Aims. The mass-loss process of red supergiant (RSG) and asymptotic giant branch (AGB) stars and its relation to variability are poorly constrained. We aim to study the photosphere and near-surface atmospheric structure, where the mass-loss is initiated. For this purpose, we studied two oxygen-rich evolved stars: the Mira-type AGB star R Car and the RSG VX Sgr. Methods. We used the VLTI-GRAVITY instrument operating in the near-infrared K-band. Our sample comprises 54 VLTI-GRAVITY snapshots (18 R Car, 36 VX Sgr) taken over about 7 years, making it the largest VLTI time series dataset to date. We determined the angular diameter as a function of time for the continuum (photosphere) and selected atomic and molecular bands, i.e., lines of Ti I and Sc I as well as bands of H2O and CO. Furthermore, we compared the variability and atmospheric structure to state-of-the-art radiative-hydrodynamics CO5BOLD 3D simulations. Results. The radii of photosphere (R⋆) and extended atmospheric layers are variable and relate to the light curve with phase shifts. The near-photospheric layers show a maximum radius near visual brightness minima (φvis ∼ 0.4–0.6). Inner atomic (Ti I, Sc I) and molecular (H2O) layers are further phase-shifted by ∆φvis ~ 0.05. The more extended CO layers show longer, irregular periods and maximum extensions of ∼1.3–1.7 R⋆ for R Car and of ∼1.5–2.2 R⋆ for VX Sgr. Comparison with synthetic interferometric data of an AGB model based on several pulsation cycles in CO5BOLD simulations revealed a similar behavior. The photosphere shows regular pulsations, but with maximum diameters preceding minimum brightness (φvis < 0.5). The H2O layer showed a much weaker extension compared to our observations, while CO showed a good agreement. Furthermore, during the 2020–2021 season, VX Sgr exhibited an extreme mass-loss event similar to that of Betelgeuse, preceded by two strong shocks and culminating with the extreme expansion of H2O and CO layers, both up to ∼2.2 R⋆. Unexpectedly, during this event, we also detected Brackett γ in interferometric data as well as strong Balmer emission in optical spectra, both of which are also signatures of a shock propagating through the atmosphere. Conclusions. The Mira R Car showed an estimated photospheric radius of R⋆ = 280 ± 25 R⊙, with a regular fundamental mode (FM) pulsation amplitude of ∼13% of R⋆. During its active cycle, the extreme RSG VX Sgr showed R⋆ = 1556 ± 110 Re, with an FM pulsation amplitude of ∼13% of R⋆, the same as R Car. During its quiescent cycle, it showed a smaller value, R⋆ = 1456 ± 108 R⊙, and low-amplitude pulsations near the first overtone (O1), only ∼4% of R⋆. This supports a steady mass-loss process for Mira stars related to stable large-amplitude FM pulsation, whereas the mass-loss process for RSGs may be dominated by extreme events connected to changes in the pulsation mode from low-amplitude O1 to large-amplitude FM pulsations.

Estimations of stellar angular diameters can be determined based on the surface brightness-colour relation (SBCR) and photometry. In the context of the PLATO space mission, the SBCR is considered to be an independent empirical alternative for estimating the stellar radii of FGK stars. We implemented a homogeneous approach, not only for calibrating the SBCR for FGK-IV/V stars, but also for determining their fundamental parameters. This made it possible to reliably place the stars on the Hertzsprung-Russell (HR) diagram and to study their impact on the SBC relations. We performed interferometric observations of 18 quiescent FGK-IV/V stars in the Gaia magnitude range of . For the first time, we used three different interferometric instruments operating in the , , and bands to measure the polychromatic limb-darkened angular diameters. In parallel, by using public domain spectra, we were able to estimate the stellar parameters (T_ . 3.088 łeq G łeq 5.498 R H K łog g, and Z) by using the open Python tool iSpec We achieved an average accuracy of 2.3% for the limb-darkened angular diameters based on polychromatic observations. However, we observed that our SBCR relation does not follow the calibration of the SBC relations in Gaia found in the literature. Furthermore, we found that the łog g and Z parameters have no impact on this relationship. Given the characteristics of our sample of quiescent stars, it constitutes an ideal set of targets for conducting a new SBCR calibration within the framework of the PLATO space mission. In this context, we report a SBCR calibration with σ_ RMS = 0.012, 0.009, and 0.009 in the G, G_ BP , and G_RP Gaia bands, respectively. This work is part of a series of papers reporting the first results obtained using a polychromatic approach to measure angular diameters, employing a fully homogeneous methodology for determining the fundamental parameters of stars and measuring the limb-darkened angular diameters

Aims. Parsec-scale jets of blazars have so far been spatially resolved only in millimeter and submillimeter wavelengths, where very long baseline interferometry can be used to obtain milliarcsecond-scale images of the jets. We have attempted to spatially resolve the near-infrared emission in jet-dominated blazars for the first time. Methods. We used the VLTI-GRAVITY instrument to obtain milliarcsecond-scale near-infrared interferometric observations of a flaring blazar Ton 599. Additionally, we observed four non-flaring blazars using the GRAVITY-wide mode, where a nearby bright star is used as a fringe tracker. Results. We modeled the squared visibilities of Ton 599 and found that they are incompatible with a single unresolved point source unless there is a significant amount of additional unknown coherence loss in the instrument. With the present data, we cannot distinguish between a model with an unresolved point source and extended emission or coherence loss and a model with a single Gaussian component. This suggests that we are seeing the unresolved or only partially resolved jet-base in near-infrared wavelengths. The wide-field mode of GRAVITY was challenging for the additional relatively faint targets, resulting in either non-detections or poor-quality data that could not be modeled. Conclusions. Our observations demonstrate that it is possible to detect the compact jet emission in blazars with near-infrared interferometry, suggesting that with the improved GRAVITY+ instrument it will be possible to spatially resolve and image the near-infrared emission of blazar jets.

The estimation of stellar angular diameters can be performed from the surface brightness - color relation (SBCR) and photometry. The SBCR have been considered by the PLATO space mission as an independent empirical alternative for estimating stellar radii of FGK stars. In this context, we have implemented an homogeneous approach not only for calibrating the SBCR for FGK-IV/V stars but, also for determining their fundamental parameters in order to place the stars reliably on the HR diagram and to study their impact on the SBCR. We have performed interferometric observations of 18 quiescent FGK-IV/V stars in the Gaia color range of 3.088 ≤ G ≤ 5.498. For the first time, we used 3 different interferometers operating in the R, H , and K bands to measure polychromatic limb-darkened angular diameters (LDAD). In parallel, by using public domain spectra we have estimated the stellar parameters (T_eff, log g and Z) by using the open python tool iSpec. We achieved an average accuracy of 2.3

Carbon-rich asymptotic giant branch (AGB) stars are among the most important contributors of enriched materials to the interstellar medium due to their strong stellar winds. To fully characterize mass loss on the AGB, it is necessary to determine the distributions of dust and gas around the stars, where the dust begins to condense from the gas, and how this extended atmospheric structure evolves over the pulsational period of the star. We present an analysis of L -band (2.8–4.2 μ m) interferometric observations of the carbon-rich AGB star V Oph made with the MATISSE instrument at the Very Large Telescope Interferometer at the maximum and minimum of the star’s visual light curve. Using the radiative transfer software RADMC-3D, we model the circumstellar dust shell, and find stellar radii of 395 and 495 R _⊙ at the two phases, and dust radii of 790 and 742.5 R _⊙ at the two epochs, respectively. By adding C _2 H _2 and HCN gas to the RADMC-3D models, we are able to fit the visibility spectra well, with some deviations at the 3.11 μ m feature. Reasons for this deviation and interpretation of the best fitting models are discussed in the text, and we discuss motivations for follow-up imaging observations of V Oph.

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 star R Car to test different theoretical predictions based on mixing-length theory from solar models. Methods. We used infrared low-spectral resolution (R similar to 35) interferometric data in the H-band (similar to 1.76 mu m) 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.

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), looking for signatures of its companion in the dust forming region of the atmosphere. The star was observed in the L- and N-bands with the VLTI/MATISSE instrument at low spectral resolution. We reconstructed images of V Hya's photosphere and surroundings using the two bands and compared our interferometric observables with VLTI/MIDI and VISIR archival data. To constrain the dust properties, we used DUSTY to model the spectral energy distribution. The star is dominated by dust emission in the L- and N- bands. The VISIR image confirms the presence of a large-scale dusty circumstellar envelope surrounding V Hya. 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, likely to be of photospheric origin. In the N-band, we found a 20 mas extension North-East 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 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 years ago, supporting the idea that the particle enhancement is related to the dusty clump moving along with the companion. The MATISSE images unveil the presence of a dust enhancement at the companion position, opening new doors for further analysis on the binary interaction with an AGB component.

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.

V838 Mon is a stellar merger remnant that erupted in 2002 in a luminous red novae event. Although it is well studied in the optical, near infrared and submillimeter regimes, its structure in the mid-infrared wavelengths remains elusive. We observed V838 Mon with the MATISSE (LMN bands) and GRAVITY (K band) instruments at the VLTI and also the MIRCX/MYSTIC (HK bands) instruments at the CHARA array. We geometrically modelled the squared visibilities and the closure phases in each of the bands to obtain constraints on physical parameters. Furthermore, we constructed high resolution images of V838 Mon in the HK bands, using the MIRA and SQUEEZE algorithms to study the immediate surroundings of the star. Lastly, we also modelled the spectral features seen in the K and M bands at various temperatures. The image reconstructions show a bipolar structure that surrounds the central star in the post merger remnant. In the K band, the super resolved images show an extended structure (uniform disk diameter $\sim 1.94$ mas) with a clumpy morphology that is aligned along a north-west position angle (PA) of $-40^\circ$. Whereas in the H band, the extended structure (uniform disk diameter $\sim 1.18$ mas) lies roughly along the same PA. However, the northern lobe is slightly misaligned with respect to the southern lobe, which results in the closure phase deviations. The VLTI and CHARA imaging results show that V838 Mon is surrounded by features that resemble jets that are intrinsically asymmetric. This is also confirmed by the closure phase modelling. Further observations with VLTI can help to determine whether this structure shows any variation over time, and also if such bipolar structures are commonly formed in other stellar merger remnants.

Context. Mass loss plays a crucial role in the lives of massive stars, especially as the star leaves the main sequence and evolves to the red supergiant (RSG) phase. Despite its importance, the physical processes that trigger mass-loss events in RSGs are still not well understood. Recently, we showed that adding a semi-empirical wind to atmosphere models can accurately reproduce observed extensions in the atmospheres of RSGs, where the mass-loss events are taking place, particularly in the CO and water lines. Aims. By adding a static wind to a MARCS atmospheric model, we computed synthetic observables that match new interferometric data of the RSGs AH Sco, KW Sgr, V602 Car, CK Car, and V460 Car obtained with the VLTI/MATISSE and VLTI/GRAVITY instruments between August 2022 and February 2023. We also used archival VLTI/AMBER data of KW Sgr and VLTI/GRAVITY data of AH Sco. The MATISSE wavelength range includes the SiO molecule at 4.0 mu m with a spectral resolution of R similar to 500. Methods. The model intensities with respect to the line-of-sight angle (mu) as well as the spectra and visibilities were computed using the stellar radiative transfer code TURBOSPECTRUM. We found the best-fit model, mass-loss rate, and best-fit angular Rosseland diameter for the observations. We simultaneously matched our model to the data, covering a wavelength range of 1.8-5.0 mu m, which corresponds to the K, L, and M bands. 4Results. Our models reproduce the spectro-interferometric data over this wide wavelength range, including extended atmospheric layers of CO, H2O, and SiO. We obtain a range of Rosseland angular diameters between 3.0 < theta(Ross) < 5.5 mas and a range of mass-loss rates of -6.5 < log (M) over dot/M-circle dot yr(-1) < -4 for our five targets. In our best-fit models, the partial pressure of SiO relative to the gas pressure, P-SiO/P-g, and the SiO 4.0 mu m line intensity increase between 2 and 3 stellar radii. The relative intensity depends on the luminosity used for our models, since the more luminous models have a higher mass-loss rate. Conclusions. This work further demonstrates that our MARCS+wind model can reproduce the observed physical extension of RSG atmospheres for several spectral diagnostics spanning a broad wavelength range. We reproduce both spectra and visibilities of newly obtained data as well as provide temperature and density stratifications that are consistent with the observations. With the MATISSE data, we newly include the extension of SiO layers as a precursor of silicate dust.

SPICA (Stellar Parameters and Images with a Cophased Array) is a 6-telescope (6T) visible instrument for the CHARA Array (Center for High Angular Resolution in Astronomy) at Mount Wilson Observatory. It uses single mode fibers for feeding the interferometric spectrograph, which offers three different spectral resolutions: R=140, R=4000, and R=14000. CHARA/SPICA has been mainly designed for large programs (surveys) in the domain of stellar fundamental parameters but also permits fast imaging thanks to the 15 baselines and the large number of spectral channels (60 in low resolution mode). SPICA is made of the visible instrument SPICA-VIS and of a new H-band, 6T, ABCD combiner performing group delay and phase delay tracking. In this paper, we present the first light results of SPICA.

Context. The mass-loss mechanisms in M-type asymptotic giant branch (AGB) stars are still not well understood; these include, in particular, the formation of dust-driven winds from the innermost gaseous layers around these stars. One way to understand the gas-dust interaction in these regions and its impact on the mass-loss mechanisms is through the analysis of high-resolution observations of the stellar surface and its closest environment. Aims. We aim to characterize the inner circumstellar environment (~3 R * ) of the M-type Mira star R Car in the near-infrared at different phases of a pulsation period. Methods. We used GRAVITY interferometric observations in the K band obtained during two different epochs over 2018. Those data were analyzed using parametric models and image reconstruction of both the pseudo-continuum and the CO band heads observed. The reported data are the highest angular resolution observations on the source in the K band. Results. We determined sizes of R Car’s stellar disk of 16.67 ± 0.05 mas (3.03 au) in January 2018 and 14.84 ± 0.06 mas (2.70 au) in February, 2018, respectively. From our physical model, we determined temperatures and size ranges for the innermost CO layer detected around R Car. The derived column density of the CO is in the ~9.18×10 18 –1×10 19 cm −2 range, which is sufficient to permit dust nucleation and the formation of stable dust-driven winds. We find that magnesium composites, Mg 2 SiO 4 and MgSiO 3 , have temperatures and condensation distances consistent with the ones obtained for the CO layer model and pure-line reconstructed images, which are the dust types most likely to be responsible for wind formation. Our reconstructed images show evidence of asymmetrical and inhomogeneous structures, which might trace a complex and perhaps clumpy structure of the CO molecule distribution. Conclusions. Our work demonstrates that the conditions for dust nucleation and thus for initialising dust-driven winds in M-type AGB stars are met in R Car, and we identify magnesium composites as the most probable candidates. We find structural changes between two observing epochs (which are separated by ~10% of the full pulsation period of the star) and evidence of the effects of asymmetries and clumpiness. This observational evidence is crucial to constraining the role of convection and pulsation in M-type stars.

Red supergiants (RSGs) are evolved massive stars in a stage preceding core-collapse supernova. The physical processes that trigger mass loss in their atmospheres are still not fully understood. Based on observations of $\alpha$ Ori, a new semi-empirical method to add a wind to hydrostatic model atmospheres of RSGs was recently developed. We use this method of adding a wind to a MARCS model atmosphere to compute synthetic observables, comparing the model to spatially resolved interferometric observations. We present a case study to model published data of HD 95687 and V602 Car obtained with VLTI/AMBER. We compute model intensities, spectra and visibilities for different mass-loss rates using the radiative transfer code Turbospectrum. The models are convolved to match the different spectral resolutions of the VLTI instruments, studying a wavelength range of $1.8-5\,\mathrm{\mu m}$ corresponding to the $K$, $L$ and $M$-bands for GRAVITY and MATISSE data. We compare the model spectra and visibilities with the published VLTI/AMBER data. The synthetic visibilities reproduce observed drops in the CO, SiO, and water layers that are not shown in visibilities based on MARCS models alone. For the case studies, we find that adding a wind to MARCS with simple radiative equilibrium dramatically improves the agreement with the visibilities and the spectra. Our results reproduce observed extended atmospheres up to several stellar radii. This paper shows the potential of our model to describe extended atmospheres in RSGs: it can reproduce the shapes of the spectra and visibilities with better accuracy in the CO and water lines than previous models. The method can be extended to other wavelength bands for both spectroscopic and interferometric data. We provide temperature and density stratifications that succeed for the first time in reproducing observed interferometric properties of RSG atmospheres.

The mass-loss mechanisms in M-type AGB stars are not well understood, in particular, the formation of dust-driven winds from the innermost gaseous layers around these stars. One way to understand the gas-dust interaction in these regions and its impact on the mass-loss mechanisms is through the analysis of high-resolution observations of the stellar surface and its closest environment. We aim at characterizing the inner circumstellar environment (~3 R*) of the M-type Mira star R Car in the near-infrared at different phases of a pulsation period. We used GRAVITY interferometric observations in the K-band obtained at two different epochs over 2018. Those data were analyzed using parametric models and image reconstruction of both the pseudo-continuum and the CO band-heads observed. The reported data are the highest angular resolution observations on the source in the K-band. We determine sizes of R Car's stellar disk of 16.67 +- 0.05 mas (3.03 au) in January 2018 and 14.84+-0.06 mas (2.70 au) in February 2018, respectively. From our physical model, we determined temperatures and size ranges for the innermost CO layer detected around R Car. We find that magnesium composites, Mg2SiO4 and MgSiO3, have temperatures and condensation distances consistent with the ones obtained for the CO layer model and pure-line reconstructed images, being them the most plausible dust types responsible of wind formation. Our reconstructed images show evidence of asymmetrical and inhomogeneous structures, which might trace a complex and perhaps clumpy structure of the CO molecule distribution. Our work demonstrates that the conditions for dust nucleation and thus for initialising dust-driven winds in M-type AGB stars are met in R Car and we identify Magnesium composites as the most probable candidates. This observational evidence is crucial to constrain the role of convection and pulsation in M-type stars.

With a possible angular resolution down to 0.1-0.2 millisecond of arc using the 330 m baselines and the access to the 600-900 nm spectral domain, the CHARA Array is ideally configured for focusing on precise and accurate fundamental parameters of stars. CHARA/SPICA (Stellar Parameters and Images with a Cophased Array) aims at performing a large survey of stars all over the Hertzsprung-Russell diagram. This survey will also study the effects of the different kinds of variability and surface structure on the reliability of the extracted fundamental parameters. New surface-brightness-colour relations will be extracted from this survey, for general purposes on distance determination and the characterization of faint stars. SPICA is made of a visible 6T fibered instrument and of a near-infrared fringe sensor. In this paper, we detail the science program and the main characteristics of SPICA-VIS. We present finally the initial performance obtained during the commissioning.

ESO's VLT interferometer (VLTI) is a general-user optical/infrared interferometric facility. Its operations scheme is fully integrated into the well-established scheme of all VLT instruments and profits enormously from this experience and the implemented infrastructure to offer a unique service to the community. Based on the greatly improved capabilities of the 2nd generation VLTI instruments and taking advantage of a further development of ESO's Observation Handling Tools, we have evolved the VLTI operations scheme as well. We have offered to VLTI investigators the possibility to indicate baseline configurations in a more flexible way and have introduced nested scheduling containers to better formalize the observational strategy. We have prepared for dedicated support of different types of interferometric observations. For imaging observations specifically, we have introduced an improved workflow to fill the uv plane and to handle time-critical imaging.

ESO’s La Silla Paranal Observatory uses a set of integrated tools for preparation and execution of Service and Visitor Mode (SM and VM, respectively) observations. The web interface for the observation preparation (p2) provides a versatile and robust environment for users to efficiently design their observations. The software architecture of p2 enabled implementation of new services, modeled according to the instruments’ specifications and operational standards. The automatic creation of Finding Charts is integrated within p2 and the Observation Preparation (ObsPrep) tool enables interactive observing strategy configuration including for example fine-tuning of the science field pointings, selection of blind offset and guide stars as well as selection of auxiliary stars for instruments using Adaptive Optics. Through the Visitor Execution Sequence, observers can plan and monitor in real-time their (on-site or remote) observations. For Service Mode runs the use of scheduling containers, recently extended to include nesting of containers, enables design of complex observing strategies that are machine readable, which allows programmatic preparation of short term scheduling for execution and planning of the night at the observatory.

After the pause imposed by the pandemic, VLTI resumed science operations and restarted technical activities aiming to close commissionings of different modes. While the community develops projects of visiting instruments, the VLTI infrastructure is about to be significantly upgraded with new visible AO and laser guide star systems by the GRAVITY+ project. VLTI operations also evolve, in particular to support imaging programmes, but also towards a more automated and integrated model. In this context, we will present a review of current capabilities, ongoing activities and future plans for the VLTI.

AGB stars are one of the main sources of dust production in the Galaxy. However, it is not clear what this process looks like and where the dust is condensing in the circumstellar environment. 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. We observed the carbon star R Scl with the VLTI-MATISSE instrument in L- and N-bands. The high angular resolution of the VLTI observations, 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: MIRA image reconstruction and the 1D code RHAPSODY. We found evidence of C2H2 and HCN molecules between 1 and 3.4 Rstar which is much closer to the star than the location of the dust (between 3.8 and 17.0 Rstar). We also estimated a mass-loss rate of 1.2+-0.4x10-6 Msun per yr. 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 micron. 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 Rstar. Furthermore, with the available dynamic range and angular resolution, we did not detect the presence of a binary companion. Additional observations combining MATISSE and SAM-VISIR instrument should enable this detection in future studies.

Context. VX Sgr is a cool, evolved, and luminous red star whose stellar parameters are difficult to determine, which affects its classification. Aims. We aim to spatially resolve the photospheric extent as well as the circumstellar environment. Methods. We used interferometric observations obtained with the MATISSE instrument in the L (3 to 4 {\mu}m), M (4.5 to 5 {\mu}m), and N (8 to 13 {\mu}m) bands. We reconstructed monochromatic images using the MIRA software. We used 3D radiation-hydrodynamics (RHD) simulations carried out with CO5BOLD and a uniform disc model to estimate the apparent diameter and interpret the stellar surface structures. Moreover, we employed the radiative transfer codes Optim3D and Radmc3D to compute the spectral energy distribution for the L, M, and N bands, respectively. Results. MATISSE observations unveil, for the first time, the morphology of VX Sgr across the L, M, and N bands. The reconstructed images show a complex morphology with brighter areas whose characteristics depend on the wavelength probed. We measured the angular diameter as a function of the wavelength and showed that the photospheric extent in the L and M bands depends on the opacity through the atmosphere. In addition to this, we also concluded that the observed photospheric inhomogeneities can be interpreted as convection-related surface structures. The comparison in the N band yielded a qualitative agreement between the N band spectrum and simple dust radiative transfer simulations. However, it is not possible to firmly conclude on the interpretation of the current data because of the difficulty in constraing the model parameters using the limited accuracy of our absolute flux calibration. Conclusions. MATISSE observations and the derived reconstructed images unveil the appearance of the stellar surface and circumstellar environment across a very large spectral domain for the first time.