Evolution of BD-14 3065b (TOI-4987b) from giant planet to brown dwarf as possible evidence of deuterium burning at old stellar ages

The present study confirms BD-14 3065b as a transiting planet-brown dwarf in a triple-star system, with a mass near the deuterium-burning boundary. BD-14 3065b has the largest radius observed within the sample of giant planets and brown dwarfs around post-main sequence stars. Its orbital period is 4.3 days and it transits a subgiant F-type star with a mass of M_∗ = 1.41 ± 0.05 M_⊙, a radius of R_∗ = 2.35 ± 0.08 R_⊙, an effective temperature of T_eff = 6935 ± 90 K, and a metallicity of -0.34 ± 0.05 dex. By combining TESS photometry with high-resolution spectra acquired with the TRES and Pucheros+ spectrographs, we measured a mass of M_p = 12.37 ± 0.92 M_Jup and a radius of R_p = 1.926 ± 0.094 R_Jup. Our discussion of potential processes that could be responsible for the inflated radius led us to conclude that deuterium burning is a plausible explanation for the heating taking place in BD-14 3065ba's interior. Detections of the secondary eclipse with TESS photometry enabled a precise determination of the eccentricity, e_p = 0.066 ± 0.011, and reveal that BD-14 3065b has a brightness temperature of 3520 ± 130 K. With its unique characteristics, BD-14 3065b presents an excellent opportunity to study its atmosphere via thermal emission spectroscopy.