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

TL;DR

By combining asteroseismic and gyrochronology ages for HR 7672A with a dynamical mass for HR 7672B, this work provides an exceptionally precise benchmark near the stellar/substellar boundary. Six brown dwarf cooling grids are tested against the measured mass, age, and luminosity; the CBPD2023 models with the updated equation of state show the best overall agreement, within $<0.3\sigma$ of all observations. Other models generally lie within $1$–$3\sigma$ and some struggle to reach the observed low luminosity for HR 7672B. The near-simultaneous KPF/TESS observations also reveal a solar-like oscillation amplitude ratio consistent with theory, underscoring the reliability of the asteroseismic-rotational approach. These results solidify HR 7672AB as a powerful benchmark and foreshadow tighter discrimination among cooling models with Gaia DR4 and JWST data.

Abstract

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) 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 $1.87\pm0.65$ Gyr. We also determine a gyrochronological age of $2.58\pm0.47$ Gyr, and we combine these two results for a final age of $2.26\pm0.40$ 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 $1.111\pm0.017$ $M_\odot$ for HR 7672A and $75.39\pm0.67$ $M_{\rm Jup}$ for HR 7672B. This places the companion near the stellar/substellar boundary and 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 Chabrier et al. (2023) models, which incorporate a new equation of state, resulting in predictions that agree within $<$0.3$σ$ with all the observations. The other 5 sets of models agree at the 1--3$σ$ level depending on the particular test, and some models struggle to predict a sufficient low luminosity for HR 7672B at any age given its dynamical mass. Finally, we 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.

Figures (15)

• Figure 1: Radial velocity observations of HR 7672A over three consecutive nights using the Keck Planet Finder. (a) Radial-velocity time series after filtering out signals with periods longer than 1.2 hours. (b) Power spectrum of the RV time series, weighted by the reported RV uncertainties, displaying a clear power excess around 3000 $\mu{\rm Hz}$. The inset shows the spectral window. (c) Power spectrum after deconvolution against the spectral window, followed by reconvolution with a Gaussian filter of width $1/T_{\rm obs}{$T_ obs$}{}$ for clarity.
• Figure 2: Replicated échelle diagrams showing structures of regular frequency spacings from KPF data. (a): échelle diagrams of the original power spectrum. (b): échelle diagram after deconvolving with the spectral window and then smoothing with a 1 $\mu$Hz-width Gaussian filter. (c): collapsed échelle diagram, by summing the power along the y-axis of panel b. (d): same as panel b, but highlighting the oscillation modes. (e): deconvolved power spectrum of the residual time series after subtracting oscillation modes.
• Figure 3: Top: Time series of S-index measured from Ca H & K emissions. Bottom: Amplitude spectra of S-index measurements from various sites, showing clear signals around 14.8 days corresponding to rotation.
• Figure 4: Estimates of stellar ages derived from various methods, including six asteroseismic modeling ages, one rotational modeling age, two rotational empirical ages, and one asteroseismic and rotational joint modeling age.
• Figure 5: Correlation coefficients of stellar model input parameters ${\mathbf x} = (M, Y_{\rm init}{$Y_ init$}{}, \rm Age{$ Age$}, \alpha_{\rm MLT}{$α_ MLT$}{}, {\rm [M/H]}{$ [M/H]$}{})$ constrained using asteroseismology, estimated from models implemented by Team 1.