HIP 61637 b: a TESS Brown Dwarf in a Near-circular Orbit around a Massive A-type Star

We present the characterization of HIP 61637 b (TOI-5401 b), a brown dwarf discovered by TESS to transit an A-type star. HIP 61637 is the most massive and the brightest star known to host a transiting brown dwarf to date. The companion lies in the middle of the "brown dwarf desert". We perform a joint analysis of light curves from NASA's TESS mission and our high-resolution spectroscopy from the Tillinghast Reflector Echelle Spectrograph. We determine that HIP 61637 b has a radius of R BD = 1.149+0.049-0.038 R J, a mass of M BD = 47.8+1.5-1.4 M J, and transits its host star every 6.829104 plus or minus 0.000011 days in a near-circular orbit (e = 0.054 plus or minus 0.013). The host star has a mass of 2.86 plus or minus 0.12,M, a radius of 4.33 plus or minus 0.17, R, and an effective temperature of T eff = 9180+240-230 K. We find that the host is nearing the end of its time on the main sequence and has begun to evolve, allowing for a precise age estimation of 396 plus or minus 46 Myr for the system using stellar evolution models. This adds an important data point to the handful of well-characterized transiting brown dwarfs with reliable age estimates, allowing us to test the latest substellar evolution models. Theory of tidal evolution predicts that tidal dissipation mechanisms have circularized the orbit, consistent with the observed near-zero eccentricity.