Shining Too Bright: Testing Brown Dwarf Models with HD 4747 B and HD 19467 B
Brown dwarfs, objects too big to be planets but too small to shine like stars, are tricky to understand. Their brightness depends not only on their mass but also on their age, and this makes it hard for astronomers to build accurate models of how they evolve. In this paper, Wood and collaborators focus on two special brown dwarfs, HD 4747 B and HD 19467 B, which orbit Sun-like stars. Because their host stars can be studied in detail, the ages and other properties of the brown dwarfs can be pinned down much more precisely. This makes them valuable “benchmarks” for testing substellar evolutionary models (SSEMs).
Measuring the Stars to Understand the Companions
The team used the CHARA Array, a powerful network of telescopes in California, to measure the physical sizes of the host stars HD 4747 A and HD 19467 A. By combining these measurements with data from the Gaia mission, they determined the radii, temperatures, and luminosities of the stars. This information was then fed into different stellar evolution models (Dartmouth, MIST, and Yonsei-Yale) to estimate stellar ages. The adopted ages turned out to be about 10.7 billion years for HD 4747 A and about 10 billion years for HD 19467 A.
Calculating the Brightness of the Brown Dwarfs
Assuming the brown dwarfs are the same ages as their host stars, the authors next calculated their intrinsic brightness, known as bolometric luminosity. For HD 4747 B, the luminosity was measured to be about 3.7 × 10⁻⁵ times the Sun’s, and for HD 19467 B about 6.5 × 10⁻⁶ times the Sun’s. These measurements serve as a direct test for the theoretical models of brown dwarf cooling and evolution.
Testing the Models: A Mismatch
When Wood and colleagues compared their measurements to predictions from commonly used models (COND03, BHAC15, and SM08), they found a clear mismatch. The models consistently predicted brown dwarfs that were too dim compared to the observed values. For HD 4747 B, the models under-predicted brightness by about 0.75 dex (roughly a factor of 6), while for HD 19467 B the difference was about 0.5 dex (a factor of 3). This also meant that the models were overestimating the masses of these objects, by about 12% for HD 4747 B and 30% for HD 19467 B.
Clouds, Metallicity, and Missing Physics
The team explored why the models might be off. One possibility is that the models don’t fully account for the effects of clouds in brown dwarf atmospheres. For HD 4747 B, which sits at the “L/T transition” where clouds start forming, including clouds in the models did improve the match, reducing the discrepancy to about 0.6 dex. However, this still didn’t solve the problem completely. Another factor could be metallicity, the abundance of elements heavier than hydrogen and helium, which affects how light passes through a brown dwarf’s atmosphere. Current models only test a limited range of metallicities, so new model sets, like the upcoming Sonora models, will be important for further progress.
Looking Forward
Overall, the study shows that brown dwarfs appear to be brighter than expected for their measured ages and masses. This suggests that today’s models are still missing important pieces of physics. Future improvements in measuring brown dwarf masses and host star ages, especially with new data from Gaia and possible asteroseismology studies with TESS, could help resolve the remaining puzzles.
Source: Wood
