Julien Drevon

Aims. We mapped the dust and molecular gas emission distributions in the vicinity (a few stellar radii) of the asymptotic giant branch (AGB) star EP Aquarii, which has a previously reported stellar companion at ∼50 au, to search for binary-induced asymmetries. Methods. We present an analysis of new high-angular-resolution (≳4 mas) images obtained with MATISSE (in the wavelength range ∼3–8 μm) as well as archival high-angular-resolution (≳20 mas) ALMA observations, including ¹² CO ( J = 2-1) line and continuum emission at ∼230 GHz. We describe the circumstellar morphology and kinematics of the molecular gas on these spatial scales. Results. The MATISSE images display a bright central source and two slightly extended emission features (‘blobs’) located on opposite sides of the AGB star (at radial distances of ∼4–10 au). These structures are consistent with previous mid-infrared interferometric observations, suggesting stability over decades. The central source is elongated along the direction defined by the blobs, in both the MATISSE and ALMA continuum images. The ALMA ¹² CO ( J = 2-1) maps display an asymmetric inner wind with a velocity gradient. Conclusions. The observations are consistent with an edge-on dusty equatorial overdensity (disc-like) and thus hint at the presence of a close (≲4 au) companion. The inferred symmetry axis is perpendicular to the CO velocity gradient and the orbital plane proposed for the distant (∼50 au) companion. EP Aquarii may represent the first misaligned hierarchical triple system with a central AGB star.

Avec l'avènement de l'interférométrie optique et la construction d'instruments interférométriques de plus en plus performants, notre vision des environnements d'étoiles évoluées au courant des dernières décennies a considérablement été modifiée. Il faut savoir qu'une étoile évoluée est la principale usine de fabrication des éléments lourds que nous retrouvons dans l'Univers. Au sein de cette thèse, j'appellerai élément lourd tous les éléments possédant un numéro atomique supérieur à 2, c'est à dire plus lourd que l'hydrogène et l'hélium. Sans étoiles évoluées, les matériaux nécessaires à la formation d'astéroïdes, de planètes et de tout ce qui nous entoure sur Terre incluant la vie elle-même basée sur le carbone, n'existeraient pas. Ainsi comprendre comment ces éléments se forment autour des étoiles, c'est aussi comprendre en quelque sorte notre propre origine, en d'autres termes, "nous sommes tous des poussières d'étoiles". Mon travail au sein de cette thèse a justement été d'étudier cette "poussière d'étoiles" afin de mieux contraindre et comprendre les milieux dans lesquels ces éléments se forment.

Aims. We investigate how the presence of a binary companion appears to affect dust and molecule formation in the circumstellar environment of a star on the asymptotic giant branch (AGB). Methods.L - and N -band observations obtained over the course of one month with the multi aperTure mid-infrared spectroScopic experiment instrument (MATISSE) at the very large telescope interferometer (VLTI) were used to constrain the distribution of dust and molecules in the close environment of π ¹ Gru through image reconstruction. The reconstructed images were fit to the interferometric observables, that is, the visibilities and closure phases, using the two Python-based tools Python for MiRA ( PYRA ) and mean astrophysical images with PYRA ( MYTHRA ) built around the multi-aperture image reconstruction algorithm ( MiRA ). Results. Our observations support (i) a wind Roche-lobe overflow (WRLOF) scenario, where mass transfer from the AGB star to its companion produces a possible circumcompanion disk in the L band around π ¹ Gru C, with a central cavity that likely traces dust sublimation and a circumcompanion envelope in the N band. (ii) A main-sequence nature for the companion: Because both Atacama large millimeter/submillimeter array (ALMA) and the VLTI/MATISSE N -band observations show emission, we favor a thermal infrared emission from the main-sequence star over free-free emission from a with dwarf companion. Finally, (iii) a plume-like structure extending from π ¹ Gru C, likely marking the onset of the spiral observed at larger scales. Together, this provides direct evidence that links small-scale mass transfer and disk formation through WRLOF to the global circumstellar morphology. Conclusions. These results highlight the atmospheric deformation induced by the companion, reveal ongoing mass transfer between the evolved star and its companion, and indicate a circumcompanion disk-like structure.

Context . The physical conditions and processes taking place in the innermost regions of protoplanetary disks are essential for planet formation and general disk evolution. In this context, we study the T-Tauri type young stellar object RY Tau, which exhibits a dust-depleted inner cavity characteristic of a transition disk. Aims . The goal of this study is to analyze spectrally resolved interferometric observations in the L, M , and N bands of the RY Tau protoplanetary disk obtained with MATISSE. We aim to provide constraints on the spatial distribution and mineralogy of dust in the inner few astronomical units by producing synthetic observations fitting the interferometric observables. Methods . We employed a 2D temperature gradient disk model to estimate the orientation of the inner disk. Successively, we analyzed the chemical composition of silicates depending on spatial region in the disk. Finally, we sampled the parameter space of a viscous accretion disk model via Monte Carlo radiative transfer simulations to investigate the actual 3D dust density distribution of RY Tau. Results . We constrained the orientation of the inner disk of RY Tau, finding no evidence of significant misalignment with respect to its outer disk. We identified several silicate species commonly found in protoplanetary disks and observed a depletion of amorphous dust grains toward the central protostar. By simultaneously considering the observed visibilities and the spectral energy distribution (SED), we found that an accretion disk and an optically thin envelope enshrouding the protostar fits the observations best. Radiative transfer simulations show that hot dust close to the protostar and in the line of sight (LOS) to the observer, either in the uppermost disk layers of a strongly flared disk or in a dusty envelope, is necessary to model the observations. The shadow cast by a dense innermost disk midplane on the dust further out explains the observed closure phases in the L band and (to some extent) in the M band. However, the closure phases in the N band are underestimated by our model, hinting at an additional asymmetry in the flux density distribution that is not visible at shorter wavelengths.

Aims . The goal of this study is to further the understanding of the wind formation mechanism in asymptotic giant branch (AGB) stars through the analysis of the close environment (within a few stellar radii) of the carbon star X TrA. Methods . X TrA was observed for the first time with the Mid-Infrared SpectroScopic Experiment instrument (MATISSE) in the L and N bands in low spectral resolution mode (R=30), and its close surroundings were mapped in specific wavelength ranges corresponding to specific molecules (C ₂ H ₂ and HCN, at 3.1 and 3.8 μm) and dust (amorphous carbon and, for example, SiC at 11.3 μm), via image reconstruction techniques. Results . The angular diameter of the star ranges from 10 mas in the L band pseudo-continuum (3.5 μm) to 20 mas at 3.1 and 11.3 μm. The reconstructed images show some mild elongated features (along the east-west direction) and asymmetric protrusions, which are most evident around 3.1 μm. Imaging results highlight the clumpy nature of the circumstellar environment, starting from the photospheric region up to more distant layers. Conclusions . The angular diameters found for X TrA in the image data are in agreement with previous photospheric diameter estimates (following VLTI/MIDI 8–13 μm observations), and their wavelength dependence is similar to values found for other carbon stars observed with MATISSE (R Scl and V Hya). The 3.1 μm images presented here show highly asymmetric features, another case of a C-rich star with irregular morphologies close to the stellar disk; this supports the notion that the C ₂ H ₂ + HCN abundance distribution usually originates from a clumpy layer around carbon stars.

Context. Asymptotic giant branch (AGB) stars are one of the main sources of dust production in the Galaxy. However, it is not yet clear what this process looks like and where the dust happens to be condensing in the circumstellar environment. Aims. By characterizing the location of the dust and the molecules in the close environment of an AGB star, we aim to achieve a better understanding the history of the dust formation process. Methods. We observed the carbon star R Scl with the thermal-infrared VLTI/MATISSE instrument in L- and N-bands. The high angular resolution of the VLTI observations (as small as 4.4 mas in the L-band and 15 mas in the N-band with ATs), combined with a large uv-plane coverage allowed us to use image reconstruction methods. To constrain the dust and molecules’ location, we used two different methods: one using MIRA image reconstruction algorithm and the second using the 1D code RHAPSODY. Results. We found evidence of C₂H₂ and HCN molecules between 1 and 3.4 R_* which is much closer to the star than the location of the dust (between 3.8 and 17.0 R_*). We also estimated a mass-loss rate of 1.2 ± 0.4 × 10⁻⁶ M_⊙ yr⁻¹. In the meantime, we confirmed the previously published characteristics of a thin dust shell, composed of amorphous carbon (amC) and silicon carbide (SiC). However, no clear SiC feature has been detected in the MATISSE visibilities. This might be caused by molecular absorption that can affect the shape of the SiC band at 11.3 µm. Conclusions. The appearance of the molecular shells is in good agreement with predictions from dynamical atmosphere models. For the first time, we co-located dust and molecules in the environment of an AGB star. We confirm that the molecules are located closer to the star than the dust. The MIRA images unveil the presence of a clumpy environment in the fuzzy emission region beyond 4.0 R_*. Furthermore, with the available dynamic range and angular resolution, we did not detect the presence of a binary companion. To solve this problem, additional observations combining MATISSE and SAM-VISIR instrument should enable this detection in future studies.

RHAPSODY is an intensity profile reconstruction code built to handle 2D centro-symmetric structures using interferometric data. RHAPSODY has been built to provide the community with a code requiring less parameters than classical image reconstruction code. This has as consequence leading to less solution non-uniqueness problems, a better convergence, and a better dynamic range assuming a centro-symmetric source at all wavelengths. RHAPSODY main steps are organized as following: In the first place, at each wavelengths, the code build a unique 1D structure made of concentric discrete uniform rings. Then, it applies the Hankel transform to reconstruct the equivalent visibility profile. Next, a change of coordinate is used on the 1D visibility profile to simulate the inclination and the rotation of the structure in the 2D Fourier plane. After a fitting process on the interferometric observations based on a χ² and Bayesian method, RHAPSODY apply an inverse Fourier transform and reconstruct the equivalent structure in the 2D image plane. According to preliminary tests made on RHAPSODY, the code is able to well reproduce 1D and 2D centrosymmetric structures up to a dynamic range of 0.5%.

Carbon-rich dust is known to form in the atmosphere of the semiregular variable star R Sculptoris. Such stardust, as well as the molecules and gas produced during the lifetime of the star, will be spread into the Galaxy via the mass-loss process. Probing this process is crucial to understand the chemical enrichment of the Galaxy. R Scl was observed using the ESO/VLTI MATISSE instrument in December 2018. Here we show the first images of the star between 3 and 10 R_*. Using the complementary MIRA 3D image reconstruction and the RHAPSODY 1D intensity profile reconstruction code, we reveal the location of molecules and dust in the close environment of the star. Indeed, the C₂H₂ and HCN molecules are spatially located between 1 and 3.4 R_* which is much closer to the star than the location of the dust. The R Scl spectrum is fitted by molecules and a dust mixture of 90% of amorphous carbon and 10% of silicone carbide. The inner boundary of the dust envelope is estimated by DUSTY at about 4.6 R_*. We derive a mass-loss rate of 1.2 ± 0.4 × 10⁻⁶M_⊙ yr⁻¹however no clear SiC forming region has been detected in the MATISSE data.