The finding, led by University of Arizona Steward Observatory postdoctoral researcher Thomas Stuber and published in the Astronomical Journal, is the highest-contrast detection of a stellar companion yet made with the European Southern Observatorys MATISSE interferometric instrument. Researchers see the Kappa Tucanae A system as a natural laboratory for studying hot exozodiacal dust, which complicates efforts to image Earth-like exoplanets around other stars.
Hot exozodiacal dust consists of very fine particles, comparable in size to smoke, orbiting so near their host stars that intense heat and radiation pressure should remove them quickly. Stuber said that, given the large amounts of dust observed, it must either be replenished at a high rate or sustained by a mechanism that significantly lengthens its lifetime.
This dust is especially important for future missions such as NASAs planned Habitable Worlds Observatory, scheduled for launch in the 2040s, which will rely on coronagraphs to block starlight and reveal faint exoplanets. Hot dust produces scattered light that can leak through such coronagraphs, potentially hiding the signatures of habitable worlds and making it necessary to understand the dusts properties to guide exoplanet observing strategies.
Stubers team used interferometry, combining light from multiple telescopes to achieve the resolution of a much larger instrument, to monitor Kappa Tucanae A between 2022 and 2024. Instead of only tracking the dust, they identified a companion star on a highly eccentric orbit that passes within about 0.3 astronomical units of the primary star, closer than any planet orbits in the solar system.
This elongated orbit turns the system into a dynamic laboratory, as the companion moves from the outer regions back into the dust-rich inner zone. Steward Observatory associate astronomer Steve Ertel, a co-author on the paper, said, "Theres basically no way that this companion is not somehow connected to that dust production. It has to be dynamically interacting with the dust."
The work extends Steward Observatorys long-standing role in interferometry, building on the Large Binocular Telescope Interferometer on Mount Graham in Arizona, funded by NASA. The LBTI was developed to search for warm exozodiacal dust with high stability and sensitivity, and its performance helped position Steward as a central hub for dust and exoplanet studies, drawing support from NASA, the National Science Foundation and private funders.
Experience gained with the LBTI is now feeding into development of a new European nulling interferometer that is expected to be about 50 times more sensitive than earlier instruments. That project is led by Denis Defrere, who trained as a postdoctoral researcher at Steward and contributed to the construction of the LBTI before moving to Europe.
Ertel said, "Steward has established itself as the global leader to this kind of research, which is really critical for exo-Earth imaging," noting that he obtained a NASA grant to use the new interferometer to investigate exozodiacal dust. The planned observations will focus on both hot and warm dust around other stars, work intended to inform techniques for directly imaging small, potentially habitable planets.
The Kappa Tucanae A system presents multiple opportunities to probe the physics of hot dust. By following the companions orbit and its interaction with surrounding material, researchers aim to determine the dusts origin, composition, grain sizes and spatial distribution in extreme inner-system environments.
Several scenarios are under consideration for sustaining the dust. Prior work by Steward researchers George Rieke and Andras Gaspar suggests that stellar magnetic fields might trap charged dust particles, while research by co-author Virginie Faramaz-Gorka points to cometary material that continuously resupplies the inner regions; other physical mechanisms may also contribute.
The discovery raises the possibility that more hot dust systems harbor unseen stellar companions that drive similar processes. Steward astronomers plan to revisit previously observed targets to search for such companions, reassessing data in light of the new results from Kappa Tucanae A.
As NASAs Habitable Worlds Observatory concept advances, systems like Kappa Tucanae A provide baseline knowledge needed to interpret observations of exoplanetary environments affected by bright inner dust. Stuber said, "Considering the Kappa Tucanae A system was observed many times before, we did not even expect to find this companion star. This makes it even more exciting to now have this unique system that opens up new pathways to explore the enigmatic hot exozodiacal dust."
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Stellar Chemistry, The Universe And All Within It
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