A pair of very close ultracool dwarf stars has been discovered

Artist's concept of Scholz's star with its companion, a binary system that might be similar to LP 413-53AB (Image Michael Osadciw/University of Rochester)
Artist’s concept of Scholz’s star with its companion, a binary system that might be similar to LP 413-53AB (Image Michael Osadciw/University of Rochester)

An article published in “The Astrophysical Journal Letters” reports the discovery that the system cataloged as LP 413-53AB consists of two ultracool dwarf stars that orbit each other in just 17 hours. A team of researchers led by Chih-Chun “Dino” Hsu of Northwestern University used observations conducted with the Keck Observatory telescopes to resolve two stars so small and so close. Previously, three binary systems composed of ultracool dwarfs had been discovered but they were young stars in astronomical terms while the pair of LP 413-53AB is estimated to be some billion years old. We don’t know how difficult these pairs are to find due to their weak emissions, and explaining the existence of LP 413-53AB is difficult.

Binary systems are the most common in the universe but gravitational interactions between two companion stars tend to slow down their rotation and at the same time increase their distance. For this reason, two young stars may be close but two close stars that are a few billion years old are an anomaly.

Ultracool dwarf stars are the smallest in existence and sometimes barely have enough mass to trigger nuclear fusion. For this reason, their electromagnetic emissions are concentrated in the infrared band and adequate instruments are needed to detect them even in the cosmic neighborhood.

In the case of the LP 413-53AB system, the researchers used in particular the Near-InfraRed SPECtrometer (NIRSPEC) instrument on the Keck II telescope to detect its infrared. Initially, the researchers thought that this system about 120 light-years from Earth had only one star but its emissions changed very quickly, a very strange event.

Five observation campaigns conducted between March 2022 and January 2023 showed the presence of two stars, both very small, with a mass less than a tenth of the Sun’s. The article leaves open the possibility that one of the two companions is actually a brown dwarf, a kind of failed star halfway between star and planet. The distance between the two companions is estimated to be around one-hundredth of that between the Sun and the Earth and that’s what surprised the researchers the most.

By reconstructing the dynamics of the pair of LPs 413-53AB in the simplest way, we should conclude that at their birth they were practically on top of each other. There are alternative explanations: it’s possible that actually, the two stars were born far from each other and only over time they got close enough to form today’s bond or it was originally a triple system with a third star that was ejected by the gravitational interactions bringing the other two to their current distance.

Ultracool dwarf stars are very common, also because they consume their hydrogen extremely slowly and consequently have an extremely long life. This makes it difficult to accurately estimate their age, and the pair of LPs 413-53AB has an age estimated loosely at some billions of years. These stars have become particularly interesting after the confirmation that one of them, TRAPPIST-1, has seven rocky planets and that at least on some of them there could be favorable conditions to host life forms similar to the Earth’s ones.

If there are planets orbiting the pair of LP 413-53AB, they must be significantly farther from them than their system’s habitable zone. Astronomers have just started understanding the habitability potential of ultracool dwarf star systems thanks to TRAPPIST-1 and are slowly gathering information about the tiny stars discovered in the cosmic neighborhood.

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