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

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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 (TI 80611440), located in an underpopulated region of the BD mass–period space. These two systems have comparable masses but different ages. The younger and larger BD is TOI-4776 b with 32 . 3 − 1.7 + 1.8 M Jup and 1.01 ± 0.05 R Jup , orbiting a late-F star about 4 . 7 − 2.5 + 3.1 Gyr old in an ∼10.41 day period. The older TOI-5422 b has 28 . 0 − 1.2 + 1.6 M Jup and 0.81 ± 0.03 R Jup in an ∼5.38 day orbit around a subgiant star about 7 . 6 − 2.6 + 2.5 Gyr old. Compared with substellar mass–radius evolution models, TOI-4776 b has an inflated radius. In contrast, TOI-5422 b 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 10.8 ± 0.5 days found by rotation analysis, and the stellar inclination angle is obtained to be I ⋆ = 7 6 − 12 + 10 deg. Therefore, it is likely that TOI-5422 b is spinning up the host star and its orbit is aligned with the stellar spin axis.

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