Rare star system may solve the mystery of brown dwarfs

Space just dealt astronomers a curveball – one that’s 82 light-years from home, potentially capable of answering fundamental questions about some of the strangest objects in our galaxy: brown dwarfs.

Brown dwarfs are neither stars nor planets. They’re caught in between. Too small to fuel the nuclear fusion that powers true stars, but too massive to be planets, they’ve always been difficult to define.

Now, a strange new quadruple system might give researchers exactly what they’ve needed to make sense of these in-between objects.

Two stars, two dwarfs, one orbit

Astronomers have discovered a system with not just one, but four objects locked together in space. Two red dwarf stars orbit each other on one side.

On the opposite, two brown dwarfs are in another close pair. And these two pairs, in concert, orbit a common center of mass – like an intergalactic waltz taking more than 100,000 years to complete a full rotation.

This unusual system is called UPM J1040−3551 AabBab. It’s located in the constellation Antlia, about 82 light-years away from us. That might sound far, but on a cosmic scale, it’s relatively close.

The researchers who found the system used data from the European Space Agency’s Gaia satellite and NASA’s WISE mission. These tools helped them spot the signs of two separate objects moving in sync through space.

Because the orbit is so slow, scientists couldn’t see it directly. Instead, they matched angular velocity – basically, the speed and direction of the objects’ motion.

Red dwarfs meet brown dwarfs

The system breaks down into two main parts. The brighter pair, UPM J1040−3551 Aab, consists of two red dwarfs. These are small, cool stars that appear orange in visible light.

You’d never spot them with your naked eye – not even the closest red dwarf, Proxima Centauri, is visible without a telescope. This pair is about 100,000 times dimmer than Polaris, the North Star.

Then there’s the dimmer pair, UPM J1040−3551 Bab. They are brown dwarfs, and they produce hardly any visible light. They’re only visible in the near-infrared part of the spectrum and are about 1,000 times fainter than their red dwarf stars. That makes them extremely difficult to observe.

The red dwarf pair was initially indicated by a tiny “wobble” observed in Gaia’s data. It was later verified when astronomers saw it was roughly 0.7 magnitudes brighter than one red dwarf would be if it were alone at that distance. That extra brightness was a tip-off – it meant two stars were glowing together.

The brown dwarf binary was identified in the same manner. They were brighter in the infrared than one object should be, which caused scientists to suspect, and then confirm, that two brown dwarfs in close orbit were present.

What makes this system special

The researchers used the SOAR Telescope in Chile to gather more data. Dr. Felipe Navarete led the work on the ground, using optical and near-infrared spectrographs to learn more about the stars and brown dwarfs.

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

The red dwarfs turned out to be M-type stars, each with temperatures around 3,200 Kelvin (about 2,900°C). They’re each about 17% the mass of the Sun.

The brown dwarfs are even more extreme. They’re T-type, with temperatures of 820 Kelvin (550°C/1,020°F) and 690 Kelvin (420°C/790°F). They’re about the size of Jupiter, but with masses 10 to 30 times greater. At the low end of that scale, they’re brushing up against what scientists call “planetary mass.”

“This is the first quadruple system ever discovered with a pair of T-type brown dwarfs orbiting two stars,” said Dr. MariCruz Gálvez-Ortiz. “The discovery provides a unique cosmic laboratory for studying these mysterious objects.”

Brown dwarfs help measure age

One of the biggest puzzles with brown dwarfs is figuring out their age and mass. That’s not easy, because brown dwarfs cool down over time. That cooling changes how they appear in telescopes.

So when scientists spot a brown dwarf with a certain temperature, they can’t immediately tell whether it’s young and small, or old and large. This is called the “age-mass degeneracy problem.” Basically, temperature alone doesn’t tell the full story.

“Brown dwarfs with wide stellar companions whose ages can be determined independently are invaluable at breaking this degeneracy as age benchmarks,” said Professor Hugh Jones.

“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.”

Orbiting dwarfs refine science

Since the brown dwarfs revolve around one another, astronomers one day expect to follow their path and determine their precise masses.

That would allow scientists to have an unusual opportunity to tune the models that scientists employ to forecast the long-term evolution of the brown dwarfs.

“This system offers a dual benefit for brown dwarf science,” said 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.”

For now, UPM J1040−3551 AabBab is one of the best natural laboratories in space for testing ideas about how stars and brown dwarfs form and evolve. It’s a rare find, and for researchers hunting answers about the universe’s strangest objects, it couldn’t have come at a better time.

The full study was published in the journal Monthly Notices of the Royal Astronomical Society.

Image Credit: Jiaxin Zhong/ Zenghua Zhang

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