arXiv subjects

J. Varga

Publications and source records attributed to J. Varga.

At least 19 recordsLinked to original sources

arXiv2026

MINDS survey of silicates in T Tauri disks: Correlation between dust and gas

Context. Silicates are key constituents of planet-forming disks and major building blocks of rocky planets. Mid-infrared spectral features of micron-sized silicate grains trace grain growth, mineralogy, and disk chemistry. Aims. We characterized the dust mineralogy in T Tauri disks using James Webb Space Telescope (JWST)/Mid-Infrared Instrument (MIRI) observations and investigated the connections between the dust and molecular gas compositions. Methods. We analyzed JWST/MIRI spectra of 26 disks from the MIRI mid-Infrared Disk Survey (MINDS). Using our DustComp spectral decomposition tool, we inferred the mass fractions of individual dust species. The fits included Mg$_2$SiO$_4$ (forsterite), MgSiO$_3$ (enstatite), and SiO$_2$ (silica), together with amorphous silicates of corresponding stoichiometry. Results. Mg-rich (and Fe-poor) silicates reproduce the data well, with residuals typically within $\pm3\%$. Grain size distributions are skewed toward sizes larger than $2\mu$m, indicating significant growth. The average dust composition is dominated by Mg$_2$SiO$_4$-stoichiometry grains ($\sim60\%$), followed by MgSiO$_3$ ($\sim30\%$) and SiO$_2$ ($\sim10\%$). Crystalline mass fractions are typically in the $5$-$24\%$ range, with a mean of $14\%$. Annealed silica is robustly detected in nine objects, with cristobalite as the main polymorph. We found a correlation between dust and molecular gas composition: disks with strong annealed silica features show stronger CO$_2$ emission, while forsterite-rich disks display stronger H$_2$O emission. Disks with annealed silica features may also have elevated gas-phase C/O ratios. Conclusions. The observed dust-gas correlation may provide the first indication that the molecular gas composition regulates the availability of dust species in the inner disk.

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arXiv2026

Unlocking the dynamics of Young Stellar Objects: Time-Domain Interferometry with six 4-m class telescopes

The dynamics of the inner regions of young stellar objects (YSOs) is driven by a variety of physical phenomena, from magnetospheres and accretion to the dust sublimation rim and inner disk flows. These inner environments evolve on timescales of hours to days, exactly when bursts, dips, and rapid structural changes carry the most valuable information about star and planet formations, but remain hardly reachable with current facilities. A better reactive infrastructure with six or more telescopes, combined with alerts from large time-domain surveys (e.g., at the era of LSST/Rubin type facilities), and equipped with instruments spanning from the V-band to the thermal infrared (N), would provide the instantaneous uv-coverage and spectral diagnostics needed to unambiguously interpret and image these events as they happen. Such a world's first time-domain interferometric observatory would enable qualitatively new science: directly linking optical and infrared variability to spatially resolved changes in magnetospheric accretion, inner-disk geometry, and dust and gas dynamics in the innermost astronomical unit. Crucially, connecting these processes to outer-scale unresolved information from JWST, ALMA, and the ELT would yield a complete tomography of the planet-forming region.

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arXiv2026

The asymmetric structure of the inner disc around HD 142527 A with VLTI/MATISSE

Circumstellar discs, and especially their inner regions, covering ranges from <1 au to a few astronomical units, are the birthplaces of terrestrial planets. The inner regions are thought to be similarly diverse in structure as the well-observed outer regions probed by ALMA. Combining data and results from previous studies of the VLTI/PIONIER and VLTI/GRAVITY instruments with new, multi-epoch VLTI/MATISSE observations, we aim to provide a comprehensive picture of the structure of the inner regions of the circumstellar disc around the F-type Herbig Ae/Be star HD 142527 A, the primary of a binary star system. We model the multi-wavelength interferometric data using a parametrised, geometrically thin disc model, allowing for azimuthal asymmetry, exploring a first-order disc modulation and an off-centre Gaussian component. We find time-variable structures in the N-band observables, which we reproduce with time-dependent models. This variability manifests as azimuthally asymmetric emission, evidenced by strong, non-zero closure phases in the N-band data. Fits to individual epochs of the N-band observations yield better $\chi^2_\text{r}$ values than fits to all epochs simultaneously. This suggests substantial changes in the geometry of the inner disc emission from ~1 au up to a few astronomical-unit scales from one year to the next. Moreover, our models produce a very close-in inner disc rim $R_\text{rim}\approx0.1$ au. All together, we find a very complex, substantially non-point symmetric and temporally-variable disc ($r_\text{out}\lesssim6$ au) around the primary. The very close-in inner rim indicates the presence of material inside the typical wall-like sublimation radius $R_\text{rim,literature}\approx0.3$ au. The complex, temporally variable inner-disc geometry is likely affected or even caused by the close passing (~5 au) and short orbit ($P\approx24$ yr) of the companion HD 142527 B.

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arXiv2025

The complex inner disk of the Herbig Ae star HD 100453 with VLTI/MATISSE

The inner regions of planet-forming disks hold invaluable insights for our understanding of planet formation. The disk around the Herbig star HD 100453 presents one such environment, with an inner disk that is significantly misaligned with respect to the outer disk. This paper expands the existing H-band (PIONIER) and K-band (GRAVITY) interferometric studies of the HD 100453 inner disk to the L-band with the MATISSE VLTI instrument. With snapshot data spanning approximately four years we aim for a more comprehensive understanding of the inner disk structures and their potential time evolution. Based on the MATISSE data obtained, we use a combination of analytical models and image reconstruction to constrain the disk structure. Additionally, we fit a temperature gradient model to the selected wavelength range of PIONIER, GRAVITY and MATISSE to derive physical properties of the inner regions. Our parametric model finds an inclination of $\sim 47.5^\circ$ and a position angle of $\sim 83.6^\circ$, which corroborates the case of strong inner-outer disk misalignment. From the symmetric temperature gradient we derive an inner disk radius around $\sim0.27$ au, with dust surface densities of $\Sigma_{\rm{subl}} \approx 10^{-3.2}$ g/cm$^2$ and vertical optical depth $\tau_{\rm{z, subl}} \approx 0.1-0.06$. Same-night MATISSE and GRAVITY observations indicate the necessity for higher-order asymmetries to explain the interferometric signals, which is further supported by a MATISSE snapshot image reconstruction. The chromatic interferometric data reveal the likely need for higher-order asymmetries to explain the inner disk of HD~100453, suggesting a possible origin in dynamic interactions or disk instabilities. Coordinated multi-wavelength infrared interferometric observations with GRAVITY and MATISSE will be crucial to confirm these findings and uncover its underlying nature.

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arXiv2025

The MATISSE view of the inner region of the RY Tau protoplanetary disk

The T-Tauri type young stellar object RY Tau exhibits a dust depleted inner cavity characteristic of a transition disk. We constrain the spatial distribution and mineralogy of dust in the RY Tau protoplanetary disk in the inner few astronomical units using spectrally resolved interferometric observations in the L, M, and N bands obtained with VLTI/MATISSE. Employing a 2D temperature gradient model we estimate the orientation of the inner disk finding no evidence of significant misalignment between the inner and outer disk of RY Tau. Successively, we analyze the chemical composition of silicates depending on spatial region in the disk and identify several silicate species commonly found in protoplanetary disks. Additionally, a depletion of amorphous dust grains toward the central protostar is observed. Monte Carlo radiative transfer simulations show that hot dust close to the protostar and in the line of sight 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 not visible at shorter wavelengths.

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arXiv2025

Multi-band infrared imaging reveals dusty spiral arcs around the binary B[e] star 3 Puppis

3 Puppis is the brightest known B[e] star. Recent work classifies this A-type object as a supergiant, yet the impact of its binarity on the circumstellar environment (CE) remains hard to characterize. To resolve its dusty region at 5-10 mas, we obtained mid-IR interferometric observations with VLTI/MATISSE over 3-12 {\mu}m. Because the (u,v) coverage supports imaging, we introduce a statistical interferometric-imaging workflow based on MiRA to generate averaged images: this systematic approach enables the selection of an optimal set of reconstructions, improving the robustness and fidelity of the recovered features. We also use SPARCO, an independent tool well suited to bright central objects embedded in fainter extended emission. Images from both tools in the L, M, and N bands agree and reveal an asymmetric, elongated feature ~17 mas (~10 au at 631 pc) southeast of the star with ~20% density contrast. A second northwest asymmetry and a skewed inner rim are detected. Simple geometric modelling, guided by the MATISSE images, constrains the morphology, location, and flux of the CE and its asymmetries. The images are consistent with earlier VLTI measurements but expose a more complex CE with large-scale clumps in the southeast and northwest parts of the disc. Hydrodynamic modelling indicates that tidal spiral-wake perturbations from the central binary, dynamically excited at Lindblad resonances in the circumbinary disc, best explain the radial extent and curvature of the elongated structures seen in all bands.

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arXiv2025

An interferometric mid-infrared study of the eruptive star binary Z CMa with MATISSE/VLTI. I. Imaging the protoplanetary disks during the 2023 outburst

The mid-infrared (MIR) emitting regions of the individual protoplanetary disks in the binary system Z CMa are resolved by MATISSE/VLTI. The observations were obtained during a serendipitous large outburst of the HBe star that lasted more than 100 days, while the FUor companion is presumed to be in quiescence. The size of the MIR-emitting disk region of the more massive HBe star increases toward longer wavelengths from $<14$ mas at 3.5$\mu m$ to $\ll 50$ mas at 11.5$\mu m$ . The lack of substructures in the HBe disk might suggest that it is a continuous disk; however, this could be due to observational constraints. We also note a radial variation of the silicate absorption feature over the disk, where the optical depth increases inwards of $<$40~au radii. This contradicts the scenario of a carved, dusty cocoon surrounding the HBe star. In the case of the less massive FUor companion, the MIR-emitting region is much smaller with an angular size $\leq$15 mas (or else a physical radius $<9$ au) in all bands, suggesting a compact disk. Both disks are aligned within uncertainties, and their orientation agrees with that of the known jets. Furthermore, MATISSE data place the binary's separation at $117.88 \pm 0.73$ mas and a position angle of $139.16^o\,\pm\,0.29^o$ east of north. Our estimates for the orbital elements gave an eccentric orbit ($e\sim0.17$) with a moderate inclination ($i\sim 66$\degr). The derived total mass is $M_{\rm total} = 16.4^{+2.1}_{-2.3}$ M$_\odot$, while the period is approximately 950 years. Our MATISSE imaging of the Herbig disk during outburst indicates a temperature gradient for the disk, while imaging of the FUor companion's disk corroborates previous studies showing that FUor disks are rather compact in the MIR. We cannot infer any misalignment between the MATISSE results and earlier ALMA/JVLA data, nor can we infer any influence from the alleged flyby event.

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arXiv2025

Time-resolved protoplanetary disk physics in DQ Tau with JWST

Accretion variability is ubiquitous in YSOs. While large outbursts may strongly affect the disk, the effects of moderate bursts are less understood. We aim to study the physical response of the disk around the eccentric binary system DQ Tau to its periodic accretion changes. We organized a multi-wavelength campaign centered on four JWST/MIRI spectra. We targeted three periastrons (high accretion state) and one apastron (quiescence). We used optical and near-infrared spectroscopy and photometry to measure how the accretion luminosity varies. We decomposed the multi-epoch SEDs into stellar, accretion, and rim components. We fitted the solid-state features using various opacity curves and the molecular features using slab models. We find the inner disk of DQ Tau to be highly dynamic. The temperature, luminosity, and location of the inner dust rim vary in response to the movement of stars and the L_acc variations. This causes variable shadowing of the outer disk, leading to an anti-correlation between the rim temperature and the strength of the silicate feature. The dust mineralogy remains constant, dominated by large amorphous olivine and pyroxene grains, with smaller fractions of crystalline forsterite. The excitation of CO (1550-2260 K), HCN (880-980 K), and hot H2O (740-860 K) molecules as well as the luminosity of the [NeII] line correlate with the accretion rate, while the warm (650 K) and cold (170-200 K) H2O components are mostly constant. CO emission, originating from a hot (>1500 K) region likely within the dust sublimation radius, is most sensitive to L_acc changes. In comparison with other T Tauri disks, DQ Tau is highly C-poor and displays moderately inefficient pebble drift. We conclude that even moderate accretion rate changes affect the thermal structure in the planet-forming disk regions on short timescales, providing a crucial benchmark for understanding disk evolution.

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arXiv2025

The mid-infrared spectrum of $\beta$ Pictoris b. First VLTI/MATISSE interferometric observations of an exoplanet

Few spectra of directly-imaged exoplanets have been obtained in the mid-infrared (> 3 $\mu$m). This region is particularly rich in molecular spectral signatures, whose measurements can help recover atmospheric parameters and provide a better understanding of giant planet formation and atmospheric dynamics. In the past years, exoplanet interferometry with the VLTI/GRAVITY instrument has provided medium-resolution spectra of a dozen substellar companions in the near infrared. The 100-meter interferometric baselines allow for the stellar and planetary signals to be efficiently disentangled at close angular separations (< 0.3''). We aim to extend this technique to the mid-infrared using MATISSE, the VLTI's mid-infrared spectro-interferometer. We take advantage of the fringe tracking and off-axis pointing capabilities recently brought by the GRA4MAT upgrade. Using this new mode, we observed the giant planet $\beta$ Pictoris b in L and M bands (2.75-5 $\mu$m) at a spectral resolution of 500. We developed a method to correct chromatic dispersion and non-common paths effects in the fringe phase and modelled the planet astrometry and stellar contamination. We obtained a high-signal-to-noise spectrum of $\beta$ Pictoris b, showing the planet continuum in L (for the first time) and M bands, which contains broad absorption features of H$_2$O and CO. In conjunction with a new GRAVITY spectrum, we modelled it with the ForMoSA nested sampling tool and the Exo-REM grid of atmospheric models, and found a solar carbon-to-oxygen ratio in the planet atmosphere. This study opens the way to the characterization of fainter and closer-in planets with MATISSE, which could complement the JWST at angular separations too close for it to obtain exoplanet spectra. Starting in 2025, the new adaptive optics system brought by the GRAVITY+ upgrade will further extend the detection limits of MATISSE.

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arXiv2025

T CrA has a companion: First direct detection of T CrA B with VLTI/MATISSE

T CrA is a Herbig Ae-type young star in a complex circumstellar environment; it includes a circumstellar disk, accretion streamers, jets, and outflows. It has long been suspected to be a binary. However, until now, there has been no direct detection of a companion. Here we present new VLTI/MATISSE L- and N-band observations of T CrA taken between 2023 May and 2024 August with the aim of testing the binary nature of the system. We modeled the data with a geometric model using the Python tool oimodeler. We detected a companion (T CrA B) with a projected separation of $\Delta r = 153.2 \pm 1.2$ mas ($\approx 23$ au) toward the west direction at a position angle of $275.4 \pm 0.1^\circ$, in 2024 May-August. Our results support that the companion has a nearly edge-on orbit that is highly misaligned with respect to the circumprimary disk. Such a configuration could cause warping and tearing of the disk around the primary, which has been proposed by recent studies. In the L band the companion is extended, with a full width at half maximum (FWHM) size of $\sim 1$ au, suggesting that the emission comes from a disk around the secondary star. The companion flux is 0.2-0.3 Jy in the L band, and 0.2-0.7 Jy in the N band, accounting for 4-20% of the total emission at those wavelengths. The SED of the companion is compatible with thermal radiation of warm dust (600-800 K).

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arXiv2024

GRAVITY for MATISSE -- Improving the MATISSE performance with the GRAVITY fringe tracker

Context: MATISSE, the mid-infrared spectro-imaging instrument of VLTI, was designed to deliver its advertised performance when paired with an external second generation fringe tracker. Science observation started in 2019, demonstrating imaging capabilities and faint science target observations. Now, The GRAVITY fringe tracker stabilizes the MATISSE fringes which allows using all spectroscopic modes and improves sensitivity and data accuracy. Aims: We present how the MATISSE and GRAVITY instruments were adapted to make the GRAVITY fringe tracker work with MATISSE, under the umbrella of the aptly-named GRA4MAT project, led by ESO in collaboration with the two instrument consortia. Methods: We detail the software modifications needed to implement an acquisition and observing sequence specific to GRA4MAT, including simultaneous fringe tracking and chopping and a narrow off-axis capability inspired by the galactic center and exoplanet capability of GRAVITY. We explain the modified data collection and reduction processes. We show how we leveraged the recent fringe tracker upgrade to implement features specific to its use with MATISSE, e.g. fringe jumps mitigation with an improved group delay control and simultaneous fringe tracking and chopping with a new state machine. Results: We successfully demonstrate significant improvements to the MATISSE instrument. Observations can now be performed at higher spectral resolutions of up to $R\sim3300$ and across the full LM bands at once. Long detector integration times, made possible with stabilized fringes, have improved the LM-bands sensitivity by a factor of 10. Low flux biases in coherently-reduced N-band data have been eliminated. The L-band transfer function is now higher and more stable. We finally illustrate the scientific potential of GRA4MAT with a preview of the first exoplanet observation made by MATISSE on $\beta$ Pictoris b.

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arXiv2024

Abundant sub-micron grains revealed in newly discovered extreme debris discs

Extreme debris discs (EDDs) are bright and warm circumstellar dusty structures around main sequence stars. They may represent the outcome of giant collisions occuring in the terrestrial region between large planetesimals or planetary bodies, and thus provide a rare opportunity to peer into the aftermaths of these events. Here, we report on results of a mini-survey we conducted with the aim to increase the number of known EDDs, investigate the presence of solid-state features around 10{\mu}m in eight EDDs, and classify them into the silica or silicate dominated groups. We identify four new EDDs and derive their fundamental properties. For these, and for four other previously known discs, we study the spectral energy distribution around 10{\mu}m by means of VLT/VISIR photometry in three narrow-band filters and conclude that all eight objects likely exhibit solid-state emission features from sub-micron grains. We find that four discs probably belong to the silicate dominated subgroup. Considering the age distribution of the entire EDD sample, we find that their incidence begins to decrease only after 300 Myr, suggesting that the earlier common picture that these objects are related to the formation of rocky planets may not be exclusive, and that other processes may be involved for older objects (>100 Myr). Because most of the older EDD systems have wide, eccentric companions, we suggest that binarity may play a role in triggering late giant collisions.

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arXiv2024

Images of Betelgeuse with VLTI/MATISSE across the Great Dimming

From Nov. 2019 to May 2020, the red supergiant star Betelgeuse experienced an unprecedented drop of brightness in the visible domain called the great dimming event. Large atmospheric dust clouds and large photospheric convective features are suspected to be responsible for it. To better understand the dimming event, we used mid-infrared long-baseline spectro-interferometric measurements of Betelgeuse taken with the VLTI/MATISSE instrument before (Dec. 2018), during (Feb. 2020), and after (Dec. 2020) the GDE. We present data in the 3.98 to 4.15\,$\mu$m range to cover SiO spectral features molecules as well as adjacent continuum. We have employed geometrical models, image reconstruction, as well as radiative transfer models to monitor the spatial distribution of SiO over the stellar surface. We find a strongly in-homogeneous spatial distribution of SiO that appears to be looking very different between our observing epochs, indicative of a vigorous activity in the stellar atmosphere. The contrast of our images is small in the pseudo-continuum for all epochs, implying that our MATISSE observations support both cold spot and dust cloud model.

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arXiv2024

Mid-infrared evidence for iron-rich dust in the multi-ringed inner disk of HD 144432

Context. Rocky planets form by the concentration of solid particles in the inner few au regions of planet-forming disks. Their chemical composition reflects the materials in the disk available in the solid phase at the time the planets were forming. Aims. We aim to constrain the structure and dust composition of the inner disk of the young star HD 144432, using an extensive set of infrared interferometric data taken by the Very Large Telescope Interferometer (VLTI), combining PIONIER, GRAVITY, and MATISSE observations. Methods. We introduced a new physical disk model, TGMdust, to image the interferometric data, and to fit the disk structure and dust composition. We also performed equilibrium condensation calculations with GGchem. Results. Our best-fit model has three disk zones with ring-like structures at 0.15, 1.3, and 4.1 au. Assuming that the dark regions in the disk at ~0.9 au and at ~3 au are gaps opened by planets, we estimate the masses of the putative gap-opening planets to be around a Jupiter mass. We find evidence for an optically thin emission ($\tau<0.4$) from the inner two disk zones ($r<4$ au) at $\lambda>3\ \mu$m. Our silicate compositional fits confirm radial mineralogy gradients. To identify the dust component responsible for the infrared continuum emission, we explore two cases for the dust composition, one with a silicate+iron mixture and the other with a silicate+carbon one. We find that the iron-rich model provides a better fit to the spectral energy distribution. Conclusions. We propose that in the warm inner regions ($r<5$ au) of typical planet-forming disks, most if not all carbon is in the gas phase, while iron and iron sulfide grains are major constituents of the solid mixture along with forsterite and enstatite. Our analysis demonstrates the need for detailed studies of the dust in inner disks with new mid-infrared instruments such as MATISSE and JWST/MIRI.

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arXiv2023

The disk of the eruptive protostar V900 Mon; a MATISSE/VLTI and MUSE/VLT perspective

In this work, we study the silicate dust content in the disk of one of the youngest eruptive stars, V900 Mon, at the highest angular resolution probing down to the inner 10 au of said disk, and study the historical evolution of the system traced in part by a newly discovered emission clump. We performed high-angular resolution mid-infrared interferometric observations of V900 Mon with MATISSE/VLTI with a spatial coverage ranging from 38-m to 130-m baselines, and compared them to archival MIDI/VLTI data. We also mined and re-analyzed archival optical and infrared photometry of the star to study its long-term evolution since its eruption in the 1990s. We complemented our findings with integral field spectroscopy data from MUSE/VLT. The MATISSE/VLTI data suggest a radial variation of the silicate feature in the dusty disk, whereby at large spatial scales ($\geq10$ au) the protostellar disk's emission is dominated by large-sized ($\geq1\,\mu m$) silicate grains, while at smaller spatial scales and closer to the star ($\leq5$ au), silicate emission is absent suggesting self-shielding. We propose that the self-shielding may be the result of small dust grains at the base of the collimated CO outflow previously detected by ALMA. A newly discovered knot in the MUSE/VLT data, located at a projected distance approximately 27,000 au from the star, is co-aligned with the molecular gas outflow at a P.A. of $250^o$ ($\pm5^o$) consistent with the position angle and inclination of the disk. The knot is seen in emission in H$\alpha$, [N II], and the [S II] doublet and its kinematic age is about 5150 years. This ejected material could originate from a previous eruption.

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arXiv2023

Brightness and mass accretion rate evolution during the 2022 burst of EX~Lupi

EX Lupi is the prototype by which EXor-type outbursts were defined. It has experienced multiple accretion-related bursts and outbursts throughout the last decades, whose study have greatly extended our knowledge about the effects of these types of events. This star experienced a new burst in 2022. We used multi-band photometry to create color-color and color-magnitude diagrams to exclude the possibility that the brightening could be explained by a decrease in extinction. We obtained VLT/X-shooter spectra to determine the Lacc and Macc during the peak of the burst and after its return to quiescence using 2 methods: empirical relationships between line luminosity and Lacc, and a slab model of the whole spectrum. We examined the 130 year light curve of EX Lupi to provide statistics on the number of outbursts experienced during this period of time. Our analysis of the data taken during the 2022 burst confirmed that a change in extinction is not responsible for the brightening. Our two approaches in calculating the Macc were in agreement, and resulted in values that are 2 orders of magnitude above what had previously been estimated, thus suggesting that EX Lupi is a strong accretor even when in quiescence. We determined that in 2022 March the Macc increased by a factor of 7 with respect to the quiescent level. We also found hints that even though the Macc had returned to almost its pre-outburst levels, certain physical properties of the gas had not returned to the quiescent values. We found that the mass accreted during this three month event was 0.8 lunar masses, which is approximately half of what is accreted during a year of quiescence. We calculated that if EX Lupi remains as active as it has been for the past 130 years, during which it has experienced at least 3 outbursts and 10 bursts, then it will deplete the mass of its circumstellar material in less than 160000 yr.

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arXiv2022

The disk of FU Orionis viewed with MATISSE/VLTI: first interferometric observations in $L$ and $M$ bands

The disk of FU Orionis is marginally resolved with MATISSE, suggesting that the region emitting in the thermal infrared is rather compact. An upper limit of $\sim1.3\pm0.1$ mas (in $L$) can be given for the diameter of the disk region probed in the $L$ band, corresponding to 0.5 au at the adopted Gaia EDR3 distance. This represents the hot, gaseous region of the accretion disk. The $N$-band data indicate that the dusty passive disk is silicate-rich. Only the innermost region of said dusty disk is found to emit strongly in the $N$ band, and it is resolved at an angular size of $\sim5$ mas, which translates to a diameter of about 2 au. The observations therefore place stringent constraints for the outer radius of the inner accretion disk. Dust radiative transfer simulations with RADMC-3D provide adequate fits to the spectral energy distribution from the optical to the submillimeter and to the interferometric observables when opting for an accretion rate $\dot{M}\sim 2\times 10^{-5}\, M_\odot$ yr$^{-1}$ and assuming $M_*=0.6\, M_\odot$. Most importantly, the hot inner accretion disk's outer radius can be fixed at 0.3 au. The outer radius of the dusty disk is placed at 100 au, based on constraints from scattered-light images in the literature. The dust mass contained in the disk is $2.4\times10^{-4}\, M_\odot$, and for a typical gas-to-dust ratio of 100, the total mass in the disk is approximately 0.02 $M_\odot$. We did not find any evidence for a nearby companion in the current interferometric data, and we tentatively explored the case of disk misalignment. For the latter, our modeling results suggest that the disk orientation is similar to that found in previous imaging studies by ALMA. Should there be an asymmetry in the very compact, inner accretion disk, this might be resolved at even smaller spatial scales ($\leq1$ mas).

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arXiv2021

The extended atmosphere and circumstellar environment of the cool evolved star VX Sagittarii as seen by MATISSE

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.

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