Kavli Affiliate: Sara Seager
| First 5 Authors: , , , ,
| Summary:
The orbital eccentricities of brown dwarfs encode valuable information of
their formation and evolution history, providing insights into whether they
resemble giant planets or stellar binaries. Here, we report the discovery of
TOI-5575b, a long-period, massive brown dwarf orbiting a low-mass M5V star
($rm 0.21pm0.02,M_odot$) delivered by the TESS mission. The companion has a
mass and radius of $rm 72.4pm4.1,M_J$ and $rm 0.84pm0.07,R_J$ on a 32-day
moderately eccentric orbit ($e=0.187pm0.002$), making it the third
highest-mass-ratio transiting brown dwarf system known to date. Building on
this discovery, we investigate the eccentricity distributions of a sample of
transiting long-period ($10leq Plesssim 1000$ days, $sim$0.1-1.5 AU) giant
planets, brown dwarfs and low-mass stars. We find that brown dwarfs exhibit an
eccentricity behavior nearly identical to that of giant planets: a preference
for circular orbits with a long tail toward high eccentricities. Such a trend
contrasts sharply with direct imaging findings, where cold (5-100 AU) brown
dwarfs and giant planets display distinct eccentricity distributions. Our
results suggest that transiting long-period brown dwarfs and giant planets
probably 1) form in different routes at exterior orbits but undergo analogous
dynamical evolution processes and migrate inwards; or 2) both contain two
sub-groups, one with widely spread eccentricities while the other has circular
orbits, that jointly sculpt the eccentricity distributions. The low-mass-star
systems appear to be a distinctive population, showing a peak eccentricity at
about 0.3, akin to more massive stellar binaries.
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