An Oasis in the Brown Dwarf Desert: Confirmation of Two Low-mass Transiting Brown Dwarfs Discovered by TESS

Abstract

As the intermediate-mass siblings of stars and planets, brown dwarfs (BDs) are vital to study for a better understanding of how objects change across the planet-to-star mass range. Here, we report two low-mass transiting BD systems discovered by TESS, TOI-4776 (TIC 196286578) and TOI-5422 (TIC 80611440), located in an under-populated region of the BD mass-period space. These two systems have comparable masses but different ages. The younger and larger BD is TOI-4776b with $32.0_{-1.8}^{+1.9}{M}_{Jup}$ and $1.018_{-0.043}^{+0.048}{R}_{Jup}$, orbiting a late-F star about $5.4_{-2.2}^{+2.8}$ Gyr old in a 10.4138$\pm$0.000014 day period. The older TOI-5422b has $27.7_{-1.1}^{+1.4}{M}_{Jup}$ and $0.815_{-0.026}^{+0.031}{R}_{Jup}$ in a 5.3772$\pm$0.00001 day orbit around a subgiant star about $8.2\pm 2.4$ Gyr old. Compared with substellar mass-radius (M-R) evolution models, TOI-4776b has an inflated radii. In contrast, TOI-5422b is slightly "underluminous" with respect to model predictions, which is not commonly seen in the BD population. In addition, TOI-5422 shows apparent photometric modulations with a rotation period of 10.75$\pm$0.54 day found by rotation analysis, and the stellar inclination angle is obtained to be $I_{\star }=75.52_{-11.79}^{+9.96}$${}^{\circ }$. Therefore, it is likely that TOI-5422b is spinning up the host star and its orbit is aligned with the stellar spin axis.