The OATMEAL Survey – III. An aligned transiting warm brown dwarf and evidence for quiescent brown dwarf migration

ABSTRACT We present the first measurement of the sky-projected orbital obliquity of a benchmark transiting brown dwarf host, HIP 33609, as a part of the Orbital Architectures of Transiting Massive Exoplanets And Low-mass stars (OATMEAL) survey. HIP 33609 b is an eccentric, 68 $M_{\rm J}$ brown dwarf orbiting a 10 300 K, A-type star with an orbital period of 39 d. Its host star is a known member of the 150 Myr old MELANGE-6 moving group, making it an excellent laboratory for testing sub-stellar evolutionary models. Using in-transit spectra collected by the Planet Finder Spectrograph (PFS) on the Magellan II Clay 6.5 m telescope, we measured a sky-projected orbital obliquity of $|\lambda | = 12.7 \pm 1.3 {}^{\circ }$. The mass of the brown dwarf is most consistent with a stellar-like fragmentation formation history followed by a period of migration. Given the eccentricity ($e=0.557$) but low orbital obliquity of the brown dwarf, we claim that coplanar high eccentricity tidal migration seems to be the most plausible pathway; however, it remains difficult to conclusively rule out other migration mechanisms. The low orbital obliquity for HIP 33609 is consistent with previous measurements of high mass-ratio companions, and bears a striking resemblance to the obliquity distribution of transiting warm Jupiters. We suggest brown dwarfs may follow a dynamically quiescent migration pathway, consistent with them forming in isolated conditions.