Scientists have discovered a record-breaking binary star system consisting of two ultra-cool dwarf stars so close together that they complete an orbit in less than a day.
The stars are separated by only about 1.5 million miles, about 1% of the distance between Earth and sun, which means that a year for these stars lasts only 17 hours. This makes the star system the densest ultracool dwarf binary ever found.
The binary systemdesignated LP 413–53AB and placed in the constellation of The bullwas discovered by Northwestern University and University of California San Diego (UC San Diego) astrophysicists using the WM Keck Observatory on the slopes of the dormant Maunakea volcano in Hawaii.
“It is exciting to discover such an extreme system,” said Chih-Chun “Dino” Hsu, the leader of the team and an astrophysicist at Northwestern University in a statement. (opens in a new tab) “In principle, we knew these systems should exist, but no such systems had been identified yet.”
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Ultracool dwarfs are one of the most common star types in the country universe, but these low-mass stars are so cool that most of the light they emit is in the form of an infrared glow. This means they are invisible to the human eye and difficult to detect. Short-period binaries of these stars are even rarer.
Before the discovery of LP 413–53AB, astronomers had only discovered three ultracool dwarf binaries with short-period orbits, all of which were less than 40 million years old, practically infants compared to our 4.6 billion-year-old Sun.
The team estimates that LP 413–53AB, on the other hand, is much older than the stars in the previously known systems, and has an age of several billion years, much closer to that of the Sun. The system’s orbital period is about four times shorter than the orbital period of the densest of the previously discovered ultracool dwarf binaries.
The team first discovered this record-breaking binary while searching through archival data, and Hsu noticed something odd about the light spectrum coming from LP 413–53AB. Early observations had caught the binary as the stars aligned, meaning their spectral data overlapped and they were mistaken for a single star.
The spectral lines then split and moved in opposite directions as the stars continued in their orbits, pointing Hsu to the fact that he was actually looking at two stars. The astrophysicist also realized that these stars must be locked in a very tight binary system.
Following up on this revelation with the Keck Observatory’s Near-Infrared Spectrograph (NIRSPEC), Hsu and the team conducted several observations of the system between March 2022 and January 2023. These observations confirmed Hsu’s prediction that the distance between the stars is about 1.48 million miles .
“When we made this measurement, we could see things change within a couple of minutes of observation,” Hsu’s Ph.D. adviser and astrophysicist at UC San Diego Adam Burgasser, who co-authored the study, said in the statement. “Most of the binaries we follow have orbital periods of years. So you get a measurement every few months. Then, after a while, you can put the puzzle together. With this system, we could see the spectral lines move apart in real time. It’s amazing to see something happen in the universe on a human time scale.”
The team theorizes that the stars may have migrated toward each other during their lifetimes as they evolved. Alternatively, they may have been part of a three-body star system, moving together when the third companion was ejected. Confirming these theories will require further observations of the LP 413–53AB system.
Hsu believes that studying other ultracool dwarf binaries could help us learn more about potentially habitable worlds beyond the solar systemextrasolar planets, or “exoplanets.” Because ultracool dwarfs are far less hot than the Sun, the region around these stars where liquid water can exist is known as “habitable zone,” would be much closer to the star.
However, life existing in a habitable zone around LP 413–53AB is almost out of the question. This is because the habitable zone distance for these stars will be close to the size of their orbits, meaning that planets are unlikely to form because the motion of the stars and their radiation will disperse the material needed to collapse and form planets.
“These ultracool dwarfs are neighbors of our sun,” Hsu said. “To identify potentially habitable hosts, it is useful to start with our nearest neighbors. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found.”
Hsu and Burgasser will investigate this possibility by continuing the search for additional short-lived ultracool dwarf binaries and thereby building a larger data sample than the very sparse one that currently exists.
“These systems are rare, but we don’t know if they’re rare because they rarely exist or because we just don’t find them,” said Chris Theissen, a researcher at the UC San Diego Center for Astrophysics and Space Sciences. “That’s an open question. Now we have one data point that we can start building on. This data had been in the archive for a long time. Dino’s tools will enable us to look for more binaries like this.”
The team’s research is available as a preprint on the website ArVix (opens in a new tab) and has been accepted for publication in The Astrophysical Journal Letters.
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