Astronomers report discovery of record-breaking pair of ultracool brown dwarfs. Brown dwarfs are small stellar objects that were never massive enough to become full-fledged stars, and among them are the ultra-cool failed stars, which have temperatures just above the boiling point of water. The LP 413-53AB pair is record-breaking on two counts. They are the oldest known pair, orbiting each other in 20.5 hours.
It’s important to discuss a few things about these stellar objects to understand how different this pair is. Before LP 413-53AB there were three other known pairs of ultracool dwarfs. All three were very young. They were at most 40 million years old. This is childhood in cosmic terms. This newly discovered pair is billions of years old. And the orbital period of the stars is at least three times shorter than that of all other ultracold dwarf binaries.
“It’s exciting to discover such an extreme system,” said Chih-Chun “Dino” Hsu, the northwestern astrophysicist who led the study, in a statement. “In principle, we knew that these systems should exist, but so far no such systems have been identified.”
The two brown dwarfs are less than one percent of the Earth-Sun distance apart. Credit: Adam Burgasser/University of California San Diego
The discovery of the system’s existence was made using archival data based on an algorithm developed by Hsu. The pair was then followed up with observations at the WM Keck Observatory. And thanks to those observations, the team realized how fast these two brown dwarfs orbited each other.
“When we did this measurement, we could see how things changed within a few minutes of observation,” added co-author Professor Adam Burgasser of UC San Diego. “Most of the binary stars 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 in real time how the spectral lines were moving apart. It’s amazing to see something happening in the universe on a human time scale.”
Size and distance comparison of the two brown dwarfs. Credit: Adam Burgasser/University of California San Diego
Brown dwarfs cool and shrink as they age. So turning back the clock means the stars were literally on top of each other if they stayed in their current positions. The team believes these two stars either migrated inward as they evolved, or maybe they enjoyed ejecting a third star.
Interest in these peculiar systems is high, and researchers hope to find many more out there.
“These systems are rare,” said Chris Theissen, co-author of the study and the Chancellor’s postdoctoral fellow at UC San Diego. “But we don’t know if they’re rare because they rarely exist or because we just can’t find them. That’s an open question. Now we have a data point to build on. This data has been in the archive for a long time. Dino’s tool will allow us to search for more binaries like this one.”
Hsu will present this research during a press conference at the 241st meeting of the American Astronomical Society in Seattle.