Claudia Paladini

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.

Context. High-precision astrometric observations at the microarcsecond level require a consistent relativistic framework for modelling both the dynamics of test particles and the propagation of light in gravitational fields. Aims. We aim to formulate the post-Newtonian (PN) expansion in the test body limit in a manner that yields simple and broadly applicable expressions for the motion of massive and massless particles in generic, spherically symmetric spacetimes. In particular, we intend to formulate it such that the formalism suits the modelling of observations of stars orbiting the Galactic centre (GC). Methods. We employed the PN expansion scheme to derive equations of motion for test particles and applied a perturbative solution method to solve the boundary value problem of identifying the geodesic connecting a given emitter–observer pair. Results are validated against numerical integration of the photon geodesic equation. Results. We derive the PN equations of motion for test particles in a generic, spherically symmetric spacetime. We derive analytical expressions for the apparent position of stars as measured by an observer on Earth and show the importance of this formalism for the GC science case. We compare our results with other approaches taken in the literature and discuss its implications for the modelling of orbital motion around Sgr A*. Conclusions. The proposed framework is well suited for the analysis of astrometric observations of stars orbiting the GC with microarcsecond accuracy. In this context, measurements performed with the GRAVITY+ instrument at the Very Large Telescope Interferometer can be used to place constraints on metric theories of gravity through the combined modelling of stellar dynamics and relativistic light propagation near Sgr A*.

Context . Stars with masses between roughly 1 and 8 M _⊙ end their lives on the asymptotic giant branch (AGB), when intense mass loss takes place, with major consequences for the chemical evolution of the universe. The mechanism responsible for the outflows is generally accepted to be radiation pressure acting on dust grains that form in the dense extended atmospheres of AGB stars. Dust formation is enabled, or at least dramatically enhanced, by the action of convection and stellar pulsations. The complex physics underlying convection, stellar pulsations, and dust nucleation precludes predicting AGB mass loss from first principles. Aims . Our aim was to characterize the recent mass ejections of the AGB star Mira A using observations of the inner envelope. In particular, we studied two lobes observed to be expanding away from Mira A to obtain empirical insights into the mass-ejection process. Methods . We investigated the evolution of the lobes using images of polarized light obtained at six epochs using the Spectro-Polarimetric High-contrast Exoplanet REsearch (SPHERE) on the Very Large Telescope and of molecular emission at two epochs obtained with the Atacama Large Millimeter/submillimeter Array (ALMA). Six lines of SO ₂ were used to investigate the excitation temperature and column density of SO ₂ in the lobes. We used the ¹³ CO J = 3-2 line and radiative transfer models to constrain the column density of the gas, which allowed us to infer the abundances of SO, SO ₂ , AlO, AlF, and PO in the lobes. Results . While dust seems confined almost exclusively to the edges of the lobes, gas fills the lobes and displays higher densities than expected at the observed radial distances based on the large-scale mass-loss rate of Mira A, with a total gas mass in the lobes of ∼2 × 10 ⁻⁵ M _⊙ . We find the expansion of the lobes to be consistent with both a constant velocity (ejection time in 2010 or 2011) or a decelerating expansion (ejection time in 2012). If ejection events with a similar magnitude happen periodically, we derive periods between 50 and 200 years to account for the mass-loss rate of Mira A. This periodicity is very uncertain because of the complexity of the circumstellar environment that hampers accurate determinations of the mass-loss rate. We find abundances in the lobes of ∼1.5 × 10 ⁻⁶ and ∼2.5 × 10 ⁻⁶ for SO and SO ₂ , respectively, when accounting for radiative transfer effects and of 2 × 10 ⁻¹⁰ , 6.5 × 10 ⁻¹⁰ , and 4 × 10 ⁻⁷ for AlO, AlF, and PO assuming LTE and optically thin emission. The strong variation in brightness of the different features identified in the polarized-light images is puzzling. We suggest that an asymmetric stellar radiation field preferentially illuminates specific regions of the circumstellar envelope at a given time, producing a lighthouse-like effect.

Context . This study presents the first application of high-precision astrometry to search for exomoons around substellar companions, as this field remains largely unexplored. Aims . We investigate whether the orbital motion of the companion HD 206893 B exhibits astrometric residuals consistent with the gravitational influence of an exomoon or binary planet. Methods . Using the VLTI/GRAVITY instrument, we monitored the astrometric positions of HD 206893 B and c on short (days to months) and long (yearly) timescales. This enabled us to isolate potential residual wobbles in the motion of component B attributable to an orbiting moon. Results . Our analysis reveals tentative astrometric residuals in the HD 206893 B orbit. If interpreted as an exomoon signature, these residuals correspond to a candidate (HD 206893 B I) with an orbital period of approximately 0.76 years and a mass of ~0.4 Jupiter masses. However, the origin of these residuals remains ambiguous and could be due to systematics. Complementing the astrometry, our analysis of GRAVITY R = 4000 spectroscopy for HD 206893 B confirms a clear detection of water, but no CO was found using cross-correlation. We also found that AF Lep b, and β Pic b are the best short-term candidates to look for moons with GRAVITY+. Conclusions . Our observations demonstrate the transformative potential of high-precision astrometry in the search for exomoons and proves the feasibility of the technique to detect moons with masses lower than Jupiter and potentially down to less than Neptune in optimistic cases. Crucially, further high-precision astrometric observations with VLTI/GRAVITY are essential to verify the reality and nature of this signal and apply this technique to a range of planetary systems.

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.

Context. Long secondary periods (LSPs) occur in roughly one third of evolved stars, yet their origin remains uncertain. Two leading hypotheses are oscillatory convective modes and a binary companion enshrouded in dust. Aims. We investigate the LSP in the red giant RT Pav using multiwavelength interferometry to test these competing hypotheses. Methods. Observations of RT Pav were obtained with the VLTI instruments PIONIER, GRAVITY, and MATISSE spanning 1.5–5.0 μm, near the expected phase of maximum projected separation under a binary hypothesis. These data were complemented by photometric data and Gaia DR3 astrometry to constrain companion mass, orbital geometry, and photometric amplitude. Monte Carlo simulations evaluated expected interferometric signatures under both scenarios. Parametric models, including uniform-disk, limb-darkened, uniform-ellipse, binary, and oscillatory convective dipole representations, were fitted to squared-visibility and closure-phase data, informing image reconstructions. Results. Gaia constrains any potential companion to a mass whose Roche-lobe volume is smaller than the minimum extent required by the observed photometric modulation, implying that any obscuring or scattering region capable of producing the observed variability would lie beyond the gravitationally bound zone of such a companion. Binary models often return the lowest χ _ν ² , yet fitted positions are not consistent across wavelength, closure phases do not increase with wavelength as a dusty companion would predict, and we only find significant (> 3 σ ) detections occurring in two of the four tested instrumental wavebands, which is inconsistent with a coherent companion signal. Furthermore simulations and theoretical estimates indicate that a companion with a ∼1% flux ratio, at LSP-consistent separations should be consistently detectable (near or above our 3 σ limits) for standard O-rich asymptotic giant branch (AGB) dust via scattering and/or thermal emission, which is not found. Conversely, an oscillatory convective dipole with a ∼200 K temperature contrast reproduces the H band morphology and the visible light-curve amplitude without violating Gaia or photometric constraints. Finally, significant short wavelength companion signals are completely removed when fitting the residuals of the best fit dipole model. Conclusions. Our interferometric snapshot of RT Pav, acquired near the phase of maximum projected separation under the binary hypothesis, supports oscillatory convective modes as the most physically consistent explanation for its LSP. A logical next step will be time-resolved spectro-interferometric monitoring across the LSP cycle.

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. The star 3 Puppis is the brightest known to exhibit the B[e] phenomenon. Although recent studies have classified this A-type star within the supergiant group, the influence of its binary nature on the circumstellar environment (CE) remains difficult to model. Aims. To resolve its dusty regions at angular scales of 5-10 milliarcseconds (mas), we conducted high angular resolution interferometric observations with the mid-infrared beam combiner VLTI/MATISSE across the 3-12 μm wavelength range. Methods. Since the ( u , v ) coverage enables image reconstruction, we present an innovative statistical interferometric imaging technique based on the MiRA software to produce averaged images. Applied to MATISSE data, this systematic approach facilitates the selection of an optimal set of reconstructions, improving the robustness and fidelity of the recovered features. We also applied SPARCO, an independent imaging tool well suited to systems with a bright central source surrounded by a fainter and extended CE. Results. The images obtained with both tools in the L, M, and N spectral bands show good agreement and clearly reveal an asymmetric elongated structure located at ∼17 mas (∼10 au at 631 pc) to the south-east of the image centre, with a density contrast of ∼20%. A second asymmetry to the north-west and a skewed inner rim are also detected. Simple geometric modelling, inspired by the reconstructed images, provides quantitative constraints on the morphology, position, and flux contribution of the CE and its asymmetries. Conclusions. Our final MATISSE images are consistent with previous VLTI observations but also reveal a more complex CE with large-scale clumps in the south-east and north-west disc regions of 3 Puppis. Finally, based on a hydrodynamic simulation, we conclude that the tidal spiral wake perturbations driven by the central binary, which are dynamically excited at Lindblad resonances within the circumbinary disc, provide the best interpretation for the radial extent and curvature of the elongated structures observed across all bands.