Hints of Disk Substructure in the First Brown Dwarf with a Dynamical Mass Constraint

We present new high-angular-resolution ALMA Band 7 observations of the Class II brown dwarf 2MASS J04442713+2512164 (2M0444), one of the brightest and most extended brown dwarf disks known to date. The observations combine 0.89 mm continuum emission with ¹²CO (3–2) and ¹³CO (3–2) molecular line data, reaching a spatial resolution of 0.046″(~6.4 au). These data allow us to explore the structure and kinematics of the disk at unprecedented scales in the substellar regime.

The ¹²CO emission reveals a rotating Keplerian disk, enabling the first dynamical mass determination for this source directly from ALMA observations. Depending on the fitting methodology, we derive a central mass between 0.043 and 0.092 M_⊙, placing 2M0444 at or near the substellar boundary and making it the lowest-mass object with a dynamical mass constraint derived from ALMA disk kinematics. The observations also reveal a striking difference between the radial extent of the gas and dust components. Visibility-plane analysis indicates a gas-to-dust size ratio larger than 6, significantly higher than expected from optical depth effects alone and consistent with efficient radial drift of millimeter-sized grains toward pressure maxima within the disk.

Our continuum visibility analysis reveals tentative evidence for annular substructure within the inner disk. Independent modeling using both the nonparametric frank framework and parametric galario fitting recovers a possible gap and ring pair located at approximately 14–16 au. The visibility profile departs from a smooth Gaussian morphology and shows oscillatory behavior consistent with unresolved substructure. In addition, new ALMA observations obtained at even higher angular resolution (~0.020″) reveal, in the image plane, a second candidate gap-ring pair located at a radius of approximately 50 au. These preliminary results suggest that the disk may host multiple substructures across a broad range of spatial scales. If confirmed, these detections would represent some of the first resolved disk substructures identified in the brown dwarf regime.

Assuming the inner gap is produced by an embedded companion, empirical relations suggest a planet mass between 0.3 and 7.7 M_⊕, compatible with rocky planet formation through core accretion. Such a scenario would imply that even very low-mass disks are capable of forming planetesimals and potentially terrestrial planets, despite the severe limitations imposed by rapid dust drift and low disk masses.

These results provide new insight into dust evolution, disk dynamics, and planet formation around substellar objects. They also highlight the importance of very high spatial resolution ALMA observations for probing the architecture of brown dwarf disks and testing whether the mechanisms shaping disks around solar-type stars extend into the lowest-mass regime.