Solid carbon in space plays a major role in planetary formation, as it efficiently absorbs ultraviolet to infrared light, influencing the structure and thermochemical evolution of proto-planetary disks (PDDs), the cradles for future planets. In our Solar System, the inner regions (0.5-5 AU) show a strong depletion of solid carbon compared to the outer regions (> 40 AU, Dartois et al. 2018). Understanding whether this carbon/silicon gradient is universal is paramount to determine whether rocky exoplanets tend to be silicates-rich rather than carbon-rich,  which has major consequences for the habitability of these planets because of a much faster cooling with consequences of an earlier continents drift stopping (Hakim et al. 2019).

The first goal of the PhD is to characterize the spatial distribution and abundance of solid carbon in PPDs, a topic very little explored to date (only partially in two disks, Habart et al. 2021, Devinat et al. 2022). Thermal IR observations are particularly suitable because of the spectral signatures of hydrocarbons and carbonaceous grains in this range. The most sensitive, highest spectral resolution IR spectral imaging data ever taken have been recently captured by JWST and allow for an unprecedentedly detailed view of the carbon emission features (e.g. Chown et al. 2024). But with a spatial resolution limited to 0.2’’ at 5 μm, JWST cannot angularly resolve their emission at the few AU scale. The VLTI interferometric instrument, MATISSE, thanks to its spectral range (2.8-13 μm) and very high angular resolution (1 AU), makes it possible for the first time to probe the internal regions (<10 AU).

The PhD will analyse JWST and MATISSE observations obtained on a selected sample of ten young stellar objects covering a range of stellar luminosities and masses, with disks having varying morphologies, gaps, and cavities of different sizes, and carbon and silicate emission features. Modeling will be performed using state-of-the-art dust models (THEMIS, “The Heterogeneous dust Evolution Model for Interstellar Solids”, Jones et al. 2013, Ysard et al. 2024). She/he will benefit of the expertise of the team at IAS for dust modeling and from the “Centre d’expertise JWST/MIRI”, hosted at IAS and CEA. She/he will also have the technical support of the Integrated data and Operation Center (IDOC) at IAS for JWST data access and high level processing, as well as, high quality of reduced observables from MATISSE at OCA. The PhD contract is particularly crucial to ensure the analysis and modelisation of the observed carbon emission features and the publication of initial results that will serve as proof of concept to test the universality of the C/Si gradient in young extrasolar systems.

The doctoral contract will be rolled out in three phases: 1) Computation of THEMIS dust modeling grids. Unlike other dust models, THEMIS calculates grain structure and optical properties directly from their composition and size using solid-state physics theory, which is crucial since the IR spectral features of carbonaceous grains are highly sensitive to these parameters. The PhD will explore variations in properties on the entire grain size distribution, from nanometer-scale to bulk material, with changes in hydrogenation state (ranging from aromatic to aliphatic-rich materials). 2) IR data collection and fitting. The PhD student will collect the JWST and VLTI spectroscopic data with clear carbon spectral features. The PhD student will adjust the model parameters of carbonaceous dust properties (size, composition) to reproduce the observations, taking into account the spatial distribution of the continuum, the aromatic and aliphatic bands, and the integrated IR spectral energy distribution. He/she will use THEMIS coupled with a transfer code thus ensuring consistency with the dust location in radiative transfer models and the local radiation field. 3) The PhD will measure the relative abundance of carbon and silicate dust as a function of the distance to the star and discuss  the results in the context of Solar System C/Si measured gradient. Deliverables and timeline for the publications: a paper on the dust modeling of observations in an iconic PPD (HD169142) (12 months), a JWST proposal (Oct. 2027, Oct. 2028) to survey disks with NIRSpec to complement MIRI (2 months), a paper on the spatial distribution and abundance of solid carbon in a sample of PPDs (18 months), a THEMIS models grid to be made publicly available to the community (4 months).

This PhD will be partly funded by the recently accepted ANR “Carbon in Young PRE-planetary SystemS”. The doctoral project is critical to secure French leadership in the emerging domain of inner PDDs characterisation. The PhD student will gain expertise in modern observational techniques and in numerical simulations. The knowledge she/he will acquire will be paramount for the scientific exploitation of IR space instrumentand synergies with ground based telescopes.

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For more Information about the topics and the co-financial partner (found by the lab!); contact Directeur de thèse - emilie.habart@universite-paris-saclay.fr

Then, prepare a resume, a recent transcript and a reference letter from your M2 supervisor/ engineering school director and you will be ready to apply online  before March 13th, 2026 Midnight Paris time!