arXiv subjects

T. A. Stuber

Publications and source records attributed to T. A. Stuber.

7 recordsLinked to original sources

arXiv2026

Asgard/NOTT: Status of laboratory nulling performance

Nulling interferometry enables the direct detection of faint companions and circumstellar structures at angular separations unresolvable by classical, diffraction-limited imagers, whilst dramatically improving the measurable contrast. The Asgard/NOTT nulling instrument aims to achieve a contrast performance of 10^-5 in the L' wavelength band (3.5 - 4.0 {\mu}m), enabling observation and characterization of young giant exoplanets near the snowline and hot exozodiacal dust. Previous studies have verified the nulling capabilities, of the chip in ambient conditions and of the test bench in cryogenic conditions. This work aims to add the first ambient performance assessment of the test bench with spectrally dispersed light. Necessary revisions are made to the data acquisition and calibration pipeline and fringe scans are carried out, modeled and fitted. The splitting ratios of the 4-telescope nulling beam combiner, a photonic Gallium Lanthanum Sulfide (GLS) chip, are moreover characterized on the bench, showing tentative agreement with previous chip characterization. The null performance has worsened, the achieved contrast of ~ 10^-1 being one order of magnitude higher than what earlier characterized performance showed. Multiple future changes to the test bed and to the approach taken promise an improved characterization of performance. In particular, the input beam intensities will be deliberately mismatched to account for the imbalanced splitting ratios of the directional couplers. With the installation of the final cryostat and camera, the developed tools will be leveraged to re-assess the performance in ambient and cryogenic conditions.

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arXiv2026

Performance Analysis of the Asgard/NOTT Nulling Interferometer: Optimizing Observing Modes for High-contrast Detection

We evaluate the performance of three beam-combination schemes, single-Bracewell, asymmetric dual-Bracewell, and symmetric dual-Bracewell, for the forthcoming Asgard/NOTT nulling interferometer at the Very Large Telescope Interferometer. Utilizing the SCIFYsim end-to-end simulator, we assess the instrument's performance by deriving the precision of calibrated null measurements as a function of stellar magnitude and simulating observations of varying hot exozodiacal dust (HEZD) distributions and a hot Jupiter. The study reveals distinct trade-offs for each observing mode. The single-Bracewell mode provides high throughput and preserves spatial information but suffers from poor error suppression. The asymmetric dual-Bracewell mode offers the strongest error suppression for detecting point-like sources, but it inherently suppresses symmetric astrophysical signals such as expected from HEZD. The symmetric dual-Bracewell mode provides a middle ground with modest error suppression while being sensitive to symmetric emission. We conclude that utilizing a combination of all three observing modes provides a robust strategy for detecting HEZD, constraining the structure of its distribution, and identifying false positives from stellar companions.

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arXiv2026

Spectrally dispersed non-redundant aperture-masking interferometry in the thermal infrared with LBTI/ALES

Non-redundant aperture-masking interferometry maximizes the angular resolution of a telescope by turning it into an interferometric array using pupil plane masks. If the interference pattern on the detector is spectrally dispersed, deeper contrasts of the astrophysical scenery and an improved coverage of its spatial Fourier modes is achieved. We combined a non-redundant aperture mask with the integral field spectrograph ALES of LMIRcam at the Large Binocular Telescope Interferometer (LBTI). With this setup, we recorded spectrally dispersed non-redundant aperture-masking observations in the $L$ band ($\sim$3-4$\,\mu$m). We present commissioning data with which we recovered a known binary star and derived contrast detection limits for further point-source components of ${\sim} 5 \times 10^{-3}$ for separations of ${\gtrsim} 80\,$mas. This is the first time that an integral field spectrograph was combined with a non-redundant aperture mask in the $L$ band. Opening up the thermal infrared for this technique, this new capability enables the LBTI to efficiently survey nearby, bright stars for faint companions, which is of great importance for studies on hot exozodiacal dust and target vetting for future exo-Earth imaging missions such as NASA's Habitable Worlds Observatory. A further application is aperture synthesis of extended sources such as active galactic nuclei or evolved stars.

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arXiv2026

Asgard/NOTT: Cryogenic characterization of the mid-infrared chip

NOTT is part of the new visitor instrument suite Asgard for the Very Large Telescope Interferometer (VLTI), and the first long-baseline nulling interferometer that will be operational in the southern hemisphere. It is an L'-band (3.5-4$\,\mu$m) instrument optimized for imaging hot exozodiacal dust and young giant planets orbiting around the snowline of nearby main-sequence stars. For planet imaging, the L' band has the advantage of relaxing the requirements on the star-planet contrast to $\sim 10^{-5}$ while limiting the level of background noise compared with longer wavelengths. Nulling interferometry in the L'-band was made possible by the development of mid-infrared integrated optics with high throughput. NOTT uses a photonic beam combiner made of Gallium Lanthanum Sulfide (GLS), manufactured at Macquarie University and characterized at ambient temperatures at Universit\"at zu K\"oln. This first characterization showed that the chip could achieve the broadband contrast requirement for exoplanet imaging. Using the test bench of the NOTT instrument assembled at KU Leuven, and its test cryostat, we successfully cooled the chip down to $\sim 138\,$K and performed its first characterization at cryogenic temperatures. The results show a raw broadband contrast of $\sim1\,\%$, similar to the previous measurements done at ambient temperatures. The splitting ratios of the different couplers are also shown to remain stable at cryogenic temperatures, with less than $\sim 2\,\%$ uncertainty compared to ambient measurements. These results thus show that the beam-combining properties and splitting ratios are behaving as expected at 138$\,$K. The current maximum throughput of the chip is estimated at $\sim37\,\%$. Future work will investigate an anti-reflection coating to reduce its Fresnel losses and increase its throughput to $\sim50\,\%$.

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arXiv2025

Interferometric Detection and Orbit Modeling of the Subcomponent in the Hot-dust System $\kappa$ Tuc A: A Low-mass Star on an Eccentric Orbit in a Hierarchical-quintuple System

The system $\kappa$ Tuc A is part of a hierarchical-quintuple system and is a prime target for studies of hot-exozodiacal dust, because a time-variable near-infrared excess has been detected. We observed the system with the Multi Aperture mid-Infrared Spectroscopic Experiment (MATISSE) and GRAVITY at the Very Large Telescope Interferometer, and detected the stellar companion to the primary $\kappa$ Tuc Aa that was previously inferred by astrometry, $\kappa$ Tuc Ab. Its $L$-band flux ratio to the primary is 1.32% and its signature in the MATISSE closure phases is mostly smaller than +/- 2{\deg}, which makes $\kappa$ Tuc Ab the highest-contrast companion ever detected with MATISSE closure phases. We verified with GRAVITY that relative astrometry with milliarcsecond precision can be retrieved from MATISSE closure phases. Using multiple epochs of observations, we obtain a full orbital solution for $\kappa$ Tuc Ab. Its orbit has an eccentricity of 0.94 and a semi-major axis of 4.8au. The orbit of $\kappa$ Tuc Ab and the orbit of the wider separation companion $\kappa$ Tuc B are mutually inclined. Based on the measured flux ratio of $\kappa$ Tuc Ab to Aa and their dynamical mass, we estimate the spectral type of $\kappa$ Tuc Ab to be M3.5V to M4.5V. While the then unknown star $\kappa$ Tuc Ab might have caused the putative detection of hot-exozodiacal dust around $\kappa$ Tuc Aa in 2012 and 2014, this cannot be for the detection in 2019, giving rise to an intriguing system architecture. This motivates studies investigating the interplay of the low-mass star on an eccentric orbit, the hot-exozodiacal dust, and a possible planetesimal reservoir.

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arXiv2023

How much large dust could be present in hot exozodiacal dust systems?

An infrared excess over the stellar photospheric emission of main-sequence stars has been found in interferometric surveys, commonly attributed to the presence of hot exozodiacal dust (HEZD). While submicrometer-sized grains in close vicinity to their host star have been inferred to be responsible for the found near-infrared excesses, the presence and amount of larger grains as part of the dust distributions are weakly constrained. We quantify how many larger grains (above-micrometer-sized) could be present in addition to submicrometer-sized grains, while being consistent with observational constraints. This is important in order to distinguish between various scenarios for the origin of HEZD and to better estimate its observational appearance when observed with future instruments. We extended a model suitable to reproduce current observations of HEZD to investigate a bimodal size distribution. By deriving the characteristics of dust distributions whose observables are consistent with observational limits from interferometric measurements in the $K$ and $N$ bands we constrained the radii of sub- and above-micrometer-sized grains as well as their mass, number, and flux density ratios. In the most extreme cases of some of the investigated systems, large grains $\gtrsim 10\,μ$m might dominate the mass budget of HEZD while contributing up to 25$\,$% of the total flux density originating from the dust at a wavelength of 2.13$\,μ$m and up to 50$\,$% at a wavelength of 4.1$\,μ$m; at a wavelength of 11.1$\,μ$m their emission might clearly dominate over the emission of small grains. While it is not possible to detect such hot-dust distributions using ALMA, the ngVLA might allow us to detect HEZD at millimeter wavelengths. Large dust grains might have a more important impact on the observational appearance of HEZD than previously assumed, especially at longer wavelengths.

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arXiv2022

Using debris disk observations to infer substellar companions orbiting within or outside a parent planetesimal belt

Aims. We analyze whether the effects of secular perturbations, originating from a substellar companion, on the dust dynamics in a debris disk can be investigated with spatially resolved observations. Methods. We numerically simulated the collisional evolution of narrow and eccentric cold planetesimal belts around a star of spectral type A3V that are secularly perturbed by a companion that orbits either closer to or farther from the star than the belt. Based on the resulting spatial dust distributions, we simulated spatially resolved maps of their surface brightness in the $K$, $N$, and $Q$ bands and at wavelengths of 70$\mu$m and 1300$\mu$m. Results. Assuming a nearby debris disk seen face-on, we find that the brightness distribution varies significantly with observing wavelength, for example between the $N$ and $Q$ band. This can be explained by the varying relative contribution of the emission of the smallest grains near the blowout limit. The orbits of both the small grains that form the halo and the large grains close to the parent belt precess due to the secular perturbations induced by a companion orbiting inward of the belt. The halo, being composed of older grains, trails the belt. The magnitude of the trailing decreases with increasing perturber mass and hence with increasing strength of the perturbation, a trend we recovered in synthetic maps of surface brightness by fitting ellipses to lines of constant brightness. Systems with an outer perturber do not show a uniform halo precession since the orbits of small grains are strongly altered. We identified features of the brightness distributions suitable for distinguishing between systems with a potentially detectable inner or outer perturber, especially with a combined observation with JWST/MIRI in the $Q$ band tracing small grain emission and with ALMA at mm wavelengths tracing the position of the parent planetesimal belt.

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