A Test of Substellar Evolutionary Models with High-precision Ages from Asteroseismology and Gyrochronology for the Benchmark System HR 7672AB

We present high-precision measurements for HR 7672AB, composed of a Sun-like (G0V) star and an L dwarf companion. Three nights of precise (70 cm s⁻¹) radial velocity (RV) asteroseismology with the Keck Planet Finder clearly detect 5 minute oscillations from the primary, HR 7672A, and modeling of the frequency spectrum yields an asteroseismic age of 1.87 ± 0.65 Gyr. We also determine a gyrochronological age of 2.58 ± 0.47 Gyr, and we combine these two results for a final age of 2.26 ± 0.40 Gyr. In addition, we obtained new RVs for HR 7672A and new astrometry for the companion HR 7672B. From a joint orbit fit, we measured a dynamical mass of 1.111 ± 0.017 M_⊙ for HR 7672A and 75.39 ± 0.67 M_Jup for HR 7672B. This places the companion near the stellar/substellar boundary and is thus particularly sensitive to differences in model predictions. The joint precision in host star age (18% uncertainty) and companion mass (0.9% uncertainty) makes HR 7672AB an exceptional substellar benchmark. Combined with the companion’s luminosity, we use these measurements to test predictions from six brown dwarf cooling models. The best agreement occurs with the G. Chabrier et al. models, which incorporate a new equation of state, resulting in predictions that agree within <0.3σ with all the observations. The other five sets of models agree at the 1σ–3σ level, depending on the particular test, and some models struggle to predict a sufficiently low luminosity for HR 7672B at any age given its dynamical mass. We also detected a weak seismic signal in near-simultaneous TESS photometry of HR 7672A, with the resulting RV-to-photometry oscillation amplitude ratio consistent with solar values.