Asteroseismology and Dynamics Reveal Interior Structure and Coeval Evolution in the Triply Post-Main-Sequence system DG Leo

$δ$ Scuti stars in binary or multiple systems serve as crucial probes for studying stellar pulsation and evolution. However, many such systems are not ideal for asteroseismology due to uncertainties in mass transfer with close companions and the challenges of dynamically measuring all components’ physical properties. The triple system DGLeo, comprising an inner binary and a distant $δ$ Scuti star, is an ideal target due to its well-separated pulsator. By combining new \textit{TESS} photometry with archival spectroscopy, our dynamical analysis shows that the system’s three components share similar masses, radii, and luminosities within errors, occupying coincident Hertzsprung–Russell diagram positions, indicative of coeval evolution. By fitting seven observed $δ$ Scuti frequencies through asteroseismic modeling with dynamically constrained theoretical grids, we simultaneously trace the pulsating star’s evolution and constrain the triple system’s evolutionary stage, with the derived fundamental parameters showing consistency with the dynamical solutions. Our analysis reveals that all three components of DGLeo are in the post-main-sequence phase, with a system age of $0.7664^{+0.1402}{-0.1258}$~Gyr. Additionally, the $δ$ Scuti component shows multiple non-radial modes with significant mixed-character frequencies, providing precise constraints on its convective core extent ($R{\mathrm{cz}}/R = 0.0562^{+0.0137}_{-0.0021}$).

💡 Research Summary

DG Leo is a hierarchical triple system composed of a close spectroscopic binary (components Aa and Ab) and a widely separated visual companion (component B) that exhibits δ Scuti pulsations. The authors combine four sectors of high‑cadence TESS photometry (120‑second sampling) with archival high‑resolution spectroscopy to derive a comprehensive dynamical and asteroseismic model of the system.

First, the TESS light curves are phase‑folded on the inner binary’s 4.146‑day orbital period, and the dominant ellipsoidal variation (frequency ≈0.482 d⁻¹) is modeled using the Wilson–Devinney (W‑D) code in detached mode. Fixed parameters include the spectroscopically measured mass ratio (q = 1.000), primary effective temperature (T₁ = 7470 K), bolometric albedos (A₁ = A₂ = 1.0), and gravity‑darkening coefficients (g₁ = g₂ = 1.0). Free parameters (inclination, potentials, secondary temperature, luminosity ratio) are optimized for each of the four TESS sectors, yielding consistent solutions: i ≈ 66–69°, component masses M₁ ≈ M₂ ≈ 2.26 M⊙, radii R₁ ≈ 3.35 R⊙ and R₂ ≈ 3.26 R⊙. Using the spectroscopic total mass and the derived mass ratio between the visual companion and the inner pair, the authors infer M_B ≈ 2.39 M⊙, R_B ≈ 2.95 R⊙, and L_B ≈ 26 L⊙.

After subtracting the binary model, the residual light curve is analyzed with Period04. Forty frequencies are extracted, of which seven are independent δ Scuti modes (F₁–F₇) spanning 9.45–12.75 c d⁻¹ (periods 0.07–0.11 d). The remaining frequencies are identified as linear combinations or side‑lobes. Pulsation constants Q are calculated using the mean density of component B (ρ ≈ 0.094 ρ⊙), yielding Q ≈ 0.018–0.022 d, consistent with low‑order radial and non‑radial p‑modes.

For asteroseismic modeling, a grid of MESA evolutionary tracks (mass 2.0–2.5 M⊙, Z ≈ 0.014) coupled with GYRE pulsation calculations is constructed. The dynamical constraints (mass, radius, luminosity) are imposed as priors, and a χ² minimization is performed to match the seven observed frequencies simultaneously. The best‑fitting model places all three stars on the post‑main‑sequence, with a common age of 0.766 Gyr (±0.13 Gyr). The convective‑core boundary is tightly constrained, giving a fractional core radius R_cz/R = 0.0562 (+0.0137/‑0.0021), indicating modest core overshoot.

The convergence of dynamical and seismic solutions demonstrates that DG Leo’s components share nearly identical masses, radii, and luminosities, occupying the same position on the Hertzsprung–Russell diagram. This co‑evolution strongly supports a simultaneous formation scenario for the triple system. Moreover, the study resolves the long‑standing ambiguity regarding component B, confirming it as a normal δ Scuti pulsator rather than an ultra‑short‑period Cepheid.

The paper highlights the power of combining precise orbital dynamics with space‑based asteroseismology in systems where mass transfer is negligible. Such an approach yields sub‑percent level constraints on fundamental stellar parameters and interior mixing processes, providing a benchmark for stellar evolution theory. The authors suggest that future high‑resolution spectroscopic monitoring and longer baseline photometry could refine rotation profiles, magnetic effects, and possible non‑linear mode coupling, further exploiting DG Leo as a laboratory for testing theories of tidal interaction and stellar interior physics.