inclusions (Ysard et al. 2015). However, it should be noted that the carbon abundance is likely highly variable and sensitive to the local density and radiation field conditions (Jones et al. 2013; Jones et al. 2014; Jones et al. 2016; Ysard et al. 2016).

In the transition to molecular clouds the THEMIS diffuse ISM CM grains sequentially accrete carbon from the gas to form secondary a-C:H mantles (CMM grains), coagulate into aggregate particles (AMM) and finally accrete ice mantles (AMMI) in the densest regions (Jones et al. 2014; Koehler et al. 2015; Jones et al. 2016; Ysard et al. 2016).

In quiescent diffuse and dense clouds, where the dust resides for millions of years, the dust processing time-scales are short with respect to the ISM cycling time-scales and the dust is likely in an quasi-equilibrium state. However, within HII, photon-dominated and shocked regions the dynamical time-scales are significantly shorter and the dust is out of equilibrium with the local conditions and therefore undergoing significant spatial and temporal evolution (Bocchio et al. 2014).

The central tenets of the THEMIS modelling framework are therefore that interstellar dust evolves in response to its local environment, that it is not the same everywhere and that its constituents are mixed within heterogeneous core/mantle particles (CM, CMM) and aggregate grains (AMM, AMMI).

Related links

  • DustEM
  • DustEM is a numerical tool that computes the extinction, the emission, and the polarisation of interstellar dust grains heated by photons. It is written in Fortran 95 and includes the THEMIS diffuse ISM CM grains by default.

  • DustPedia
  • DustPedia was a project funded by the EU, from 2014 to 2018, under the heading 'Exploitation of space science and exploration data'. It was a collaboration of six European institutes with a primary goal of exploiting existing data in the Herschel Space Observatory and Planck Telescope databases. These data were combined with other available data from both ground based and space based telescopes to make the most extensive and intensive study of galaxies in the nearby Universe. The THEMIS team was partly funded by the DustPedia project.

  • Institut d'Astrophysique Spatiale (IAS)
  • IAS, located in Orsay (France), is the home institute of THEMIS.

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