Oldest known pair of ultracool dwarf stars discovered

Using the W. M. Keck Observatory in Hawaii, researchers have discovered the tightest ultracool dwarf binary system known to date. The two stars in this newly-discovered system are so close that it takes them less than one Earth day to revolve around each other.

The team, comprising Northwestern University and the University of California San Diego (UC San Diego) astrophysicists, first discovered the strange binary system while exploring archival data. The system, named LP 413-53AB, comprises a pair of ultracool dwarfs, a type of low-mass star that are much cooler and dimmer than the Sun and emit their light primarily in the infrared, making them completely invisible to the human eye.

Located in the constellation Taurus, LP 413-53AB is estimated to be billions of years old like our Sun, making it the oldest binary dwarf known to date. However, it has an orbital period that is about four times shorter than all the ultracool dwarf binaries discovered so far.

"It's exciting to discover such an extreme system. In principle, we knew these systems should exist, but no such systems had been identified yet," said the lead-author Chih-Chun “Dino” Hsu, a Northwestern astrophysicist.

Hsu noticed something strange while examining the spectral data of LP 413-53AB. Spectral data is the information obtained by analyzing the light emitted or absorbed by an object.

Initially, the system was observed when the stars were in a roughly aligned position and their spectral lines overlapped. This gave the impression that there was only one star. However, as the stars progressed in their orbit, the spectral lines began to shift in opposite directions and were eventually separated into pairs in later spectral data. Thereafter, Hsu realized that the system was actually composed of two stars that orbit each other closely.

On March 13, 2022, the team turned the Keck II telescope toward the Taurus constellation and observed the system for two hours, followed by more observations in July, October, and December 2022 as well as January 2023.

The team speculates that the stars either migrated toward each other as they evolved, or they could have come together after the ejection of a third, now lost, stellar member.

According to the team, there are two possible scenarios that could have led to the formation of the binary system: either the stars moved closer to each other over time as they evolved or they were brought together after a third star, which is no longer present, was ejected from the system.

"These ultracool dwarfs are neighbours of our Sun. To identify potentially habitable hosts, it's helpful to start with our nearby neighbours. But if close binaries are common among ultracool dwarfs, there may be few habitable worlds to be found," Hsu said.

More details can be found here.