Kavli Affiliate: Sara Seager
| First 5 Authors: Elina Y. Zhang, Theron W. Carmichael, Daniel Huber, Keivan G. Stassun, Akihiko Fukui
| Summary:
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 $32.0^{+1.9}_{-1.8}M_{Jup}$ and
$1.018^{+0.048}_{-0.043}R_{Jup}$, orbiting a late-F star about
$5.4^{+2.8}_{-2.2}$ Gyr old in a 10.4138$pm$0.000014 day period. The older
TOI-5422b has $27.7^{+1.4}_{-1.1}M_{Jup}$ and $0.815^{+0.031}_{-0.026}R_{Jup}$
in a 5.3772$pm$0.00001 day orbit around a subgiant star about $8.2pm2.4$ 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^{+9.96}_{-11.79}$$^{circ}$. Therefore, it is likely that
TOI-5422b is spinning up the host star and its orbit is aligned with the
stellar spin axis.
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