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The exciting discovery of an extremely rare quadruple star system could significantly advance our understanding of brown dwarfs, astronomers say. These mysterious objects are too big to be considered a planet but also too small to be a star because they lack the mass to keep fusing atoms and blossom into fully-fledged suns.

In research in the Monthly Notices of the Royal Astronomical Society, astronomers have identified an extremely rare hierarchical quadruple star system consisting of a pair of cold brown dwarfs orbiting a pair of young red dwarf stars, located 82 light-years from Earth in the constellation Antlia.

The system, named UPM J1040−3551 AabBab, was identified by an international research team led by Professor Zenghua Zhang of Nanjing University.

The researchers made their discovery using common angular velocity measured by the European Space Agency's Gaia astrometric satellite and NASA's Wide-field Infrared Survey Explorer (WISE), followed by comprehensive spectroscopic observations and analysis.

That's because this wide binary pair takes more than 100,000 years to complete one orbit around each other, so their orbital motion cannot be seen in years. Researchers therefore had to analyze how they are moving in the same direction with the same angular velocity.

In this system, Aab refers to the brighter stellar pair Aa and Ab, while Bab refers to the fainter substellar pair Ba and Bb.

"What makes this discovery particularly exciting is the hierarchical nature of the system, which is required for its orbit to remain stable over a long time period," said Professor Zhang.

"These two pairs of objects are orbiting each other separately for periods of decades, while the pairs are also orbiting a common center of mass over a period of more than 100,000 years."

The two pairs are separated by 1,656 astronomical units (au), where 1 au equals the Earth-sun distance. The brighter pair, UPM J1040−3551 Aab, consists of two nearly equal-mass red dwarf stars, which appear orange in color when observed in visible wavelengths.

With a visual magnitude of 14.6, this pair is approximately 100,000 times fainter than Polaris (the North Star) in visible wavelengths. In fact, no red dwarf star is bright enough to be seen with the naked eye—not even Proxima Centauri, our closest stellar neighbor at 4.2 light-years away. To make UPM J1040−3551 Aab visible without optical aid, this binary pair would need to be brought to within 1.5 light-years of Earth, placing it closer than any star in our current cosmic neighborhood.

The fainter pair, UPM J1040−3551 Bab, are two much cooler brown dwarfs that emit virtually no visible light and appear roughly 1,000 times dimmer than the Aab pair when observed in near-infrared wavelengths, where they are most easily detected.

The close binary nature of UPM J1040−3551 Aab was initially suspected due to its wobbling photocenter during Gaia's observations and confirmed by its unusual brightness—approximately 0.7 magnitude brighter than a single star with the same temperature at the same distance, as the combined light from the nearly equal-mass pair effectively doubles the output.

Similarly, UPM J1040−3551 Bab was identified as another close binary through its abnormally bright infrared measurements compared to typical brown dwarfs of its spectral type. Spectral fitting analysis strongly supported this conclusion, with binary templates providing a significantly better match than single-object templates.

Dr. Felipe Navarete, of the Brazilian National Astrophysics Laboratory, led the critical spectroscopic observations that helped characterize the system components.

Using the Goodman spectrograph on the Southern Astrophysical Research (SOAR) Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF NOIRLab, Dr. Navarete obtained optical spectra of the brighter pair, while also capturing near-infrared spectra of the fainter pair with SOAR's TripleSpec instrument.

"These observations were challenging due to the faintness of the brown dwarfs," said Dr. Navarete, "but the capabilities of SOAR allowed us to collect the crucial spectroscopic data needed to understand the nature of these objects."

Their analysis revealed that both components of the brighter pair are M-type red dwarfs with temperatures of approximately 3,200 Kelvin (about 2,900°C) and masses of about 17% that of the sun.

The fainter pair are more exotic objects: two T-type brown dwarfs with temperatures of 820 Kelvin (550°C) and 690 Kelvin (420°C), respectively.

Brown dwarfs are small and dense low-mass objects, with the brown dwarfs in this system having sizes similar to the planet Jupiter but masses estimated to be 10 to 30 times greater. Indeed, at the low end of this range, these objects could be considered "planetary mass" objects.

"This is the first quadruple system ever discovered with a pair of T-type brown dwarfs orbiting two stars," said Dr. MariCruz Gálvez-Ortiz of the Center for Astrobiology in Spain, a co-author of the research paper. "The discovery provides a unique cosmic laboratory for studying these mysterious objects."

Unlike stars, brown dwarfs continuously cool throughout their lifetime, which changes their observable properties such as temperature, luminosity, and spectral features. This cooling process creates a fundamental challenge in brown dwarf research known as the "age-mass degeneracy problem."

An isolated brown dwarf with a certain temperature could be a younger, less massive object or an older, more massive one—astronomers cannot distinguish between these possibilities without additional information.

"Brown dwarfs with wide stellar companions whose ages can be determined independently are invaluable at breaking this degeneracy as age benchmarks," explained Professor Hugh Jones, of the University of Hertfordshire, a co-author of the research paper.

"UPM J1040−3551 is particularly valuable because H-alpha emission from the brighter pair indicates the system is relatively young, between 300 million and 2 billion years old."

The team believes the brown dwarf pair (UPM J1040−3551 Bab) could potentially be resolved with high-resolution imaging techniques in the future, enabling precise measurements of their orbital motion and dynamical masses.

"This system offers a dual benefit for brown dwarf science," said co-researcher Professor Adam Burgasser, of the University of California San Diego. "It can serve as an age benchmark to calibrate low-temperature atmosphere models, and as a mass benchmark to test evolutionary models if we can resolve the brown dwarf binary and track its orbit."

The discovery of the UPM J1040−3551 system represents a significant advancement in the understanding of these elusive objects and the diverse formation paths for stellar systems in the neighborhood of the sun.