HIP 33609 b: An Eccentric Brown Dwarf Transiting a V = 7.3 Rapidly Rotating B Star

We present the discovery and characterization of HIP 33609 b, a transiting warm brown dwarf orbiting a late B star, discovered by NASA's Transiting Exoplanet Survey Satellite as TOI-588 b. HIP 33609 b is a large (R b = 1.580−0.070+0.074 R J) brown dwarf on a highly eccentric (e = 0.560−0.031+0.029 ) orbit with a 39 days period. The host star is a bright (V = 7.3 mag), T eff = 10,400 −660+800 K star with a mass of M * = 2.383−0.095+0.10 M ⊙ and radius of R * = 1.863−0.082+0.087 R ⊙, making it the hottest transiting brown dwarf host star discovered to date. We obtained radial velocity measurements from the CHIRON spectrograph confirming the companion's mass of M b = 68.0−7.1+7.4 M J as well as the host star's rotation rate ( vsini*=55.6±1.8 km s−1). We also present the discovery of a new comoving group of stars, designated as MELANGE-6, and determine that HIP 33609 is a member. We use a combination of rotation periods and isochrone models fit to the cluster members to estimate an age of 150 ± 25 Myr. With a measured mass, radius, and age, HIP 33609 b becomes a benchmark for substellar evolutionary models.