They’re twins! The famous brown dwarf Gliese 229B, discovered 30 years ago, is not a single object – it is a pair of dwarfs, as new observations reveal. Accordingly, the system, which is only around 19 light-years away, consists of two brown dwarfs weighing 34 and 38 solar masses, which orbit each other extremely closely. This explains why Gliese 229B appeared so faint despite its large mass. At the same time, this bright discovery raises new questions, as the astronomers report in “Nature”.
Riddle about too weak luminosity
But despite its fame, the brown dwarf Gliese 229B still poses a mystery today: “The high mass of Gliese 229B contradicts all substellar development models, because according to them, an object with 71 Jupiter masses of its age would have to have a bolometric luminosity two to 20 times higher than ours measure,” explain Jerry Xuan from the California Institute of Technology and his colleagues. This raises doubts about the current models for determining the mass of brown dwarfs and large gas planets.
In order to solve the mystery of Gliese 229B, Xuan and his team have now set their sights on the nearby brown dwarf for another five months – with two particularly high-resolution instruments from the Very Large Telescope (VLT) of the European Southern Observatory in Chile. The GRAVITY instrument combines the data from the four telescope units using interferometry and thereby achieves a particularly high spatial resolution. The CRIRES+ spectrograph can make the smallest shifts in the light spectrum of a celestial object visible.
It’s a couple!
The surprising result: Gliese 229B is not just one brown dwarf, but two – it is a double system. “So we now know that we were mistaken about the nature of this object all along,” says Xuan. “It’s two instead of one. But we haven’t been able to detect this so far because of their small distance.” According to the data, Gliese 220B consists of two medium-sized brown dwarfs with 34 and 38 Jupiter masses.
Unusually small distance
But why was the double system never recognized as such before? The reason is that the two brown dwarfs Gliese 229Ba and Bb are only 0.042 astronomical units apart – which is only around 16 times the distance from the Earth to the Moon. For star-like celestial bodies, these two brown dwarfs are extremely close. “Their close separation makes them the closest pair of brown dwarfs orbiting a star,” the astronomers write.
But this raises new questions for the pair of dwarves. How such close pairs of brown dwarfs form in orbit around a star is still unclear. Astronomers suspect that the precursors of these objects are formed by the local gravitational collapse of dust and gas in the disk of material around a young star – similar to how Jupiter and other gas giants probably formed. The young brown dwarfs then either subsequently drift towards each other and become a pair or are gravitationally bound from the start.
Origin difficult to explain
However, such models have difficulty explaining extremely close pairs like Gliese 229B. “This density-limited fragmentation limits the original distances of such objects to more than ten astronomical units,” explain the astronomers. “This means that substantial dynamic and dissipative processes are necessary to form a very close binary system of brown dwarfs.” In addition, the orbits of Gliese 229 Ba and Bb are slightly tilted against the rotation axis of their central star – this is also common Difficult to explain in models.
“Even 30 years after its discovery, Gliese 229B continues to teach us new things about substellar objects,” say Xuan and his colleagues. They suspect that some other unusual massive brown dwarfs are actually close pairs. “This is the most exciting and fascinating discovery in substellar astrophysics in recent decades,” says Rebecca Oppenheimer of Caltech, who co-discovered Gliese 229B in 1995. (Nature, 2024; two: 10.1038/s41586-024-08064-x)
Quelle: Nature, California Institute of Technology
October 17, 2024 – Nadja Podbregar
