BD-14 3065b (TOI-4987b): from giant planet to brown dwarf: evidence for deuterium burning in old age?

The present study reports the confirmation of BD-14 3065b, 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_J and a radius of R_p=1.926±0.094 R_J. Our discussion of potential processes that could be responsible for the inflated radius led us to conclude that deuterium burning is a plausible explanation resulting from the heating of BD-14 3065b's interior. Detection of the secondary eclipse with TESS photometry enables a precise determination of the eccentricity e_p=0.066±0.011 and reveals 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 through thermal emission spectroscopy.