arXiv subjects

Sylvestre Lacour

Publications and source records attributed to Sylvestre Lacour.

At least 19 recordsLinked to original sources

arXiv2026

MATISSE in the era of GRAVITY+

MATISSE, the mid-infrared spectro-interferometric imager of the Very Large Telescope Interferometer (VLTI), has been feeding on the light corrected by the GRAVITY+ Adaptive Optics (GPAO) systems since the retirement of MACAO on the four Unit Telescopes in 2024. We report the assessment of the new MATISSE performance with GPAO, based on commissioning observations obtained in 2024 and 2025 and compared with archival MACAO observations of the same stellar sources. The instrumental visibility transfer function is more stable in the L band, while its typical value in the L and N bands is unchanged; the visibility signal-to-noise ratio is consistent with, or better than, the values reported at the initial MATISSE commissioning. The correlated flux increases by 20--30\% in GRA4MAT mode thanks to a better fringe tracking performance, and the total flux injected into the instrument increases by a factor of 1.3 to 1.4 (up to 1.7) in the L band at low spectral resolution, with no measurable loss in the N band. Combined with the new K-band fringe-tracking limits enabled by GPAO, this translates into a sensitivity gain of a factor 2 to 6 in the L and M bands, a confirmation of the N-band limits similar to the MACAO era. We then report the performances of MATISSE with the GPAO's laser guide stars in November and December 2025. The updated noise figures follow the exposure time calculator predictions in LM LOW and MED and come out better than them in N LOW, the L-band transfer function is stable at the few percent level between observing blocks an hour apart, and correlated fluxes, differential phases and closure phases are recovered down to a 10\,mJy target. Embedded young stellar objects and AGN are now within reach, thanks to the better performances on red targets brought by the lasers. We finally discuss the science cases opened by this gain and the case for a future upgrade of the instrument.

astro-ph.CO↗

arXiv2026

Instrumental artifacts in photonic lantern spectroastrometry and their mitigation with PLred

Spectroastrometry is a powerful spectral-differential technique for probing angular scales below the resolution limit, but it is also well known to be susceptible to instrumental artifacts that can mimic or obscure real signals. In photonic lantern spectroastrometry, recently demonstrated on-sky with Subaru/FIRST-PL, the astrometric signal is encoded in relative flux variations between lantern outputs rather than in centroid shifts along a slit. This changes the artifact landscape: some slit-based spectroastrometric artifacts are avoided, but new artifact mechanisms emerge, including detector nonlinearity and spectral extraction errors, which can produce spurious features on emission or absorption lines. These lessons directly informed the design of PLred, an open source Python package for photonic lantern data reduction and instrument-agnostic spectral-differential image reconstruction. We describe the origin of these artifacts and the key pipeline design choices used to mitigate them.

astro-ph.IM↗

arXiv2026

He'e-Lab: A modular testbed for astrophotonics and wavefront sensing development

Advanced astronomical instrumentation requires accessible, reconfigurable platforms to validate novel technologies and algorithms before on-sky deployment. We present the design, architecture, and alignment validation of the Hawaii Experimental Engineering Lab (He'e-Lab), a state-of-the-art modular testbed dedicated to two complementary research tracks: (A) the integration and characterization of astrophotonics components within a real-time computing loop, and (B) the development of advanced wavefront sensing and control (WFS&C) algorithms. The testbed features a broadband supercontinuum source (500 nm to 2 microns), a high-order 1k-actuator Boston Micromachines deformable mirror, and a 37-segment hexagonal mirror assembly providing piston-tip-tilt control to emulate segmented apertures like Keck and JWST. Downstream capabilities include a HASO 126 Shack-Hartmann sensor, a real-time computing environment driven by the CACAO package, and a modular injection platform coupled to a visible-wavelength spectrograph (R3, 000). We report on the successful system alignment and outline the roadmap for upcoming adaptive optics and photonic device validation frameworks.

astro-ph.IM↗

arXiv2026

FIRST-PL: Commissioning the first visible photonic lantern spectrograph for sub-diffraction-limit astronomy on Subaru/SCExAO

FIRST-PL (Fibered Imager foR a Single Telescope - Photonic Lantern) is a newly commissioned visible-light instrument on Subaru/SCExAO achieving spectroscopy below the diffraction limit. The instrument uses a Photonic Lantern (PL)-converting multimode fiber into 19 single-mode outputs-feeding a mid-resolution spectrograph (R 3000, 630-790 nm). On-sky performance demonstrates 40% injection efficiency at 680 nm (Strehl 30%) and 12x throughput improvement over single-mode fibers. Three operational modes enable spectro-astrometry (50 microarcseconds precision demonstrated on beta-CMi), image reconstruction, and high-contrast imaging. FIRST-PL represents a significant advancement in high-throughput photonic instrumentation.

astro-ph.IM↗

arXiv2026

Technologies and novel components for broadband splitting and coupling in pairwise and nulling interferometry

Passive and active photonic components are central to the continued development of astronomical photonic integrated circuits (PICs), although achieving broadband achromatic performance remains a significant challenge. This work presents broadband evanescent tri-couplers, tapered directional couplers, and a chromatically controlled achromatic intensity modulator for astronomical interferometry in the J- and H-bands. Silicon nitride and silicon oxide provide complementary low-loss platforms, while customised tapered components enable broadband operation across the 0.95-1.8 microns range. For the silicon nitride platform developed by STMicroelectronics, tapered tri-couplers and directional couplers were designed as replacements for conventional components in a pairwise beam combiner for the PLANETS project. Optimised tapered tri-couplers achieve less than 1% excess loss across the J-band, while tapered directional couplers provide broadband 40:60 splitting suitable for beam combination. For the silicon oxide platform from Enablence, a two-dimensional tapered tri-coupler was investigated for nulling interferometry. The device provides broadband starlight suppression while limiting exoplanet throughput loss to less than 2.2% across the H-band and simultaneously retaining broadband phase-sensing capability for fringe tracking. The chromatically controlled achromatic intensity modulator is introduced as a combination of a tapered directional coupler with a thermo-optic phase shifter, the device provides programmable broadband intensity control for applications including null-depth balancing in interferometric PICs. Future work will extend these concepts to lithographically fabricated chalcogenide glass platforms operating in the mid-infrared, enabling compact photonic beam combiners for future ground- and space-based nulling interferometers targeting Earth-like exoplanets.

astro-ph.IM↗

arXiv2026

ngVLTI: Cost and Feasibility of a Four-Telescope VLTI Expansion for Milliarcsecond-Scale Imaging

We assess the cost and technical feasibility of extending the Very Large Telescope Interferometer (VLTI) through the addition of four 8 m-class Unit Telescopes (UTs), an upgrade we refer to as the new generation VLTI (ngVLTI). Such an upgrade would provide a dense and homogeneous uv coverage with baselines up to 220 m, enabling true imaging at milliarcsecond angular resolution across science cases ranging from Solar System bodies to distant active galactic nuclei. Because each image is reconstructed at a single wavelength, repeating the reconstruction across the spectral channels of the instrument would deliver spectral-imaging cubes -- a qualitatively new capability for the VLTI, bringing it close to the imaging power of ALMA at near-infrared wavelengths. Motivated by this scientific case, we examine a compact telescope concept consisting of a fast ($f/0.64$), segmented, parabolic primary mirror feeding a subterranean coudé focus compatible with the existing VLTI infrastructure. We summarise the optical design, which achieves diffraction-limited performance over a 1 arcmin field of view and a well-matched reimaged pupil, discuss the mechanical trade-offs behind the choice of a 60-segment, 1.2 m primary -- a mirror mass of ~13.5 tons and an altitude moving mass of ~50 tons -- and quantify the gravitational flexure of the telescope structure and its resulting optical sensitivity as a function of pointing elevation. We then present a back-of-the-envelope cost estimate of order 80 MEuro per telescope (2026 prices), broken down into the segmented primary, adaptive secondary, coudé train, mount, enclosure, and ancillary instrumentation. The proposed upgrade appears both cost-competitive and technically achievable, offering a long-term perspective for Paranal Observatory in the ELT era.

astro-ph.IM↗

arXiv2026

Using VLTI/GRAVITY+ to determine the identity of a third planet candidate in the PDS 70 system

Detections of protoplanets are rare and protoplanetary disk features mischaracterized as planets are common. PDS 70 is one of only two stars known to host multiple confirmed protoplanets, PDS 70 b and c, and repeat detections of a third point-like source in the system suggest the presence of third inner planet. However, previous observations of this third source are insufficient to distinguish whether it is a planet or a concentrated dust clump in Keplerian motion. Our observations with VLTI/GRAVITY+ did not re-detect this point-like source, suggesting that it is, in fact, a dust clump and not a planet. These observations demonstrate how the angular resolving power of VLTI/GRAVITY+ can be used to distinguish between protoplanets and protoplanetary disk features.

astro-ph.EP↗

arXiv2026

Destructuring the disk of AB Aurigae: Dynamics and accretion

In this paper, we present near-IR polarized images of the AB Aur disk at three epochs spanning 3.85 years with SPHERE/IRDIS, as well as Halpha images obtained with SPHERE/ZIMPOL at a single epoch. The purpose of this study is to analyze the dynamics of the entire disk and of the various structures in near-IR polarimetry, and to identify sources of Halpha emission to derive constraints on their mass accretion rate. The dynamical study in the near-IR shows that the disk globally follows Keplerian rotation, but we observe a departure from this behavior at radii smaller than ~60au. At the smallest radius of ~25au, we measure a deviation from Keplerian rotation as large as ~12deg over 3.85 years, demonstrating sub-Keplerian rotation. The two bright spirals within the millimeter cavity have different dynamic trends, and we discuss their possible link with the identified planet candidates. We also discuss the implications of the non-Keplerian behavior, and we posit that it could be related to interactions with multiple protoplanets orbiting out of the disk plane on elliptical orbits. Furthermore, the orbital analysis of the compact sources (labeled f1, f2, and f3) suggests that their orbital planes are significantly inclined with respect to the disk plane by several tens of degrees. The variability of the shadows suggests that they are produced by optically thick regions located within ~60au. For the photometric analysis in Halpha, we derive a flux of about 8.22x10^{-15} erg/s/cm^2 for the entire feature f1, but only 6.46x10^{-16} erg/s/cm^2 at the location of AB Aur b, consistent with non-detection. If f1 were a point source and the accretion remained constant for 1Myr, it would correspond to ~5-20 Jupiter masses according to the magnetospheric accretion model or ~6-10 Jupiter masses according to the boundary layer accretion model.

astro-ph.EP↗

arXiv2026

Direct spectroscopic confirmation of the young embedded proto-planet WISPIT 2c

WISPIT 2 is a nearby young star with a multi-ringed disk which was recently confirmed to host a ~4.9 MJup gas giant planet embedded in a large (60 au) gap at a radial separation of 57 au from the host star. We confirm and characterise a second, close-in planet in the WISPIT 2 system using a combination of new VLT/SPHERE H-band dual-polarisation imaging and VLTI/GRAVITY K-band interferometric observations of the WISPIT 2 system. The GRAVITY detection is consistent with a point-like source while its extracted K-band spectrum shows CO band-head absorption at 2.3 microns and a continuum shape consistent with a young giant planet. From the GRAVITY data we extract a medium resolution K-band spectrum of the companion and fit atmospheric model grids using the species tool with nested sampling to constrain its effective temperature, radius, and luminosity. We infer Teff of 1500-2600 K, a radius of 0.91-2.2 RJup, and a luminosity of (-3.47)-(-3.63). Comparison with evolutionary tracks implies a mass range of 8-12 MJup, approximately twice as massive as the previously confirmed WISPIT 2b. The astrometry rules out a background source and marginally detects orbital motion of WISPIT 2c, which needs further follow-up observations for confirmation. WISPIT 2 now becomes an analogue to PDS 70, offering a second laboratory for studying the formation and early evolution of a multi-planet system within its natal disk.

astro-ph.EP↗

arXiv2026

Exoplanet atmospheres and demographics in the 2040s

Direct observations of exoplanets probe the demographics and atmospheric composition of young self-luminous companions, yielding insight into their formation and early evolution history. In the near future, Gaia will reveal hundreds of nearby young exoplanets amenable to direct follow-up observations. Long-baseline interferometry with current and future facilities is best capable of exploiting this unique synergy which is poised to deliver a statistical sample of benchmark planets with precise dynamical masses and in-depth atmospheric characterization. This will enable tackling the longstanding question of how giant planets form from multiple angles simultaneously, shining light on the complex physical processes underlying planet formation.

astro-ph.IM↗

arXiv2025

Exoplanets in reflected starlight with dual-field interferometry and a fifth Unit Telescope at VLTI

In this white paper, we propose an upgrade to the Very Large Telescope Interferometer (VLTI) consisting of the addition of a new 8m Unit Telescope (UT5). The primary goal of this upgrade is to optimise the VLTI for exoplanet detection by creating four additional baselines of approximately 200m oriented toward the north-west. The inclusion of this telescope would reduce the inner working angle and improve the achievable contrast of the VLTI, thereby enabling the detection of mature exoplanets in reflected light.

astro-ph.IM↗

arXiv2025

Hunting exomoons with a kilometric baseline interferometer

Despite numerous search campaigns based on a diverse set of observational techniques, exomoons - prospective satellites of extrasolar planets - remain an elusive and hard-to-pin-down class of objects. Yet, the case for intensifying this search is compelling: as in the Solar System, moons can act as proxies for studying planet formation and evolution, provide direct clues as to the migration history of the planetary hosts and, in favourable cases, offer potentially habitable environments. Here, we present an investigation into how the search for exomoons would benefit from a new interferometric facility operating in the optical wavelength domain and leveraging baselines substantially longer than the ones the VLTI is currently equipped with. We find that an interferometer providing an astrometric precision of 1$\,μ$as would be able to robustly detect Earth-mass and sub-Earth-mass exomoons on dynamically stable orbits around Jupiter-like planets at distances between 50 and 200 pc.

astro-ph.IM↗

arXiv2025

On-sky Demonstration of Subdiffraction-limited Astronomical Measurement Using a Photonic Lantern

Resolving fine details of astronomical objects provides critical insights into their underlying physical processes. This drives in part the desire to construct ever-larger telescopes and interferometer arrays and to observe at shorter wavelength to lower the diffraction limit of angular resolution. Alternatively, one can aim to overcome the diffraction limit by extracting more information from a single telescope's aperture. A promising way to do this is spatial mode-based imaging, which projects focal-plane field onto a set of spatial modes before detection, retaining focal-plane phase information crucial at small angular scales but typically lost in intensity imaging. However, the practical implementation of mode-based imaging in astronomy from the ground has been challenged by atmospheric turbulence. Here, we present the first on-sky demonstration of a subdiffraction-limited, mode-based measurement using a photonic lantern (PL)-fed spectrometer installed on the SCExAO instrument at the Subaru Telescope. We introduce a novel calibration strategy that mitigates time-varying wavefront error and misalignment effects, leveraging simultaneously recorded focal-plane images and using a spectral-differential technique that self-calibrates the data. Observing the classical Be star $β$ CMi, we detected spectral-differential spatial signals and reconstructed images of its H$α$-emitting disk. We achieved an unprecedented H$α$ photocenter precision of 50$μ$as in about 10-minute observation with a single telescope, measuring the disk's near-far side asymmetry for the first time. This work demonstrates the high precision, efficiency, and practicality of photonic mode-based imaging techniques to recover subdiffraction-limited information, opening new avenues for high angular resolution spectroscopic studies in astronomy.

astro-ph.IM↗

arXiv2025

New Orbital Constraints for YSES 1 b and HR 2562 B from High-Precision Astrometry and Planetary Radial Velocities

We present new VLTI/GRAVITY astrometry and updated orbit fits for the directly imaged companions YSES 1 b and HR 2562 B, substellar objects straddling the planet-brown dwarf boundary. Using high-precision astrometry, radial velocity (RV) data, and proper motions, we derive revised orbital parameters with orbitize! arXiv:1910.01756. For YSES 1 b, the inclusion of GRAVITY astrometry and a relative radial velocity measurement from arXiv:2409.16660 overcomes the traditional challenge of constraining eccentricities for distant companions, enabling the first orbit fit and yielding a constrained eccentricity of 0.44 (0.20). This represents the first full orbit fit for the system. Additionally, we calculate a median line-of-sight stellar obliquity of 12 (+11, -8) degrees, providing further insight into the system's dynamical architecture. For HR 2562 B, our analysis agrees with arXiv:2302.04893, confirming a low-eccentricity orbit (0.34 (0.20)) and an inclination of 87 (1) degrees. We find HR 2562 B's orbit to be nearly coplanar with the debris disk, with a mutual inclination of 3.7 (0.3) degrees. For both YSES 1 b and HR 2562 B the lower eccentricities favor an in situ formation scenario over extreme scattering or cloud fragmentation.

astro-ph.EP↗

arXiv2025

YSES 2b is a background star: Differential astrometric M-dwarf measurements in time

We wish to confirm the nature of YSES 2b, a purportedly faint companion of the young star YSES 2. We used on-sky observations from SPHERE and GRAVITY to measure the astrometric position of 2b with respect to the star YSES 2, and examined the competing hypotheses of (i) a bound substellar companion versus (ii) a distant unrelated background source with a non-zero proper motion. YSES 2b appears to be a late-type M-dwarf star over 2 kiloparsecs behind the star YSES 2. It has a transverse velocity of about 300 km/s and is located within one of the spiral arms of the Galaxy. The main discriminant was multiple epochs of GRAVITY astrometry that identified the sub-milliarcsecond parallactic motion of the star.

astro-ph.EP↗

arXiv2025

Differentiating Formation Models with New Dynamical Masses for the PDS 70 Protoplanets

Hot- and cold-start planet formation models predict differing luminosities for the young, bright planets that direct imaging surveys are most sensitive to. However, precise mass estimates are required to distinguish between these models observationally. The presence of two directly imaged planets, PDS 70 b and c, in the PDS 70 protoplanetary disk provides us a unique opportunity for dynamical mass measurements, since the masses for these planets are currently poorly constrained. Fitting orbital parameters to new astrometry of these planets, taken with VLTI/GRAVITY in the $K$~band, we find $2σ$ dynamical upper mass limits of 4.9 $M_{\rm Jup}$ for b and 13.6 $M_{\rm Jup}$ for c. Adding astrometry from the newly proposed planet candidate PDS 70 d into our model, we determine $2σ$ dynamical upper mass limits of 5.3 $M_{\rm Jup}$, 7.5 $M_{\rm Jup}$ and 2.2 $M_{\rm Jup}$ for b, c, and the candidate d respectively. However, $N$-body analysis of the orbits fit in this case suggest that the inclusion of $d$ makes the system unstable. Using the upper mass limits for b and c we rule out the coldest-start formation models for both planets, calculating minimum post-formation entropies ($S_i$) of 9.5 $k_{\rm B}/{\rm baryon}$ and 8.4 $k_{\rm B}/{\rm baryon}$ respectively. This places PDS 70 b and c on the growing list of directly-imaged planets inconsistent with cold-start formation.

astro-ph.EP↗

arXiv2025

VLTI/GRAVITY upper limit on near-infrared emission from the nearby 33 Msun black hole Gaia BH3

The recent astrometric discovery of the nearby (590 pc) massive ($33 M_\odot$) dormant black hole candidate Gaia BH3 offers the possibility to angularly resolve the black hole from its companion star by using optical interferometry. Our aim is to detect emission in the near-infrared K band from the close-in environment of Gaia BH3 caused by accretion. Gaia BH3 was observed with the GRAVITY instrument using the four 8-meter Unit Telescopes of the VLT Interferometer. We searched for the signature of emission from the black hole in the interferometric data using the CANDID, PMOIRED, and exoGravity tools. With a present separation of 18 mas, the Gaia BH3 system can be well resolved angularly by GRAVITY. We did not detect emission from the black hole at a contrast level of $Δm = 6.8$ mag with respect to the companion star, that is, $f_\mathrm{BH}/f_* < 0.2\%$. This corresponds to an upper limit on the continuum flux density of $f_\mathrm{BH} < 1.9 \times 10^{-16}$ W m$^{-2}$ $μ$m$^{-1}$ in the K band. In addition, we did not detect emission from the black hole in the hydrogen Brγ line. The non-detection of near-infrared emission from the black hole in Gaia BH3 indicates that its accretion of the giant star wind is presently occurring at most at a very low rate. This is consistent with the limit of $f_\mathrm{Edd} < 4.9 \times 10^{-7}$ derived previously on the Eddington ratio for an advection-dominated accretion flow. Deeper observations with GRAVITY may be able to detect the black hole as the companion star approaches periastron around 2030.

astro-ph.GA↗

arXiv2025

On the Orbit of the Binary Brown Dwarf Companion GL229 Ba and Bb

The companion GL229B was recently resolved by Xuan et al. (2024) as a tight binary of two brown dwarfs (Ba and Bb) through VLTI-GRAVITY interferometry and VLT-CRIRES+ RV measurements. Here, we present Bayesian models of the interferometric and RV data in additional detail, along with an updated outer orbit of the brown dwarf pair about the primary. To create a model of the inner orbit with robust uncertainties, we apply kernel phases to the GRAVITY data to address baseline redundancy in the raw closure phases. Using parallel tempering, we constrain the binary's orbit using only VLTI-GRAVITY data, despite each epoch having low visibility-plane coverage and/or SNR. We demonstrate very agreement the VLTI-GRAVITY and CRIRES+ datasets and find that the inner binary has a period of 12.1346$\pm$0.0011 days, eccentricity of 0.2317$\pm$0.0025, and total mass of 71.0$\pm$0.4 Mjup, with Ba and Bb having masses of 37.7$\pm$1.1Mjup and 33.4$\pm$1.0Mjup respectively. With new Keck/NIRC2 astrometry, we update the outer orbit GL229B around the primary. We find a semi-major axis of 42.9+3.0-2.4AU, eccentricity of 0.736$\pm$0.014, and a total mass for B of 71.7$\pm$0.6Mjup, consistent with that derived from the inner orbit. We find a mutual inclination of 31$\pm$2.5deg, below the threshold for Kozai-Lidov oscillations. The agreement on the mass of Ba+Bb between the inner and outer orbits is an important test of our ability to model RV, astrometry, and Hipparcos-Gaia proper motion anomaly. Our methodological advances in handling interferometric data with low SNR and sparse UV-coverage will benefit future observations of rapidly-orbiting companions with VLTI-GRAVITY.

astro-ph.SR↗