A rare quadruple star system that is the first of its kind to be discovered by astronomers could help better understand so-called “failed stars” or brown dwarfs.
These celestial bodies get their unfortunate nickname from the fact that, despite forming like a standard star, they “fail” to gather enough mass to trigger the nuclear fusion of hydrogen to helium in their cores, the process that defines what a star is. Brown dwarfs are somewhat mysterious because it is difficult to track how their characteristics change as they age.
This system, designated UPM J1040−3551 AabBab and located around 82 light-years from Earth, is an extremely rare hierarchical quadruple star system containing a pair of cold brown dwarfs that orbit a pair of young red dwarf stars. The two pairs of binary stellar bodies are separated by 1,656 times the distance between Earth and the sun, meaning that it takes over 100,000 Earth-years to complete an orbit.
The team discovered the nature of UPM J1040−3551 AabBab using the European Space Agency (ESA) star tracking spacecraft Gaia and NASA’s Wide-field Infrared Survey Explorer (WISE) to measure the angular velocity of its components. This allowed them to see that Aab (the stellar pair Aa and Ab) and Bab (the brown dwarf pair Ba and Bb) are moving towards the same direction with the same angular velocity.
“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,” team leader and Nanjing University researcher Zenghau Zhang said in a statement. “These two pairs of objects are orbiting each other separately for periods of decades, while the pairs are also orbiting a common centre of mass over a period of more than 100,000 years.”
Decoding a very rare star system
Even though the stars of Aab are the brightest components of this quadruple star system, they are still so faint that to be visible with the naked eye, these red dwarfs would have to be no more than 1.5 light-years from Earth. Emitting little to no visible light, the pair of brown dwarfs Bab is around 1,000 times fainter than their parent binary red dwarfs and can only be observed in infrared.
Despite their faint nature, it was the unusual relative brightness of the red dwarf binary (Aab) compared to other red dwarfs, in addition to a wobble in the center of light between the stars, that initially hinted that this is a close stellar binary and not a single star. The brown dwarf pairing of Bab was also “decoded” thanks to its unusual brightness compared to other “typical” solo failed stars.
The nature of the four components of UPM J1040−3551 AabBab was confirmed by spectroscopy performed for Aab using optical light using the Goodman spectrograph on the Southern Astrophysical Research (SOAR) Telescope at Cerro Tololo Inter-American Observatory in Chile. The nature of Bab was confirmed in infrared light with the aid of SOAR’s TripleSpec instrument.
“These observations were challenging due to the faintness of the brown dwarfs,” Navarete said. “But the capabilities of SOAR allowed us to collect the crucial spectroscopic data needed to understand the nature of these objects.”
This analysis revealed both components of the red dwarf pairing Aab have masses around 17% that of the sun, with temperatures of around 5,300 degrees Fahrenheit (2,900 degrees Celsius).
Meanwhile, the brown dwarfs of Bab have masses of between 10 to 30 times that of Jupiter (0.01 to 0.03 times the mass of the mass) with temperatures of between 1,020 degrees Fahrenheit (550 degrees Celsius) and 788 degrees Fahrenheit (420 degrees Celsius). This makes these two failed stars rare examples of T-type brown dwarfs.
“This is the first quadruple system ever discovered with a pair of T-type brown dwarfs orbiting two stars,” team member MariCruz Gálvez-Ortiz of the Center for Astrobiology in Spain said. “The discovery provides a unique cosmic laboratory for studying these mysterious objects.”
This is because brown dwarfs cool as they age, which changes their properties. This process depends on the mass of the brown dwarf, leading to a challenge in the understanding of these objects called the “age-mass degeneracy problem.”
That means a brown dwarf of a set temperature could be a young, less massive object, or an older, more massive failed star. Without additional information, astronomers can’t determine which of these possibilities is correct. But that information could be available if the brown dwarf has an associated failed star companion.
“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 [created when an electron falls from the third lowest to second lowest energy level] from the brighter pair indicates the system is relatively young, between 300 million and 2 billion years old.”
The team behind this discovery believes that the study of the Bab brown dwarf binary in UPM J1040−3551 Aab Bab could be bolstered in the future when high-resolution imaging has progressed enough to enable precise measurements of their orbital motion and masses.
“This system offers a dual benefit for brown dwarf science,” team member Adam Burgasser, of the University of California, San Diego, said. “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 team’s research was published in the Monthly Notices of the Royal Astronomical Society
