Presentation #331.02 in the session Brown Dwarfs.
We report the discovery and characterization of TOI-3662b, a sub-giant star companion on the borderline between a brown dwarf (BD) and a low-mass M-star, from the TESS mission. TOI-3662b has a moderately eccentric orbit (e = 0.38) around a late-A/early-F type star with a period of P = 9.98635 ± 0.00007 days. Our lightcurves from space-based TESS and ground-based KeplerCam on the 1.2-meter telescope at Whipple Observatory yield the BD radius of Rb = 2.69 ± 0.37 RJ.We performed high-resolution spectroscopy using the Tillinghast Reflector Echelle Spectrograph (TRES), and determined the stellar properties using the Stellar Parameter Classification (SPC) tool. The host star has a mass of 2.26 ± 0.26 Msun, a radius of 3.56 ± 0.40 Rsun, an effective temperature of Teff = 7630 ±630 K, and a metallicity of 0.134 ± 0.097 dex. TOI-3662b has a mass of 88.7 ±7.3 MJ, placing it near the border of the upper-mass limit of Brown Dwarfs (80 Jupiter Masses). We derived the age of the system using MIST stellar isochrones and EXOFASTv2 software, arriving at a bimodal solution of 4 and 760 Myr-old systems. Independent analysis of relative probabilities and other age indicators, such as Lithium absorption in spectra, lead us to accept the post-main sequence solution. Given its longer orbital period and relatively young age, we conclude that the host star has not yet had time to circularize the orbit of its companion. TOI-3662b is one of the first borderline BDs discovered around a sub-giant star, and joins the ranks of three dozen transiting Brown dwarfs studied to this day.