Kappa Tucanae A, a star located seventy light-years away, has baffled astronomers due to the presence of extremely hot dust. This dust, with a temperature exceeding 1,000 °F, is located very close to the star. At this proximity, the dust should not exist, as it would either be vaporized or quickly dispersed. Researchers at the University of Arizona may have found an explanation: a companion star orbiting through the area where the unusual dust is found. The findings were published in the Astronomical Journal.
Exozodiacal dust, depicted in this artist's illustration as a glowing white haze above the horizon of a hypothetical habitable planet orbiting another star, was found to exist in such large quantities in the Kappa Tucanae star system that it has astronomers puzzled over its origin. Image Credit: ESO/L. Calçada
The highest contrast detection of a stellar companion using the European Southern Observatory's MATISSE instrument was achieved by a team led by Thomas Stuber, a postdoctoral researcher at the U of A's Steward Observatory. This discovery offers researchers a unique opportunity to study hot exozodiacal dust, a factor that makes it more difficult to find potentially habitable, Earth-like planets.
Hot exozodiacal dust represents a fundamental obstacle to a complete understanding of planetary systems. These microscopic particles, comparable in fineness to smoke, orbit their stars at such close distances that the combined effects of high temperature and intense radiation pressure should, in theory, lead to their almost instantaneous disappearance.
If we see dust in such large amounts, it needs to be replaced rapidly, or there needs to be some sort of mechanism that extends the lifetime of the dust.
Thomas Stuber, Study Lead, University of Arizona
This dust presents a significant impediment to the search for Earth-like planets around target stars. NASA's Habitable Worlds Observatory, projected for a 2040s launch, will employ advanced coronagraphs to obscure starlight, facilitating the detection of faint exoplanets. However, this hot dust generates "coronagraphic leakage", scattered light capable of obscuring signals from potentially habitable worlds. Consequently, determining its origin and composition is essential for guiding future exoplanet investigations.
Stuber's team conducted repeated observations of Kappa Tucanae A from 2022 to 2024, utilizing interferometry, a technique that merges light from multiple telescopes to achieve the resolution of a much larger, singular instrument.
The international team, with over a decade of global leadership in exozodiacal dust research prior to this project, anticipated analyzing the dust's temporal behavior. Contrary to expectations, the researchers uncovered an unforeseen element: a stellar companion in a highly eccentric orbit, approaching the primary star within 0.3 astronomical units, a proximity greater than any planet in the solar system reaches relative to the sun.
According to Stuber, this finding redefines Kappa Tucanae A, shifting its classification from a perplexing system to an intricate stellar laboratory. The companion star's trajectory is characterized by an extremely elliptical path, extending significantly outward before re-entering the system's dust-rich inner region.
There's basically no way that this companion is not somehow connected to that dust production. It has to be dynamically interacting with the dust.
Steve Ertel, Study Co-Author and Associate Astronomer, Steward Observatory
The current advancement is founded upon Steward Observatory's extensive history of technological excellence in interferometry. The observatory's Large Binocular Telescope Interferometer, financed by NASA and constructed on Mount Graham, transformed the investigation into warm exozodiacal dust (a milder form of hot dust) due to its unmatched stability and sensitivity.
The distinctive features of the LBTI elevated Steward to a leading international position in studying exozodiacal dust. This success drew substantial financial support from NASA, the National Science Foundation, and philanthropic sources, establishing the observatory as a leader in exoplanet research. This accumulated knowledge is now being applied to develop the next generation of instruments, including a forthcoming European nulling interferometer designed to achieve 50 times greater sensitivity than prior observations.
This continuity of expertise is evident: Denis Defrère, who leads the European instrument's development, previously received training at Steward as a postdoctoral researcher, where he contributed to the construction of the LBTI.
“Steward has established itself as the global leader to this kind of research, which is really critical for exo-Earth imaging,” said Ertel, who obtained a NASA grant to study exozodiacal dust with this new instrument.
The Kappa Tucanae A discovery opens various avenues for future research. By examining the interaction between the stellar companion and the dust, astronomers aim to gain a broader understanding of hot dust's origin, composition, grain size, and distribution.
The findings could ascertain whether magnetic fields trap charged dust particles, as theorized by Steward researchers George Rieke and András Gáspár, or if cometary material continuously replenishes the supply, a concept explored by Steward researcher Virginie Faramaz-Gorka, also a co-author. Alternatively, the research might reveal entirely different physical principles governing these extreme environments.
This discovery also indicates that other hot dust systems could potentially harbor similar stellar companions. Steward researchers now intend to re-examine previously observed stars in search of companions that might have been missed.
As NASA's Habitable Worlds Observatory progresses toward realization, discoveries like this one provide the fundamental knowledge necessary to undertake the complex research anticipated.
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.
Thomas Stuber, Study Lead, University of Arizona
Journal Reference:
Stuber, A, T., et al. (2025) Interferometric Detection and Orbit Modeling of the Subcomponent in the Hot-dust System κ Tuc A: A Low-mass Star on an Eccentric Orbit in a Hierarchical-quintuple System. The Astronomical Journal. DOI 10.3847/1538-3881/adfe66. https://iopscience.iop.org/article/10.3847/1538-3881/adfe66