Andrea Chiavassa

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. Mass loss in evolved massive stars plays a critical role in shaping their circumstellar environments and enriching the interstellar medium. In binary systems, stellar interactions can further complicate this process, affecting stellar evolution, stellar yields, and nebular morphology. Aims. We aim to characterise the physical and morphological properties of the binary system AFGL 4106, which is composed of two evolved massive stars. Understanding its mass-loss processes and circumstellar environment offers insight into the late stages of stellar evolution in massive binary systems. Methods. We obtained high-angular-resolution, high-contrast imaging using VLT/SPHERE with ZIMPOL (optical) and IRDIS (near-infrared) across multiple filters. We used aperture photometry to extract the spectral energy distributions (SEDs) of each star and applied radiative-transfer modelling to study the system and its surrounding dusty environment. Results. The observations resolve both components of the binary and unveil a complex, dusty nebula featuring asymmetric structures and cavities. SED fitting yields stellar temperatures of T ₁ = 6723 ± 196 K and T ₂ = 3394 ± 264 K, along with bolometric luminosities of L ₁ = (7.9 ± 0.18)×10 ⁴ L _⊙ and L ₂ = (3.8 ± 0.11)×10 ⁴ L _⊙ . These values support the classification of the primary as being in a post-red-supergiant (post-RSG) phase and the secondary as an active red supergiant (RSG). The luminosity ratio, combined with the inferred radii, indicates that both stars are at close yet distinct stages of their evolution. The binary is surrounded by an extended shell whose asymmetric morphology and large-scale features suggest interaction with the stellar winds and interstellar medium (ISM), and possibly the presence of a third, undetected companion. Conclusions. These observations provide the first resolved view of AFGL 4106’s system and its dusty envelope. Our analysis sets constraints on the physical properties and evolutionary status of the system. This work contributes to our understanding of mass-loss processes in massive binaries and the shaping of nebulae around evolved stars.

Context. Wind dynamics play a pivotal role in governing transport processes within planetary atmospheres, influencing atmospheric chemistry, cloud formation, and the overall energy budget. Understanding the strength and patterns of winds is crucial for comprehensive insights into the physics of ultra-hot-Jupiter atmospheres. Current research has proposed different mechanisms that limit wind speeds in these atmospheres. Aims. This study focuses on unraveling the wind dynamics and the chemical composition in the atmosphere of the ultra-hot Jupiter TOI-1518 b. Methods. Two transit observations using the high-resolution (R_λ ∼ 85 000) optical (spectral coverage between 490 and 920 nm) spectrograph MAROON-X were obtained and analyzed to explore the chemical composition and wind dynamics using the cross-correlation techniques, global circulation models (GCMs), and atmospheric retrieval. Results. We report the detection of 14 species in the atmosphere of TOI-1518 b through cross-correlation analysis. VO was detected only with the new HyVO line list, whereas TiO was not detected. Additionally, we measured the time-varying cross-correlation trails for six different species, compared them with predictions from GCMs, and conclude that a strong drag is slowing the winds in TOI-1518 b’s atmosphere (τ_drag ≈ 10³−10⁴ s). We find that the trails are species dependent. Fe+ favors stronger drag than Fe, which we interpret as a sign of magnetic effects being responsible for the observed strong drag. Furthermore, we show that Ca+ probes layers above the Roche lobe, leading to a qualitatively different trail than the other species. Finally, We used a retrieval analysis to further characterize the abundances of the different species detected. Our analysis is refined thanks to the updated planetary mass of 1.83 ± 0.47 M_Jup we derived from new Sophie radial-velocity observations. We measure an abundance of Fe of log₁₀ Fe = −4.88_−0.76^+0.63 corresponding to 0.07 to 1.62 solar enrichment. For the other elements, the retrievals appear to be biased, probably due to the different K_p/V_sys shifts between Fe and the other elements, which we demonstrate for the case of VO.

Context. Since the recent discovery of the directly imaged super-Jovian planet AF Lep b, several studies have been conducted to characterize its atmosphere and constrain its orbital parameters. AF Lep b has a measured dynamical mass of 3.68 ± 0.48 M_Jup, radius of 1.3 ± 0.15 R_Jup, nearly circular orbit in spin-orbit alignment with the host star, relatively high metallicity, and near-solar to super-solar C/O ratio. However, key parameters such as the rotational velocity and radial velocity have not been estimated thus far, as they require high-resolution spectroscopic data that are impossible to obtain with classical spectrographs. Aims. AF Lep b was recently observed with the new HiRISE visitor instrument at the VLT, with the goal of obtaining high-resolution (R ≈ 140 000) spectroscopic observations to better constrain the orbital and atmospheric parameters of the young giant exoplanet. Methods. We compared the extracted spectrum of AF Lep b to self-consistent atmospheric models using ForMoSA, a forward modeling tool based on Bayesian inference methods. We used our measurements of the planet’s radial velocity to offer new constraints on its orbit. Results. From the forward modeling, we find a C/O ratio that aligns with previous low-resolution analyses and we confirm its supersolar metallicity. We also unambiguously confirm the presence of methane in the atmosphere of the companion. Based on all available relative astrometry and radial velocity measurements of the host star, we show that two distinct orbital populations are possible for the companion. We derived the radial velocity of AF Lep b to be 10.51 ± 1.03 km s⁻¹ and show that this value is in good agreement with one of the two orbital solutions, allowing us to rule out an entire family of orbits. Additionally, assuming that the rotation and orbit are coplanar, the derived planet’s rotation rate is consistent with the observed trend of increasing spin velocity with higher planet mass.

Context. The dramatic dimming episode of the red supergiant Betelgeuse in 2019 and 2020, caused by a partial darkening of the stellar disk, has highlighted gaps in the understanding of the evolution of massive stars. Aims. We analyzed numerical models to investigate the processes behind the formation of dark surface patches and the associated reduction in the disk-integrated stellar light. Methods. With the CO5BOLD code, we performed global 3D radiation-hydrodynamical simulations of evolved stars, including convection in the stellar interior, self-excited pulsations, and the resulting atmospheric dynamics with strong radiative shocks. Results. We attribute dimming phenomena to obscuring clouds of cool gas in the lower atmosphere, forming according to three different scenarios. One process transports material outward in a strong shock, similar to what occurs in 1D simulations of radially pulsating asymptotic giant branch (AGB) stars. However, in 3D models, deviations from spherical symmetry of the shock front can lead to further local density enhancements. Another mechanism is triggered by a large convective upflow structure, in combination with exceptionally strong radial pulsations. This induces Rayleigh-Taylor instabilities, causing plumes of material to be sent outward into the atmosphere. The third and rarest scenario involves large-amplitude convective fluctuations, leading to enhanced flows in deep downdrafts, which rebound and send material outward. In all cases, the dense gas above the stellar surface cools and darkens rapidly in visible light. AGB stars show localized dark patches regularly during intermediate phases of their large-amplitude pulsations, while more massive stars will only intermittently form such patches during luminosity minima. Conclusions. The episodic levitation of dense gas clumps above the stellar surface, followed by the formation of complex molecules in the cooling gas and possibly dust grains at a later stage, can account for the dark patches and strong dimming events of supergiant stars such as Betelgeuse.

Context. The complex dynamics of asymptotic giant branch (AGB) stars and the resulting stellar winds have a significant impact on the measurements of stellar parameters and amplify their uncertainties. Three-dimensional (3D) radiative hydrodynamic (RHD) simulations of convection suggest that convection-related structures at the surface of AGB star affect the photocentre displacement and the parallax uncertainty measured by Gaia. Aims. We explore the impact of the convection on the photocentre variability and aim to establish analytical laws between the photo-centre displacement and stellar parameters to retrieve such parameters from the parallax uncertainty. Methods. We used a selection of 31 RHD simulations with CO⁵BOLD and the post-processing radiative transfer code OPTIM3D to compute intensity maps in the Gaia G band [320–1050 nm]. From these maps, we calculated the photocentre position and temporal fluctuations. We then compared the synthetic standard deviation to the parallax uncertainty of a sample of 53 Mira stars observed with Gaia. Results. The simulations show a displacement of the photocentre across the surface ranging from 4 to 13% of the corresponding stellar radius, in agreement with previous studies. We provide an analytical law relating the pulsation period of the simulations and the photocentre displacement as well as the pulsation period and stellar parameters. By combining these laws, we retrieve the surface gravity, the effective temperature, and the radius for the stars in our sample. Conclusions. Our analysis highlights an original procedure to retrieve stellar parameters by using both state-of-the-art 3D numerical simulations of AGB stellar convection and parallax observations of AGB stars. This will help us refine our understanding of these giants.

Context.Stellar convection plays an important role in atmospheric dynamics, wind formation, and the mass-loss processes in asymptotic giant branch stars. However, a direct characterization of convective surface structures in terms of size, contrast, and lifespan is quite challenging, as spatially resolving these features requires the highest angular resolution.Aims.We aim to characterize the size of convective structures on the surface of the O-rich AGB starR Carto test different theoretical predictions based on mixing-length theory from solar models.Methods.We used infrared low-spectral resolution (R ∼ 35) interferometric data in theH-band (∼1.76 μm) obtained by the instrument PIONIER at the Very Large Telescope Interferometer (VLTI) to image the star’s surface at two epochs separated by approximately six years. Using a power spectrum analysis, we estimated the horizontal size of the structures on the surface of R Car. The sizes of the stellar disk at different phases of a pulsation cycle were obtained using parametric model fitting in the Fourier domain.Results.Our analysis supports that the sizes of the structures in R Car are correlated with variations in the pressure scale height in the atmosphere of the target, as predicted by theoretical models based on solar convective processes. We observed that these structures grow in size when the star expands within a pulsation cycle. While the information is still scarce, this observational finding highlights the role of convection in the dynamics of those objects. New interferometric imaging campaigns with the renewed capabilities of the VLTI are envisioned to expand our analysis to a larger sample of objects.

Context. Asymptotic giant branch (AGB) stars enrich the interstellar medium through their mass loss. The mechanism(s) shaping the circumstellar environment of mass-losing stars is not clearly understood so far. Aims. Our purpose is to study the effect of binary companions located within the first 10 stellar radii from the primary AGB star. In this work, we target the mass-losing carbon star V Hydrae (V Hya) and search for signatures of its companion in the dust-forming region of the atmosphere. Methods. The star was observed in the L and N bands with the VLTI/MATISSE instrument at low spectral resolution. We reconstructed images of the photosphere and surroundings of V Hya using the two bands and compared our interferometric observables with VLTI/MIDI and VISIR archival data. To constrain the dust properties, we used the 1D radiative transfer code DUSTY to model the spectral energy distribution. Results. The star is dominated by dust emission in the L- and N-bands. The MATISSE reconstructed images show asymmetric and elongated structures in both infrared bands. In the L band, we detected an elongated shape of approximately 15 mas that likely is of photospheric origin. In the N band, we found a 20 mas extension northeast from the star and perpendicular to the L-band elongated axis. The position angle and the size of the N-band extension match the prediction of the companion position at the MATISSE epoch. By comparing MATISSE N-band with MIDI data, we deduce that the elongation axis in the N-band has rotated since the previous interferometric measurements 13 yr ago, supporting the idea that the particle enhancement is related to the dusty clump moving along with the companion. The VISIR image confirms the presence of a large-scale dusty circumstellar envelope surrounding V Hya. Conclusions. The MATISSE images unveil the presence of a dust enhancement at the position of the companion. This opens new doors for further analyses of the binary interaction with an AGB component.