Brown dwarfs are a growing area of focus for astronomers, thanks to improved instruments that have the necessary resolution to visualize them. The term describes substellar objects that are about 13 to 80 Jupiter masses, making them too small to become stars, but massive enough to experience some nuclear fusion in their cores and produce heat. Initially theorized in the 1960s, it was not until the mid-1990s that this class of stellar object was confirmed through direct observation. And thanks to next-generation telescopes and improved data-sharing techniques, there are growing opportunities to study these objects.
As with many celestial objects that were only discovered in the past few decades, there are many questions about how and under what circumstances these objects form. Combining the power of ground-based and space-based observatories, an international team of astronomers has discovered a brown dwarf (J1446B) located about 55 light-years from Earth. What's more, this brown dwarf was spotted orbiting the nearby red dwarf star J1446, essentially making it a binary companion. This discovery is providing new insight into how stars and planets form.
Taichi Uyama, a researcher with the Astrobiology Center of Japan and California State University, Northridge, led the research. He was joined by Charles Beichman of the NASA Exoplanet Science Institute (NExSI) and the Infrared Processing and Analysis Center (IPAC) at Caltech, as well as members of the National Astronomical Observatory of Japan (NAOJ), the Institute of Science Tokyo (IScT), Instituto de Astrofísica de Canarias (IAC), the Max Planck Institute for Astronomy (MPIA), the Space Telescope Science Institute (STScI), NASA's Jet Propulsion Laboratory (JPL), and multiple universities.
The paper detailing their research findings was published on Oct. 20th in *The Astronomical Journal*.
Post-processed result of the August 2023 NIRC2 image of the red dwarf J1446 and a point-like source is detected close to the central star. Credit: Taichi Uyama (Astrobiology Center/CSUN)/W.M. Keck Observatory)
Because of their faint nature, brown dwarfs are very difficult to detect in optical light. However, the team was able to overcome this problem by combining observations from three observatories: the W.M. Keck Observatory and the Subaru Telescope (both located in Mauna Kea, Hawaii), and the ESA's Gaia Observatory. The Keck I telescopes adaptive optics (AOs) and Near-Infrared Camera (NIRC2) provided high-resolution near-infrared data, allowing them to image the brown dwarf companion directly.
Meanwhile, Subaru's InfraRed Doppler (IRD) spectrograph provided radial velocity measurements while the *Gaia Observatory* measured J1446's proper motion and velocity (aka. astrometry). The radial velocity and astrometry measurements enabled the team to detect the presence of a companion around J1446, as indicated by the wobble resulting from the companion's gravitational pull. From this, they determined that J1446B has a mass of approximately 60 Jupiters and orbits its host star at 4.3 times the distance between the Earth and the Sun (4.3 AU), with an orbital period of approximately 20 years.
But what was especially impressive about the near-infrared observations was the variations they detected in J1446B's brightness (approximately 30%). This suggests that the brown dwarf experiences atmospheric phenomena, which could include clouds and powerful storms, similar to those observed with the gas giants here in the Solar System. As co-author Beichman said in a W.M. Keck Observatory press release:
Keck’s critical contribution was to make direct images of this Brown Dwarf companion which led to characterizing the object’s orbit and physical properties such as mass and temperature. Remarkably, two Keck images showed variability in the Brown Dwarf’s brightness, suggesting the existence of clouds and weather patterns! This combined approach will become more and more powerful, reaching down to the realm of gas giant planets like our own Jupiter, as new Keck instrumentation comes into operation.
“Studying the weather on these distant objects not only helps us to understand how their atmosphere form, but also informs our larger search for life planets beyond the solar system,” said Uyama. The findings are also significant since they support the idea that M-type red dwarf stars are capable of having low-mass stellar and substellar companions. For decades, astronomers believed that most red dwarfs (more than 70%) were single-star systems. While some binaries have been detected, the intrinsically faint nature of M-type dwarfs has prevented more detailed observations from being made.
However, recent advances in instrument technology and data analysis are challenging these assumptions. New observations indicate that the number of red dwarf stars with low-mass or substellar (brown dwarf) companions may be significantly greater than previously thought. Since M-type dwarfs are the most common in the Universe, accounting for about 75% of stars in our galaxy, they are vital to our understanding of stellar and planet formation. Moreover, understanding the frequency with which these stars have companions (and their mass distribution) is essential to understanding the difference between them.
Lastly, the discovery of J1446B will provide opportunities to study the formation of brown dwarfs and atmospheric models. In the near future, follow-up observations are expected using the Keck Observatory’s new High-resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC) combined with ensuing Gaia Data Releases. These studies will enable the mapping of J1446B's weather patterns and advance our understanding of how stars and planetary systems form and evolve, which includes the Solar System.
Further Reading: Keck Observatory, The Astronomical Journal
