Results Test Brown Dwarf Theory challenging astrophysicists

Maunakea, Hawaiʻi – Astronomers using W. M. Keck Observatory on Maunakea, Hawaiʻi Island, have measured one of the most precise ages yet for a Sun-like star hosting a brown dwarf companion. The result offers a powerful new test of how brown dwarfs cool and evolve over time, helping to address a long-standing challenge in astrophysics.

The study focused on the nearby system HR 7672, which includes a Sun-like star and a faint brown dwarf companion. Using Keck Observatory’s Keck Planet Finder (KPF), the team detected subtle oscillations in the star’s surface, ripples that revealed its age to be 2.3 billion years.

Because the brown dwarf formed alongside the star, this precise stellar age serves as a benchmark for the companion’s evolution, offering a rare chance to directly test theoretical models of brown dwarf cooling.

“The 18% age uncertainty establishes the HR7672 system as a valuable benchmark for years to come,” said Yaguang Li, lead author and researcher at the University of Hawaiʻi at Mānoa.

The study, led by the University of Hawaiʻi Institute for Astronomy, is published in The Astrophysical Journal.

 A Two-Decade Scientific Arc

The HR 7672 system has played a historic role in the study of substellar objects. The companion, known as HR 7672B, was first discovered by researcher Michael Liu, co-author and professor at the University of Hawaiʻi Institute for Astronomy. HR 7672B was the first directly imaged brown dwarfs orbiting a Sun-like star.

Using Keck Observatory’s Near-Infrared Camera (NIRC2) and the telescope’s Adaptive Optics system to correct for atmospheric blurring, Liu obtained a sharper image of the brown dwarf, which is 2,000 times fainter than its bright host star.

“Pioneering observations with Keck Observatory helped illuminate the so-called “brown dwarf desert,” the scarcity of such companions around Sun-like stars at close separations,” said Liu.

Now, more than two decades later, a new generation of Keck Observatory instrumentation continues to advance that legacy. Using ultra-precise measurements of the host star with the Keck Planet Finder (KPF) instrument, astronomers detected tiny stellar pulsations that reveal the star’s internal structure and age with unprecedented precision.

“The unique fast-readout mode of the Keck Planet Finder makes it the only instrument in the Northern Hemisphere capable of sampling oscillations on such short timescales,” added Li.

Testing How Brown Dwarfs Cool Over Time

Brown dwarfs are failed stars that are too small to sustain stable hydrogen fusion, so they gradually cool and fade as they age. Their brightness, therefore, depends sensitively on both their mass and age. However, astronomers have had difficulty testing theoretical models of this cooling, in part because reliable ages are rarely available.

Now, with this new and precise age measurement, combined with HR 7672B’s well-known luminosity and mass, the system becomes an exceptional “benchmark” for testing brown dwarf evolutionary models.

Comparing the observations with six different theoretical cooling models, the team found the best agreement with the most recent models that incorporate updated interior physics. Without the new data, the team would not have been able to distinguish this model from the five other possibilities.

These results demonstrate that high-precision stellar ages are essential for understanding substellar evolution — and show that precision spectroscopy with the next generation of observations will finally provide this information.

“Yaguang’s research has made this object even more valuable for our theoretical understanding of brown dwarfs,” said Liu.

As a next step, the researchers plan to generalize this method to a broader set of benchmark systems and test brown dwarf evolutionary models across different regimes.