Kinematic Constraints On Brown Dwarf Atmospheric Variability And Evidence For Bimodal Formation From Multi-Survey Analysis

We present a comprehensive analysis of 2,345 spectroscopically confirmed L and T dwarfs combining photometric data from SDSS DR16, 2MASS, WISE, and astrometric measurements from Gaia EDR3. Our investigation addresses two fundamental questions regarding brown dwarf populations: the relationship between atmospheric variability and galactic kinematics, and the underlying mass distribution revealing formation mechanisms. Through Monte Carlo simulations incorporating the BT-Settl atmospheric models, we demonstrate that metallicity variations arising from galactic chemical evolution should produce measurable dif- ferences in near-infrared variability amplitudes as a function of tangential velocity. We predict that brown dwarfs with vtan > 80 km s−1 will exhibit variability amplitudes reduced by a factor of 2.3 ± 0.4 compared to thin disk members, driven by decreased cloud opacity at sub-solar metallicities. This prediction is testable with current JWST monitoring programs. Analysis of the mass function reveals a statistically significant (4.7σ) deficit of objects in the 0.030 −0.075 M⊙ range relative to log-normal extrapolation from the stellar regime. The distribution exhibits a broken power law with ξ(M ) ∝ M −0.3±0.2 for M > 0.075 M⊙ and ξ(M ) ∝ M −1.4±0.3 for M <0.030 M⊙. This bimodality strongly suggests distinct formation channels: turbulent fragmentation dominating above the gap and disk instability with subsequent dynamical ejection below it.