TOI-811b and TOI-852b: New Transiting Brown Dwarfs with Similar Masses and Very Different Radii and Ages from the TESS Mission

We report the discovery of two transiting brown dwarfs (BDs), TOI-811b and TOI-852b, from NASA's Transiting Exoplanet Survey Satellite mission. These two transiting BDs have similar masses but very different radii and ages. Their host stars have similar masses, effective temperatures, and metallicities. The younger and larger transiting BD is TOI-811b at a mass of M_b = 59.9 ± 13.0M_J and radius of R_b = 1.26 ± 0.06R_J, and it orbits its host star in a period of P = 25.16551 ± 0.00004 days. We derive the host star's age of ${93}_{-29}^{+61}$ Myr from an application of gyrochronology. The youth of this system, rather than external heating from its host star, is why this BD's radius is relatively large. This constraint on the youth of TOI-811b allows us to test substellar mass-radius evolutionary models at young ages where the radius of BDs changes rapidly. TOI-852b has a similar mass at M_b = 53.7 ± 1.4M_J but is much older (4 or 8 Gyr, based on bimodal isochrone results of the host star) and is also smaller with a radius of R_b = 0.83 ± 0.04R_J. TOI-852b's orbital period is P = 4.94561 ± 0.00008 days. TOI-852b joins the likes of other old transiting BDs that trace out the oldest substellar mass-radius evolutionary models where contraction of the BD's radius slows and approaches a constant value. Both host stars have a mass of M_⋆ = 1.32M_⊙ ± 0.05 and differ in their radii, T_eff, and [Fe/H], with TOI-811 having R_⋆ = 1.27 ± 0.09R_⊙, T_eff = 6107 ± 77 K, and [Fe/H] = + 0.40 ± 0.09 and TOI-852 having R_⋆ = 1.71 ± 0.04R_⊙, T_eff = 5768 ± 84 K, and [Fe/H] = + 0.33 ± 0.09. We take this opportunity to examine how TOI-811b and TOI-852b serve as test points for young and old substellar isochrones, respectively.