TOI-811b and TOI-852b: New transiting brown dwarfs with similar masses and very different radii and ages from the TESS mission

Kavli Affiliate: Andrew M. Vanderburg

| First 5 Authors: Theron W. Carmichael, Samuel N. Quinn, George Zhou, Nolan Grieves, Francois Bouchy

| Summary:

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 $M_b = 55.3 pm
3.2{rm M_J}$ and radius of $R_b = 1.35 pm 0.09{rm R_J}$ and it orbits its
host star in a period of $P = 25.16551 pm 0.00004$ days. Its age of
$93^{+61}_{-29}$ Myr, which we derive from an application of gyrochronology to
its host star, is why this BD’s radius is relatively large, not heating from
its host star since this BD orbits at a longer orbital period than most known
transiting BDs. This constraint on the youth of TOI-811b allows us to test
substellar mass-radius isochrones where the radius of BDs changes rapidly with
age. TOI-852b is a much older (4.0 Gyr from stellar isochrone models of the
host star) and smaller transiting BD at a mass of $M_b = 53.7 pm 1.3{rm
M_J}$, a radius of $R_b = 0.75 pm 0.03{rm R_J}$, and an orbital period of $P
= 4.94561 pm 0.00008$ days. TOI-852b joins the likes of other old transiting
BDs that trace out the oldest substellar mass-radius isochrones where
contraction of the BD’s radius asymptotically slows. Both host stars have a
mass of $M_star = 1.32{rm M_odot}pm0.05$ and differ in their radii, $T_{rm
eff}$, and [Fe/H] with TOI-811 having $R_star=1.27pm0.09{rm R_odot}$,
$T_{rm eff} = 6107 pm 77$K, and $rm [Fe/H] = +0.40 pm 0.09$ and TOI-852
having $R_star=1.71pm0.04{rm R_odot}$, $T_{rm eff} = 5768 pm 84$K, and
$rm [Fe/H] = +0.33 pm 0.09$. We take this opportunity to examine how TOI-811b
and TOI-852b serve as test points for young and old substellar isochrones,

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