The cool brown dwarf Gliese 229 B is a close binary (2410.11953v1)

Abstract: Owing to their similarities with giant exoplanets, brown dwarf companions of stars provide insights into the fundamental processes of planet formation and evolution. From their orbits, several brown dwarf companions are found to be more massive than theoretical predictions given their luminosities and the ages of their host stars (e.g. Brandt et al. 2021, Cheetham et al. 2018, Li et al. 2023). Either the theory is incomplete or these objects are not single entities. For example, they could be two brown dwarfs each with a lower mass and intrinsic luminosity (Brandt et al. 2021, Howe et al. 2024). The most problematic example is Gliese 229 B (Nakajima et al. 1995, Oppenheimer et al. 1995), which is at least 2-6 times less luminous than model predictions given its dynamical mass of $71.4\pm0.6$ Jupiter masses ($M_{\rm Jup}$) (Brandt et al. 2021). We observed Gliese 229 B with the GRAVITY interferometer and, separately, the CRIRES+ spectrograph at the Very Large Telescope. Both sets of observations independently resolve Gliese 229 B into two components, Gliese 229 Ba and Bb, settling the conflict between theory and observations. The two objects have a flux ratio of $0.47\pm0.03$ at a wavelength of 2 $\mu$m and masses of $38.1\pm1.0$ and $34.4\pm1.5$ $M_{\rm Jup}$, respectively. They orbit each other every 12.1 days with a semimajor axis of 0.042 astronomical units (AU). The discovery of Gliese 229 BaBb, each only a few times more massive than the most massive planets, and separated by 16 times the Earth-moon distance, raises new questions about the formation and prevalence of tight binary brown dwarfs around stars.

• The paper presents a breakthrough by resolving Gliese 229 B into two distinct brown dwarfs, thereby explaining its previously anomalous luminosity.
• High-resolution astrometry and spectroscopy provided dynamic mass measurements and orbit details, revealing a 12.1-day period and a 0.042 AU separation.
• These findings challenge existing substellar evolution models and suggest a need to reevaluate other unresolved brown dwarfs in astrophysical research.

The Cool Brown Dwarf Gliese 229 B is a Close Binary

The paper presented offers a detailed examination of the brown dwarf Gliese 229 B, which has been resolved into a binary system, Gliese 229. Ba and Bb. This paper poses significant implications for our understanding of substellar objects, particularly in addressing the mismatch between theoretical models and observed properties of such systems. Using high-resolution techniques offered by the GRAVITY interferometer and the CRIRES+ spectrograph, the authors resolved Gliese 229 B into two distinct brown dwarfs. This resolution provides a compelling explanation for the previously noted incongruities between Gliese 229 B's modeled and observed luminosities.

Key Findings

• Resolution of Problematic Observations: Gliese 229 B, originally observed to have a significantly lower luminosity than predicted by substellar evolutionary models, is resolved into two components, Gliese 229. Ba and Bb. The dynamic mass measurements obtained through Gaia DR3 and Hipparcos data place the mass at approximately 71.4 ± 0.6 M_Jup.
• Binary Characteristics: The resolved binary system, characterized by a flux ratio of 0.47 ± 0.03 at 2 μm, comprises masses of 38.1 ± 1.0 M_Jup and 34.4 ± 1.5 M_Jup for Gliese 229. Ba and Bb, respectively. The components orbit one another with a period of 12.1 days and a semimajor axis of 0.042 AU, marking this system as an exceptionally tight brown dwarf binary.
• Astrophysical Context: This discovery raises intriguing questions about the formation mechanisms for such tight brown dwarf binaries. The configuration of Gliese 229. BaBb, particularly its compact size relative to other known systems, challenges prevailing models of star formation, which typically predict wider separations.
• Substantial Methodological Approaches: The paper utilizes rigorous astrometric and spectroscopic observations from VLTI/GRAVITY and VLT/CRIRES+, employing robust methods such as astrometric fitting and RV extraction. These techniques have proven independently effective in confirming the binarity of Gliese 229 B, with a distinct statistical significance.

Implications for Substellar Models

The discovery of Gliese 229. BaBb significantly impacts theoretical models, specifically concerning the mass-luminosity relationship in substellar objects. By aligning the observed characteristics of the system with the predictions of the ATMO 2020 substellar evolutionary model for an age of approximately 2.45 ± 0.20 Gyr, the paper resolves the long-standing mass-luminosity discrepancy associated with Gliese 229 B as a single entity. Furthermore, it suggests that other anomalously massive brown dwarfs must be reassessed, potentially as unresolved binaries, necessitating a reevaluation of model assumptions used in substellar astrophysics.

Future Directions

The paper opens promising avenues for future research. Observations aimed at resolving other similarly affected brown dwarfs into binaries will be essential. This effort will strengthen the understanding of substellar evolutionary processes and refine the models used to infer properties of directly imaged exoplanets and brown dwarf companions lacking dynamical masses. Moreover, this paper underscores the potential for interferometry and high-resolution spectroscopy, along with advances in observational technologies, to explore and characterize such compact binary systems further.

Overall, this paper documents a pivotal step in addressing critical challenges in the modeling of brown dwarfs, positing that the resolution of tight binary systems like Gliese 229. BaBb could reshape our understanding of substellar formation and evolution.