Very Large Telescope Interferometer Research Articles

Signatures of the accelerating black holes with a cosmological constant from the Sgr [formula omitted] and [formula omitted] shadow prospects

Recently, the Event Horizon Telescope (EHT) achieved the realization of an image of the supermassive black hole Sgr A⋆ showing an angular shadow diameter D=48.7±7μas and the fractional deviation δ=−0.08−0.09+0.09(VLTI),−0.04−0.10+0.09(Keck), alongside the earlier image of M87⋆ with angular diameter D=42±3μas, deviation δ=−0.01−0.17+0.17 and deviations from circularity estimated to be ΔC≲10%. In addition, the shadow radii are assessed within the ranges 3.38≤rsM≤6.91 for M87⋆ and 3.85≤rsM≤5.72 as well as 3.95≤rsM≤5.92 for Sgr A⋆ using the Very Large Telescope Interferometer (VLTI) and Keck observatories, respectively. These values are provided with 1-σ and 2-σ measurements. Such realizations can unveil a better comprehension of gravitational physics at the horizon scale. In this paper, we use the EHT observational results for M87⋆ and Sgr A⋆ to elaborate the constraints on parameters of accelerating black holes with a cosmological constant. Concretely, we utilize the mass and distance of both black holes to derive the observables associated with the accelerating black hole shadow.First, we compare our findings with observed quantities such as angular diameter, circularity, shadow radius, and the fractional deviation from the M87⋆ data. This comparison reveals constraints within the acceleration parameter and the cosmological constant. In fact, the acceleration parameter A is found to be in the interval [0,0.475−0.0125+0.0125], the cosmological constant spans the range of Λ∈[−14.5801−1.1051×10−6+1.1051×10−6,(1.0086×10−6)−1.5396×10−4+9.2121×10−5]. Then by considering the Sgr A⋆ data, the acceleration scheme persists with the same interval range, while, the cosmological constant shifts very slightly to the following domain [−14.7266−1.1051×10−6+1.1051×10−6,(4.6200×10−5)−1.5888×10−4+6.7890×10−5] for the VLTI observatory. Moreover, within the Keck measurements, one can find Λ∈[−14.7266−1.1051×10−6+1.1051×10−6,(2.4149×10−5)−1.5094×10−4+7.2836×10−5]. Besides, for the others parameters, both black holes reveal the range of the spin parameter a in [0,1] and electrical/magnetic charges are {e,g}∈[0,0.09].Further, only the cosmological constant intervals are altered by rsM measurements, precisely within the M87⋆ data, the upper bound shifts to (Λ1-σ=1.1361×10−4,Λ2-σ=1.7822×10−4), the VLTI gives (Λ1-σ=1.5457×10−4,Λ2-σ=1.8208×10−4), while the Keck results uncover Λ2-σ=1.8015×10−4.Lastly, one cannot rule out the possibility of the negative values for the cosmological constant on the emergence of accelerated black hole solutions within the context of minimal gauged supergravity. This intriguing possibility raises the prospect of such solutions being plausible candidates for astrophysical black holes.

Chromatically modeling the parsec-scale dusty structure in the center of NGC 1068

The Very Large Telescope Interferometer (VLTI) has been providing breakthrough images of the dust in the central parsecs of active galactic nuclei (AGNs), which is thought to be a key component of the AGN unification scheme and AGN host galaxy interaction. In single infrared bands, these images can enjoin multiple interpretations, some of which could challenge the unification scheme. This is the case for the archetypal type 2 AGN of NGC\,1068, whereby the degeneracy is reduced by multi-band temperature maps that are hindered by an ambiguity in the alignment between different single-band images. We aim to solve this problem by creating a chromatic model capable of simultaneously explaining the VLTI GRAVITY+MATISSE $2\ observations of AGNs hosted by NGC\,1068. We employed a simple disk and wind geometry populated by spherical black-body emitters and dust obscuration to create a versatile multi-wavelength modelling method for chromatic IR interferometric data of dusty objects. This simple geometry is capable of reproducing the spectro-interferometric data of NGC\,1068 from the K through N bands. It explains the complex single band images with obscuration and inclination effects, and it solves the alignment problem between bands. We find that the resulting model disk and wind geometry is consistent with previous studies of comparable and larger scales. For example, compared to molecular gas emission, our model wind position angle (PA) of $23^2_2 is close to the mas-scale outflowing CO(6--5) PA of $ seen with ALMA . The equivalent $90^ offset model disk PA is also consistent with the CO(6--5) disk axis of $112^ as well as the mas-scale disk axis from CO(2--1), CO(3--2), and HCO$^+$(4--3) of $115 circ$ . Furthermore, the resulting model images visually resemble the multiple achromatic image reconstructions of the same data when evaluated at the same wavelengths. We conclude that the IR emitting structure surrounding the AGN within NGC\,1068 can indeed be explained by the clumpy disk+wind iteration of the AGN unification scheme. Within the scheme, we find that it is best explained as a type 2 AGN and the obscuring dust chemistry can be explained by a mix of olivine silicates and $16 amorphous carbon.

Heimdallr, Baldr, and Solarstein: designing the next generation of VLTI instruments in the Asgard suite

High angular resolution imaging is an increasingly important capability in contemporary astrophysics. Of particular relevance to emerging fields such as the characterization of exoplanetary systems, imaging at the required spatial scales and contrast levels results in forbidding challenges in the correction of atmospheric phase errors, which in turn drives demanding requirements for precise wavefront sensing. Asgard is the next-generation instrument suite at the European Southern Observatory’s Very Large Telescope Interferometer (VLTI), targeting advances in sensitivity, spectral resolution, and nulling interferometry. In this paper, we describe the requirements and designs of three core modules: Heimdallr, a beam combiner for fringe tracking, low order wavefront correction, and visibility science; Baldr, a Zernike wavefront sensor to correct high order atmospheric aberrations; and Solarstein, an alignment and calibration unit. In addition, we draw generalizable insights for designing such system and discuss integration plans.

Spatially resolving the AGB star V3 in the metal-poor globular cluster 47 Tuc with VLTI/GRAVITY

Context. Mass loss at the asymptotic giant branch (AGB) plays an important role not only in the final fates of stars, but also in the chemical evolution of galaxies. Nevertheless, the metallicity effects on AGB mass loss are not yet fully understood. Aims. We present spatially resolved observations of an AGB star, V3, in the metal-poor globular cluster 47 Tuc (NGC 104). Methods. The AGB star 47 Tuc V3 was observed using the GRAVITY instrument at ESO’s Very Large Telescope Interferometer (VLTI) at 2–2.45 μm, with a projected baseline length of up to 96 m. Results. The object 47 Tuc V3 has been spatially resolved and stands as the first to attempt to spatially resolve an individual star in a globular cluster. The uniform-disk fit to the observed data results in an angular diameter of ∼0.7 mas. Our modeling of the spectral energy distribution and near-infrared interferometric GRAVITY data suggests that the observed data can be explained by an optically thin dust shell with a 0.55 μm optical depth of 0.05–0.25, consisting of metallic iron grains, likely together with effects of the extended atmosphere of the central star. The dust temperature at the inner shell boundary is 500–800 K (corresponding to 23–90 stellar radii), significantly lower than observed in nearby oxygen-rich AGB stars. Radiation pressure on small (< 0.05 μm) iron grains is not sufficient to drive stellar winds. Therefore, iron grains may grow to larger sizes, even in the metal-poor environment. Alternatively, it is possible that the observed iron grain formation is a result of the mass outflow initiated by some other mechanism(s). Conclusions. The sensitivity and angular resolution of VLTI provides a new window onto spatially resolving individual stars in metal-poor globular clusters. This allows us to improve subsequent studies of the metallicity dependence of dust formation and mass loss.

The size-luminosity relation of local active galactic nuclei from interferometric observations of the broad-line region

By using the GRAVITY instrument with the near-infrared (NIR) Very Large Telescope Interferometer (VLTI), the structure of the broad (emission-)line region (BLR) in active galactic nuclei (AGNs) can be spatially resolved, allowing the central black hole (BH) mass to be determined. This work reports new NIR VLTI/GRAVITY interferometric spectra for four type 1 AGNs (Mrk 509, PDS 456, Mrk 1239, and IC 4329A) with resolved broad-line emission. Dynamical modelling of interferometric data constrains the BLR radius and central BH mass measurements for our targets and reveals outflow-dominated BLRs for Mrk 509 and PDS 456. We present an updated radius-luminosity (R-L) relation independent of that derived with reverberation mapping (RM) measurements using all the GRAVITY-observed AGNs. We find our R-L relation to be largely consistent with that derived from RM measurements except at high luminosity, where BLR radii seem to be smaller than predicted. This is consistent with RM-based claims that high Eddington ratio AGNs show consistently smaller BLR sizes. The BH masses of our targets are also consistent with the standard $M_ BH relation. Model-independent photocentre fitting shows spatial offsets between the hot dust continuum and the BLR photocentres (ranging from sim 17 mu as to 140 mu as) that are generally perpendicular to the alignment of the red- and blueshifted BLR photocentres. These offsets are found to be related to the AGN luminosity and could be caused by asymmetric K-band emission of the hot dust, shifting the dust photocentre. We discuss various possible scenarios that can explain this phenomenon.

VLTI/GRAVITY Provides Evidence the Young, Substellar Companion HD 136164 Ab Formed Like a “Failed Star”

Young, low-mass brown dwarfs orbiting early-type stars, with low mass ratios (q ≲ 0.01), appear to be intrinsically rare and present a formation dilemma: could a handful of these objects be the highest-mass outcomes of “planetary” formation channels (bottom up within a protoplanetary disk), or are they more representative of the lowest-mass “failed binaries” (formed via disk fragmentation or core fragmentation)? Additionally, their orbits can yield model-independent dynamical masses, and when paired with wide wavelength coverage and accurate system age estimates, can constrain evolutionary models in a regime where the models have a wide dispersion depending on the initial conditions. We present new interferometric observations of the 16 Myr substellar companion HD 136164 Ab (HIP 75056 Ab) made with the Very Large Telescope Interferometer (VLTI)/GRAVITY and an updated orbit fit including proper motion measurements from the Hipparcos–Gaia Catalog of Accelerations. We estimate a dynamical mass of 35 ± 10 M J (q ∼ 0.02), making HD 136164 Ab the youngest substellar companion with a dynamical mass estimate. The new mass and newly constrained orbital eccentricity (e = 0.44 ± 0.03) and separation (22.5 ± 1 au) could indicate that the companion formed via the low-mass tail of the initial mass function. Our atmospheric fit to a SPHINX M-dwarf model grid suggests a subsolar C/O ratio of 0.45 and 3 × solar metallicity, which could indicate formation in a circumstellar disk via disk fragmentation. Either way, the revised mass estimate likely excludes bottom-up formation via core accretion in a circumstellar disk. HD 136164 Ab joins a select group of young substellar objects with dynamical mass estimates; epoch astrometry from future Gaia data releases will constrain the dynamical mass of this crucial object further.

Polarization analysis of the VLTI and GRAVITY

Aims. The goal of this work is to characterize the polarization effects of the beam path of the Very Large Telescope Interferometer (VLTI) and the GRAVITY beam combiner instrument. This is useful for two reasons: to calibrate polarimetric observations with GRAVITY for instrumental effects and to understand the systematic error introduced to the astrometry due to birefringence when observing targets with a significant intrinsic polarization. Methods. By combining a model of the VLTI light path and its mirrors and dedicated experimental data, we constructed a full polarization model of the VLTI Unit Telescopes (UTs) and the GRAVITY instrument. We first characterized all telescopes together to construct a universal UT calibration model for polarized targets with the VLTI. We then expanded the model to include the differential birefringence between the UTs. With this, we were able to constrain the systematic errors and the contrast loss for highly polarized targets. Results. Along with this paper, we have published a standalone Python package that can be used to calibrate the instrumental effects on polarimetric observations. This enables the community to use GRAVITY with the UTs to observe targets in a polarimetric observing mode. We demonstrate the calibration model with the Galactic Center star IRS 16C. For this source, we were able to constrain the polarization degree to within 0.4% and the polarization angle to within 5° while being consistent with the literature values. Furthermore, we show that there is no significant contrast loss, even if the science and fringe-tracker targets have significantly different polarization, and we determine that the phase error in such an observation is smaller than 1°, corresponding to an astrometric error of 10 µas. Conclusions. With this work, we enable the use by the community of the polarimetric mode with GRAVITY/UTs and outline the steps necessary to observe and calibrate polarized targets with GRAVITY. We demonstrate that it is possible to measure the intrinsic polarization of astrophysical sources with high precision and that polarization effects do not limit astrometric observations of polarized targets.

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. Studying the dust before it gets incorporated in planets provides a valuable diagnostic for the material composition. Aims. We aim to constrain the structure and dust composition of the inner disk of the young Herbig Ae 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 to assess the hidden diversity of minerals occurring in a planet-forming disk such as HD 144432. 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 (τ < 0.4) from the inner two disk zones (r < 4 au) at λ > 3 µm. Our silicate compositional fits confirm radial mineralogy gradients, as for the mass fraction of crystalline silicates we get around 61% in the innermost zone (r < 1.3 au), mostly from enstatite, while only ~20% in the outer two zones. 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. Our GGchem calculations also support an iron-rich and carbon-poor dust composition in the warm disk regions (r < 5 au, T > 300 K). 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.

Single-mode waveguides for GRAVITY

The second generation Very Large Telescope Interferometer (VLTI) instrument GRAVITY is a two-field infrared interferometer operating in the K band between 1.97 and 2.43 µm with either the four 8 m or the four 1.8 m telescopes of the Very Large Telescope (VLT). Beams collected by the telescopes are corrected with adaptive optics systems and the fringes are stabilized with a fringe-tracking system. A metrology system allows the measurement of internal path lengths in order to achieve high-accuracy astrometry. High sensitivity and high interferometric accuracy are achieved thanks to (i) correction of the turbulent phase, (ii) the use of low-noise detectors, and (iii) the optimization of photometric and coherence throughput. Beam combination and most of the beam transport are performed with single-mode waveguides in vacuum and at low temperature. In this paper, we present the functions and performance achieved with weakly birefringent standard single-mode fiber systems in GRAVITY. Fibered differential delay lines (FDDLs) are used to dynamically compensate for up to 6 mm of delay between the science and reference targets. Fibered polarization rotators allow us to align polarizations in the instrument and make the single-mode beam combiner close to polarization neutral. The single-mode fiber system exhibits very low birefringence (less than 23°), very low attenuation (3.6–7 dB km−1 across the K band), and optimized differential dispersion (less than 2.04 µrad cm2 at zero extension of the FDDLs). As a consequence, the typical fringe contrast losses due to the single-mode fibers are 6% to 10% in the lowest-resolution mode and 5% in the medium- and high-resolution modes of the instrument for a photometric throughput of the fiber chain of the order of 90%. There is no equivalent of this fiber system to route and modally filter beams with delay and polarization control in any other K-band beamcombiner.

Polarimetry and astrometry of NIR flares as event horizon scale, dynamical probes for the mass of Sgr A*

We present new astrometric and polarimetric observations of flares from Sgr A* obtained with GRAVITY, the near-infrared interferometer at ESO’s Very Large Telescope Interferometer (VLTI), bringing the total sample of well-covered astrometric flares to four and polarimetric flares to six. Of all flares, two are well covered in both domains. All astrometric flares show clockwise motion in the plane of the sky with a period of around an hour, and the polarization vector rotates by one full loop in the same time. Given the apparent similarities of the flares, we present a common fit, taking into account the absence of strong Doppler boosting peaks in the light curves and the EHT-measured geometry. Our results are consistent with and significantly strengthen our model from 2018. First, we find that the combination of polarization period and measured flare radius of around nine gravitational radii (9Rg ≈ 1.5RISCO, innermost stable circular orbit) is consistent with Keplerian orbital motion of hot spots in the innermost accretion zone. The mass inside the flares’ radius is consistent with the 4.297 × 106 M⊙ measured from stellar orbits at several thousand Rg. This finding and the diameter of the millimeter shadow of Sgr A* thus support a single black hole model. Second, the magnetic field configuration is predominantly poloidal (vertical), and the flares’ orbital plane has a moderate inclination with respect to the plane of the sky, as shown by the non-detection of Doppler-boosting and the fact that we observe one polarization loop per astrometric loop. Finally, both the position angle on the sky and the required magnetic field strength suggest that the accretion flow is fueled and controlled by the winds of the massive young stars of the clockwise stellar disk 1–5″ from Sgr A*, in agreement with recent simulations.

High-contrast detection of exoplanets with a kernel-nuller at the VLTI

Context. The conventional approach to direct imaging is to use a single aperture coronagraph with wavefront correction via extreme adaptive optics (AO). Such systems are limited to observing beyond an inner working angle (IWA) of a few λ/D. Nulling interferometry with two or more apertures will enable detections of companions at separations at and beyond the formal diffraction limit. Aims. In this paper, we evaluate the astrophysical potential of a kernel-nuller as the prime high-contrast imaging mode of the Very Large Telescope Interferometer (VLTI). Methods. By taking into account baseline projection effects induced by Earth rotation, we introduce some diversity in the response of the nuller as a function of time. This response is depicted by transmission maps. We also determine whether we can extract the astrometric parameters of a companion from the kernel outputs, which are the primary intended observable quantities of the kernel-nuller. This then leads us to comment on the characteristics of a possible observing program for the discovery of exoplanets. Results. We present transmission maps for both the raw nuller outputs and their subsequent kernel outputs. To further examine the properties of the kernel-nuller, we introduce maps of the absolute value of the kernel output. We also identify 38 targets for the direct detection of exoplanets with a kernel-nuller at the focus of the VLTI. Conclusions. With continued upgrades of the VLTI infrastructure, which will reduce fringe tracking residuals, a kernel-nuller would enable the detection of young giant exoplanets at separations <10 AU, where radial velocity and transit methods are more sensitive.

Advances in Optical/Infrared Interferometry

After decades of fast-paced technical advances, optical/infrared (O/IR) interferometry has seen a revolution in recent years: ▪ The GRAVITY instrument at the Very Large Telescope Interferometer (VLTI) with four 8-m telescopes reaches thousand-times-fainter objects than possible with earlier interferometers, and the Center for High Angular Resolution Astronomy array (CHARA) routinely offers up to 330-m baselines and aperture synthesis with six 1-m telescopes. ▪ The observed objects are fainter than 19 mag, the images have submilliarcsecond resolution, and the astrometry reaches microarcsecond precision. ▪ This led to breakthrough results on the Galactic Center, exoplanets, active galactic nuclei, young stellar objects, and stellar physics. Following a primer in interferometry, we summarize the advances that led to the performance boost of modern interferometers: ▪ Single-mode beam combiners now combine up to six telescopes, and image reconstruction software has advanced over earlier developments for radio interferometry. ▪ With a combination of large telescopes, adaptive optics (AO), fringe tracking, and especially dual-beam interferometry, GRAVITY has boosted the sensitivity by many orders of magnitude. Another order-of-magnitude improvement will come from laser guide star AO. In combination with large separation fringe tracking, O/IR interferometry will then provide complete sky coverage for observations in the Galactic plane and substantial coverage for extragalactic targets.

Strong lensing and shadow of Ayon-Beato–Garcia (ABG) nonsingular black hole

We study nonsingular black holes viewed from the point of view of Ayon-Beato–Garcia (ABG) nonlinear electrodynamics (NLED) and present a complete study of their corresponding strong gravitational lensing. The NLED modifies the the photon’s geodesic, and our calculations show that such effect increases the corresponding photon sphere radius and image separation, but decreases the magnification. We also show that the ABG’s shadow radius is not compatible with bound estimates of Sgr A* from Keck and VLTI (Very Large Telescope Interferometer). Thus, the possibility of Sgr A* being a nonsingular ABG black hole is ruled out.

The GRAVITY young stellar object survey

Context. T Tauri stars are known to be the cradle of planet formation. Most exoplanets discovered to date lie at the very inner part of the circumstellar disk (<1 au). The innermost scale of young stellar objects is therefore a compelling region to be addressed, and long-baseline interferometry is a key technique to unveil their mysteries. Aims. We aim to spatially and spectrally resolve the innermost scale (≤1 au) of the young stellar system CI Tau to constrain the inner disk properties and better understand the magnetospheric accretion phenomenon. Methods. The high sensitivity offered by the combination of the four 8-m class telescopes of the Very Large Telescope Interferometer (VLTI) allied with the high spectral resolution (R ~ 4000) of the K-band beam combiner GRAVITY offers a unique capability to probe the sub-au scale of the CI Tau system, tracing both dust (continuum) and gas (Brγ line) emission regions. We developed a physically motivated geometrical model to fit the interferometric observables – visibilities and closure phases (CP) – and constrained the physical properties of the inner dusty disk. The continuum-corrected pure line visibilities have been used to estimate the size of the Hydrogen I Brγ emitting region. Results. From the K-band continuum study, we report a highly inclined (i ~ 70°) resolved inner dusty disk, with an inner edge located at a distance of 21 ± 2 R★ from the central star, which is significantly larger than the dust sublimation radius (Rsub = 4.3 to 8.6 R★). The inner disk appears misaligned compared to the outer disk observed by ALMA and the non-zero closure phase indicates the presence of an asymmetry that could be reproduced with an azimuthally modulated ring with a brighter south-west side. From the differential visibilities across the Brγ line, we resolved the line-emitting region, and measured a size of 4.8- 1.0+ 0.8 R★. Conclusions. The extended inner disk edge compared to the dust sublimation radius is consistent with the claim of an inner planet, CI Tau b, orbiting close in. The inner-outer disk misalignment may be induced by gravitational torques or magnetic warping. The size of the Brγ emitting region is consistent with the magnetospheric accretion process. Assuming it corresponds to the magnetospheric radius, it is significantly smaller than the co-rotation radius (Rcor= 8.8 ± 1.3 R★), which suggests an unstable accretion regime that is consistent with CI Tau being a burster.

The effect of winds on atmospheric layers of red supergiants

Context. 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, and they remain one of the key questions in stellar astrophysics. Based on observations of α Ori, a new semi-empirical method to add a wind to hydrostatic model atmospheres of RSGs was recently developed. This method can reproduce many of the static molecular shell (or ‘MOLsphere’) spectral features. Aims. We used this method of adding a semi-empirical 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 interferometric data of HD 95687 and V602 Car obtained with the AMBER instrument at the Very Large Telescope Interferometer (VLTI). Methods. We computed model intensities with respect to the line-of-sight angle (µ) for different mass-loss rates, spectra, and visibilities using the radiative transfer code TURBOSPECTRUM. We were able to convolve the models to match the different spectral resolutions of the VLTI instruments, studying a wavelength range of 1.8–5 µm corresponding to the K, L, and M bands for GRAVITY and MATISSE data. The model spectra and squared visibility amplitudes were compared with the published VLTI/AMBER data. Results. 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 onto the MARCS model with simple radiative equilibrium dramatically improves the agreement with the squared visibility amplitudes as well as the spectra, with the fit being even better when applying a steeper density profile than predicted from previous studies. Our results reproduce observed extended atmospheres up to several stellar radii. Conclusions. 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 a 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 observations. We provide temperature and density stratifications that succeed, for the first time, in reproducing observed interferometric properties of RSG atmospheres.

Spiral arms in broad-line regions of active galactic nuclei

As a major feature of the spectra of active galactic nuclei, broad emission lines deliver information on the kinematics and spatial distributions of ionized gas located in the so-called broad-line regions (BLRs) surrounding the central supermassive black holes (SMBHs). There is growing evidence for the appearance of spiral arms in BLRs. It has been shown through reverberation mapping (RM) campaigns that the characterized radius of BLRs overlaps with that of self-gravitating regions of accretion disks. In the framework of the WKB approximation, we show robust properties of observational features of the spiral arms. The resulting spiral arms lead to the characterization of various profiles of the broad emission line. We calculated the RM and differential interferometric features of BLRs withm = 1 mode spiral arms. These features can be detected with high-quality RM and differential interferometric observations via such instruments as GRAVITY on board the Very Large Telescope Interferometer (VLTI). The WKB approximation will be relaxed and universalized in the future to explore more general cases of density wave signals in RM campaigns and differential spectroastrometric observations.

First light for GRAVITY Wide

GRAVITY+ is the upgrade for GRAVITY and the Very Large Telescope Interferometer (VLTI) with wide-separation fringe tracking, new adaptive optics, and laser guide stars on all four 8 m Unit Telescopes (UTs) to enable ever-fainter, all-sky, high-contrast, milliarcsecond interferometry. Here we present the design and first results of the first phase of GRAVITY+, known as GRAVITY Wide. GRAVITY Wide combines the dual-beam capabilities of the VLTI and the GRAVITY instrument to increase the maximum separation between the science target and the reference star from 2 arcseconds with the 8 m UTs up to several 10 arcseconds, limited only by the Earth’s turbulent atmosphere. This increases the sky-coverage of GRAVITY by two orders of magnitude, opening up milliarcsecond resolution observations of faint objects and, in particular, the extragalactic sky. The first observations in 2019–2022 include the first infrared interferometry of two redshift z ~ 2 quasars, interferometric imaging of the binary system HD 105913A, and repeat observations of multiple star systems in the Orion Trapezium Cluster. We find the coherence loss between the science object and fringe-tracking reference star well described by the turbulence of the Earth’s atmosphere. We confirm that the larger apertures of the UTs result in higher visibilities for a given separation due to the broader overlap of the projected pupils on the sky and provide predictions for visibility loss as a function of separation to be used for future planning.

The dusty heart of Circinus

Context. Active galactic nuclei play a key role in the evolution of galaxies, but their inner workings and physical connection to the host are poorly understood due to a lack of angular resolution. Infrared interferometry makes it possible to resolve the circumnuclear dust in the nearby Seyfert 2 galaxy, the Circinus Galaxy. Previous observations have revealed complex structures and polar dust emission but interpretation was limited to simple models. The new Multi AperTure mid-Infrared Spectro-Scopic Experiment (MATISSE) makes it possible to image these structures for the first time. Aims. We aim to precisely map the morphology and temperature of the dust surrounding the supermassive black hole through interferometric imaging. Methods. We observed the Circinus Galaxy with MATISSE at the Very Large Telescope Interferometer (VLTI), producing 150 correlated flux spectra and 100 closure phase spectra. The novel inclusion of closure phases makes interferometric imaging possible for the first time. We reconstructed images in the N-band at ∼10 mas resolution. We fit blackbody functions with dust extinction to several aperture-extracted fluxes from the images to produce a temperature distribution of central dusty structures. Results. We find significant substructure in the circumnuclear dust: central unresolved flux of ∼0.5 Jy, a thin disk 1.9 pc in diameter oriented along ∼45°, and a ∼4 × 1.5 pc polar emission extending orthogonal to the disk. The polar emission exhibits patchiness, which we attribute to clumpy dust. Flux enhancements to the east and west of the disk are seen for the first time. We distinguish the temperature profiles of the disk and of the polar emission: the disk shows a steep temperature gradient indicative of denser material; the polar profile is flatter, indicating clumpiness and/or lower dust density. The unresolved flux is fitted with a high temperature, ∼370 K. The polar dust remains warm (∼200 K) out to 1.5 pc from the disk. We attribute approximately 60% of the 12 μm flux to the polar dust, 10% to the disk, and 6% is unresolved; the remaining flux was resolved out. The recovered morphology and temperature distribution resembles modeling of accretion disks with radiation-driven winds at large scales, but we placed new constraints on the subparsec dust. Conclusions. The spatially resolved subparsec features imaged here place new constraints on the physical modeling of circumnuclear dust in active galaxies; we show strong evidence that the polar emission consists of dust clumps or filaments. The dynamics of the structures and their role in the Unified Model remain to be explored.

Detection of eccentric close-binary supermassive black holes with incomplete interferometric data

Context.Recent studies have proposed that General Relativity Analysis via VLT InTerferometrY upgrade (GRAVITY+) on board the Very Large Telescope Interferometer (VLTI) is able to trace the circular orbit of the subparsec (≲0.1 pc) close-binary supermassive black holes (CB-SMBHs) by measuring the photo-centre variation of the hot dust emission. However, the CB-SMBHs orbit may become highly eccentric throughout the evolution of these objects, and the orbital period may be far longer than the observational time baseline.Aims.We investigate the problem of detecting the CB-SMBH with hot dust emission and high eccentricity (eCB-SMBH,e= 0.5) when the observed time baselines of their astrometric data and radial velocities are considerably shorter than the orbital period.Methods.The parameter space of the Keplerian model of theeCB-SMBH is large for exploratory purposes. We therefore applied the Bayesian method to fit orbital elements of theeCB-SMBH to combine radial velocity and astrometric data covering a small fraction of the orbital period.Results.We estimate that a number of potentialeCB-SMBH systems within reach of GRAVITY+ will be similar to the number of planned circular targets. We show that using observational time baselines that cover ≳10% of the orbit increases the possibility of determining the period, eccentricity, and total mass of aneCB-SMBH. When the observational time baseline becomes too short (~5%), the quality of the retrievedeCB-SMBH parameters degrades. We also illustrate how interferometry may be used to estimate the photo-centre at theeCB-SMBH emission line, which could be relevant for GRAVITY+ successors. Even if the astrometric signal foreCB-SMBH systems is reduced by a factor of√(1 - e2)compared to circular ones, we find that the hot dust emission ofeCB-SMBHs can be traced by GRAVITY+ at the elementary level.

MATISSE, the VLTI mid-infrared imaging spectro-interferometer

Context.Optical interferometry is at a key development stage. The Very Large Telescope Interferometer (VLTI) has established a stable, robust infrastructure for long-baseline interferometry that is usable by general astronomical observers. The present second-generation instruments offer a wide wavelength coverage and improved performance. Their sensitivity and measurement accuracy lead to data and images of high reliability.Aims.We have developed the Multi AperTure mid-Infrared SpectroScopic Experiment (MATISSE) to access, for the first time, high resolution imaging in a wide spectral domain. Many front-line topics are explored with this new equipment, including: stellar activity and mass loss; planet formation and evolution in the gas and dust disks around young stars; and environment interaction and accretion processes around super massive black holes in active galactic nuclei.Methods.The instrument is a spectro-interferometric imager in the transmission windows calledL,M, andN, from 2.8 to 13.0 microns, combining four optical beams from the VLTI’s unit or auxiliary telescopes. Its concept, related observing procedure, data reduction, and calibration approach, is the product of 30 years of instrumental research and has benefitted from the expertise developed in the frame of the VLTI’s first generation instruments. The instrument utilises a multi-axial beam combination that delivers spectrally dispersed fringes. The signal provides the following quantities at several spectral resolutions: photometric flux, coherent fluxes, visibilities, closure phases, wavelength differential visibilities and phases, and aperture-synthesis imaging.Results.This article provides an overview of the physical principle of the instrument and its functionalities. The motivation of the choice of the instrumental concept and the characteristics of the delivered signal are detailed with a description of the observing modes and of their performance limit. MATISSE offers four spectral resolutions inL&Mbands, namely 30, 500, 1000 and 3400, and 30 and 220 in theNband, and it provides an angular resolution down to 3 mas for the shortest wavelengths. The MATISSE stand-alone sensitivity limits are 60 mJy inLand 300 mJy inN. The paper gives details of the sensitivity limits for the different measurables and their related precision criteria, considering telescope configurations and spectral resolutions. We also discuss the gain provided with the GRA4MAT fringe tracker. An ensemble of data and reconstructed images illustrate the first acquired key observations.Conclusions.The instrument has been in operation at Cerro Paranal, ESO, Chile, since 2018, and has been open for science use by the international community since April 2019. The first scientific results are being published now.