JWST/NIRCam discovery of the first Y+Y brown dwarf binary: WISE J033605.05$-$014350.4

Abstract: We report the discovery of the first brown dwarf binary system with a Y dwarf primary, WISE J033605.05$-$014350.4, observed with NIRCam on JWST with the F150W and F480M filters. We employed an empirical point spread function binary model to identify the companion, located at a projected separation of 84 milliarcseconds, position angle of 295 degrees, and with contrast of 2.8 and 1.8 magnitudes in F150W and F480M, respectively. At a distance of 10$\,$pc based on its Spitzer parallax, and assuming a random inclination distribution, the physical separation is approximately 1$\,$au. Evolutionary models predict for that an age of 1-5 Gyr, the companion mass is about 4-12.5 Jupiter masses around the 7.5-20 Jupiter mass primary, corresponding to a companion-to-host mass fraction of $q=0.61\pm0.05$. Under the assumption of a Keplerian orbit the period for this extreme binary is in the range of 5-9 years. The system joins a small but growing sample of ultracool dwarf binaries with effective temperatures of a few hundreds of Kelvin. Brown dwarf binaries lie at the nexus of importance for understanding the formation mechanisms of these elusive objects, as they allow us to investigate whether the companions formed as stars or as planets in a disk around the primary.

• The paper introduces the first Y+Y brown dwarf binary using JWST/NIRCam, detailing a 0.97 au separation and a companion-to-host mass ratio of ~0.61.
• It employs an empirical PSF binary model with F150W and F480M filters, enabling precise detection of faint companions at sub-arcsecond separations.
• The discovery challenges existing formation models and paves the way for future dynamical mass and atmospheric characterization studies.

JWST/NIRCam Discovery of the First Y+Y Brown Dwarf Binary: WISE J033605.05$-$014350.4

The paper by Calissendorff et al. introduces a notable advancement in the field of substellar astronomy with the discovery of the first Y+Y brown dwarf binary system, designated WISE J033605.05$-$014350.4 (hereafter W0336). The authors utilized data from the Near Infrared Camera (NIRCam) on the James Webb Space Telescope (JWST), observing the binary through F150W and F480M filters. This discovery provides a unique opportunity to examine the characteristics and formation mechanisms of brown dwarf binaries, particularly at the cooler end of the temperature spectrum.

Key Observations and Analysis Techniques

W0336, situated approximately 10 parsecs away, was observed using an empirical point spread function (PSF) binary model. The companion was detected at a projected separation of 0.084 arcseconds, translating to a physical separation of about 0.97 astronomical units (au), and possessed contrast differences in F150W and F480M filters of 2.8 and 1.8 magnitudes, respectively. The companion's mass is estimated between 4-12.5 Jupiter masses, accompanying a primary of 7.5-20 Jupiter masses. These values imply a companion-to-host mass ratio, $q$, of approximately 0.61, a relatively low ratio compared to other ultracool dwarf binaries.

The identification of this binary system relied on advanced data processing and analysis techniques, utilizing JWST/NIRCam's capabilities far superior to past facilities due to its sensitivity in wavelengths greater than 4.5 micrometers. Specifically, the empirical PSF was constructed from contemporaneous observations of nearby Y-dwarfs, enhancing the model accuracy in detecting faint companions at small separations.

Implications for Brown Dwarf Formation and Multiplicity Studies

The detection of W0336 adds a valuable data point to the growing population of ultracool dwarf binaries. It supports the notion that lower mass binaries, with separations less than 1 au, do exist and can be detected with advanced instrumentation like JWST. The existence of such systems challenges preexisting models of binary formation, particularly concerning formation pathways for extremely low-mass objects. Brown dwarfs, located below the hydrogen-burning limit of approximately 80 $M_{\rm Jup}$, are theorized to form through processes analogous to both star formation and planetary formation in disks around more massive bodies. The relatively low mass ratio of W0336's components may provide insights into these processes, given that existing formation models generally predict the majority of brown dwarf binaries to have near-equal mass components.

Potential and Future Research Directions

The findings in this paper carry several implications for future research. Firstly, the methodology applied here could uncover more such systems, thereby refining our understanding of binary fraction statistics, mass distributions, and the relationship between separation and primary mass. Additionally, continued observation of systems like W0336 may allow for dynamical mass determination through orbital characterization, enhancing evolutionary models' predictive power for substellar objects.

Spectroscopic follow-up observations would be valuable in constraining atmospheric conditions, surface gravities, and chemical compositions, improving theoretical models of ultracool atmospheres. Continued exploration and characterization of similar systems could elucidate the boundary between planets and brown dwarfs, specifically focusing on objects straddling the deuterium-burning threshold.

In conclusion, WISE J033605.05$-$014350.4 represents a significant discovery in brown dwarf research. The detailed analysis and novel observations provided by Calissendorff et al. extend the current framework for understanding brown dwarf formation and highlight the crucial role of advanced telescopic equipment in expanding our cosmic perspective.