Astrophysicists from Northwestern University and the University of California, San Diego, have identified the densest binary system ever observed among extremely cold dwarfs. The two stars are so close that they orbit each other in less than an Earth day, meaning that a year of each star is only 20.5 hours long.

The newly discovered LP 413-53AB system consists mainly of a pair of ultracold dwarfs, very low-mass stars that emit light in the infrared, making them invisible to the human eye. Despite this, it is one of the most common types of stars in the universe.

Astronomers have previously discovered only three short-period ultracold dwarf binary systems, all relatively young – up to 40 million years ago. LP 413-53AB is estimated to be billions of years old – the same age as our Sun – but has an orbital period at least three times shorter than any extremely cold dwarf pair ever discovered.

“It’s exciting to discover such an extreme system,” said Chi-Chun “Dino” Hsu, a Northwest astrophysicist who led the study. “We knew, in principle, that these systems should exist, but no such system had yet been described.”

Hsu recently presented the research at a press conference of the American Astronomical Society as part of a session on “Stars and their Activity” at the 241st meeting of the American Astronomical Society in Seattle. Hsu is a postdoctoral fellow in physics and astronomy at Northwestern’s Weinberg College of Arts and Sciences and a fellow of Northwestern’s Center for Interdisciplinary Research and Research in Astrophysics (CIERA). She started this research as a doctorate. He is a student at the University of California, San Diego, where he was recommended by Professor Adam Burgasser.

The team first discovered the strange duo while digging through archival data. Hsu developed an algorithm that can model a star based on its spectral data. By analyzing the spectrum of light emitted by a star, astrophysicists can determine the star’s chemical composition, temperature, gravity, and spin. This analysis also shows the motion of the star as it moves towards and away from the observer, known as the radial velocity.

This drawing compares the proximity of two dwarf stars in a recently discovered binary system to other systems. Credit: Adam Burgasser/University of California, San Diego

While examining the spectral data of the LP 413-53AB, Hsu noticed something strange. Early observations caught the system when the stars were roughly aligned and their spectral lines overlap, leading Xu to believe it was just a star. But as the stars moved in their orbits, the spectral lines shifted in opposite directions, splitting into pairs in later spectral data. Hsu realized that the two stars were actually locked in an incredibly close binary system.

Using the powerful telescopes at the WM Keck Observatory, Hsu decided to observe the phenomenon himself. On March 13, 2022, the team turned the telescopes to the constellation Taurus, where the binary system is located, and observed for two hours. They then made additional observations in July, October, and December.

“When we made this measurement, we were able to see that everything changed in a few minutes of observation,” Burgasser said. Said. “Most of the binaries we watch have rotation periods of years. So you get a measurement every few months. Then, after a while, you’ll be able to put the puzzle together. With this system, we were able to see how the spectral lines diverge in real time. It’s amazing to see something happening in the universe on the human time scale.”