Populating the brown dwarf and stellar boundary: Five stars with transiting companions near the hydrogen-burning mass limit

Kavli Affiliate: Robert F. Goeke

| First 5 Authors: Nolan Grieves, Fran├žois Bouchy, Monika Lendl, Theron Carmichael, Ismael Mireles

| Summary:

We report the discovery of five transiting companions near the
hydrogen-burning mass limit in close orbits around main sequence stars
originally identified by the Transiting Exoplanet Survey Satellite (TESS) as
TESS Objects of Interest (TOIs): TOI-148, TOI-587, TOI-681, TOI-746, and
TOI-1213. Using TESS and ground-based photometry as well as radial velocities
from the CORALIE, CHIRON, TRES, and FEROS spectrographs, we found the
companions have orbital periods between 4.8 and 27.2 days, masses between 77
and 98 $mathrm{M_{Jup}}$, and radii between 0.81 and 1.66 $mathrm{R_{Jup}}$.
These targets have masses near the uncertain lower limit of hydrogen core
fusion ($sim$73-96 $mathrm{M_{Jup}}$), which separates brown dwarfs and
low-mass stars. We constrained young ages for TOI-587 (0.2 $pm$ 0.1 Gyr) and
TOI-681 (0.17 $pm$ 0.03 Gyr) and found them to have relatively larger radii
compared to other transiting companions of a similar mass. Conversely we
estimated older ages for TOI-148 and TOI-746 and found them to have relatively
smaller companion radii. With an effective temperature of 9800 $pm$ 200 K,
TOI-587 is the hottest known main-sequence star to host a transiting brown
dwarf or very low-mass star. We found evidence of spin-orbit synchronization
for TOI-148 and TOI-746 as well as tidal circularization for TOI-148. These
companions add to the population of brown dwarfs and very low-mass stars with
well measured parameters ideal to test formation models of these rare objects,
the origin of the brown dwarf desert, and the distinction between brown dwarfs
and hydrogen-burning main sequence stars.

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