Tight dance of the tiny ones: Astronomers have discovered the closest pair of ultracold dwarf stars to date at a distance of 120 light years. The two glowing dark red dwarfs are separated by less than a hundredth of the distance between the sun and the earth, and they only need 17 hours to orbit. Their orbit is four times shorter than that of the previously closest known pair of dwarfs. Because both stars could not have formed so close to each other, they probably got into their current position afterwards.
Most of the stars in the cosmos are not solitary, but part of a double or multiple system – our sun probably once had a stellar sister. Astronomical observations suggest that the proportion of binary star systems decreases as the star mass decreases: while massive blue giants are still more than 80 percent in pairs, this proportion falls to less than 20 percent in the case of low-mass red and brown dwarfs.
However: Especially very cool, faint dwarf stars are often relatively close together. This is why astronomers often find it difficult to tell whether these M dwarfs are alone or in pairs in the sky. So far, only three such ultracold dwarf pairs have been identified by transit, as reported by Chih-Chun Hsu from Northwestern University and his colleagues. Dwarf stars with surface temperatures of less than 2,100 degrees are considered ultracold.
Light spectrum reveals double nature
Now there’s another pair of dwarfs – and a record-breaking one at that. The two stars LP 413-53 A and B lie about 120 light-years away in the constellation of Taurus and appear as just one star in telescopic images. However, slight fluctuations in brightness have long aroused the suspicion that this nearby dwarf could actually be a binary system. To verify this, Chu and his team examined the system using the near-infrared spectrometer at Keck Observatory in Hawaii.
The observations revealed that LP 413-53AB must in fact be two closely spaced stars. The two dwarfs are only 0.08 and 0.06 solar masses and have surface temperatures of less than 2,100 degrees. The dual nature of this system was recognizable by the regular drifting apart of several spectral lines. This movement was amazingly fast: “We could see how the spectra changed within just a few minutes,” says co-author Adam Burgasser from the University of California in San Diego. “For most binary stars, this takes years.”
Closest pairing of any dwarf
The analyzes showed: The two dwarf stars of LP 413-53AB orbit each other with a period of only 17 hours. They are therefore less than 0.008 astronomical units apart – which corresponds to less than one hundredth of the distance between the earth and the sun. “This is the closest known pair of ultracold dwarf stars,” say the astronomers. At the same time, LP 413-53AB is also one of the closest binaries among all main sequence stars.
“Discovering such an extreme system is very exciting,” says Chu. “We knew that such binary systems exist in principle, but none of them had been discovered before.” The age of this pair of dwarfs is also unusual: the three previously known ultracold binary dwarfs were less than 40 million years old and therefore still very young and comparatively warm . But LP 413-53AB is more than a billion years old according to astronomers’ estimates, probably around the age of our Sun.
Wandered inside
But that means: In their youth, these two dwarf stars must have been significantly larger and hotter. Their surfaces almost touched at today’s distance. Given the minimal distance between the two dwarf stars, astronomers assume that they did not form in their current position. Instead, the two dwarfs likely formed at a greater distance from each other. Only afterwards did they drift towards each other due to their mutual attraction and the braking effect of tidal forces.
Alternatively, an originally third partner in the system could have brought the two dwarfs in touch. Its gravitational pull slowed the ultra-cold dwarfs before turbulence catapulted the third star out of the system. “Indeed, some close pairs of ultracold dwarf stars are known that are still in hierarchical systems of three,” Chu and his colleagues explain. (The Astrophysical Journal Letters, accepted; doi: 10.48550/arXiv.2301.07039)
Quelle: W. M. Keck Observatory
