UH discovery on Maunakea sheds light on brown dwarfs

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Astronomers at the University of Hawaiʻi have precisely measured the age of a nearby Sun-like star and its unusual companion, known as a brown dwarf, an object that falls between a planet and a star. The discovery offers new clues into how brown dwarfs grow and change over time.

Using the W. M. Keck Observatory on Maunakea, the team from the UH Institute for Astronomy (IfA) studied the HR 7672 system, composed of a Sun-like star and a faint brown dwarf companion. With an instrument called the Keck Planet Finder, they tracked tiny five-minute pulsations in the star's light and used them to estimate its age to be about 2.3 billion years. The study has been recently published in Astrophysical Journal.

Because the brown dwarf formed at the same time as the star, the star's age also reveals the companion's age, giving researchers a rare chance to check if their models of how brown dwarfs cool throughout time are correct.

"This is like finally having a reliable clock for an object we've been trying to understand for years," said IfA Parrent Fellow Yaguang Li, who led the study. "It really helps us place evolutionary models under stringent tests and determine which physical ingredients are correct."

Shaping discovery

For more than two decades, the HR 7672 system has helped shape how astronomers study brown dwarfs. Its companion, HR 7672B, was discovered in 2002 and was one of the first brown dwarfs ever directly imaged around a Sun-like star using adaptive optics (AO), a technology that sharpens images blurred by Earth's atmosphere. Those early observations helped reveal how rare brown dwarfs are around Sun-like stars at close orbital distances.

Brown dwarfs do not sustain the same energy-producing reactions as stars. Instead, they slowly cool and fade over time. But testing how that happens has been difficult, in part because scientists rarely know their exact ages.

With this new measurement, paired with what is already known about the object's energy output and mass, HR 7672B now stands out as a key reference point. The team compared their findings with several models and found the closest match with newer theories that better describe what's happening inside these objects.

Full circle

The work highlights the long impact of the Beatrice Watson Parrent Fellowship at IfA. More than 20 years ago, then-fellow Michael Liu discovered HR 7672B using Keck AO. Today, Li, the current Parrent Fellow, is building on that work with this new high-precision age-dating of the same system.

"HR 7672B was one of the first discoveries I made as a Parrent Fellow when I came to UH," said Liu, IfA faculty member and co-author of the study. "It's exciting to see new work from another Parrent Fellow make this object even more valuable for understanding how brown dwarfs evolve."