The Tale of a Hungry Subgiant and Its Brown Dwarf: Interior Radiative Damping Dominates the Tidal Evolution of TOI-5882

Technical Summary: The Tale of a Hungry Subgiant and Its Brown Dwarf: Interior Radiative Damping Dominates the Tidal Evolution of TOI-5882

Problem Statement
The post-main-sequence evolution of stars with close-in companions is governed by complex interactions between stellar expansion, mass loss, and tidal dissipation. These processes determine the orbital architecture of binary systems, potentially leading to companion engulfment, common-envelope evolution, and the formation of compact remnants. However, accurately modeling these events is challenging because existing tidal prescriptions often fail to account for the evolving internal structure of the star. Classical equilibrium tide models (e.g., Hut 1981; Hurley et al. 2002) typically assume tidal dissipation occurs solely within the convective envelope via turbulent viscosity. This approach neglects contributions from deeper radiative regions and the frequency-dependent coupling between the orbit and the star's internal mode spectrum. Consequently, these models may significantly underestimate tidal dissipation rates and mispredict the timescales for orbital decay and Roche-lobe overflow (RLOF), particularly for subgiant and red giant systems hosting massive companions like brown dwarfs.

Methodology
To address these limitations, the authors develop a self-consistent tidal evolution framework that couples the stellar evolution code MESA (Modules for Experiments in Stellar Astrophysics) with the linear tidal response code GYRE-tides.

• Coupling Mechanism: The framework operates by passing instantaneous stellar structure models from MESA to GYRE at each evolutionary timestep. GYRE solves the linearized, non-adiabatic equations of stellar pulsations to compute the full tidal response, including the excitation and damping of internal gravity waves (IGWs).
• Torque and Energy Calculation: The resulting secular torque and energy deposition rates are calculated using the full linear tidal response formalism (Townsend et al. 2018; Sun et al. 2023). These values are then fed back into MESA via specific hook functions (other_jdot_ls and other_edot_tidal) to update the orbital angular momentum and energy.
• Target System: The framework is applied to TOI-5882, a 1.3 $M_\odot$ subgiant hosting a 22 $M_{Jup}$ brown dwarf on a 7.14-day, nearly circular orbit.
• Comparative Models: The authors perform four distinct binary evolution simulations to isolate the effects of different dissipation mechanisms:
  1. MESA's standard equilibrium tide with a linear tidal reduction factor ($n=1$).
  2. MESA's standard equilibrium tide with a quadratic tidal reduction factor ($n=2$).
  3. The coupled GYRE-tides framework with viscous damping disabled (radiative damping only).
  4. The coupled GYRE-tides framework with both radiative and viscous damping enabled.

Key Results

1. Dominance of Radiative Damping: The study demonstrates that interior radiative damping of IGWs, rather than convective envelope viscosity, dominates the tidal evolution of TOI-5882. The classical equilibrium tide models significantly underestimate the star's angular momentum evolution by several orders of magnitude.
2. Accelerated Inspiral: The coupled framework predicts a 2–6 fold reduction in the engulfment timescale compared to classical prescriptions. Specifically:
   • The standard MESA equilibrium tide ($n=2$) predicts an RLOF timescale of $\sim$130 Myr.
   • The GYRE-tides framework with radiative damping only predicts RLOF in $\sim$30 Myr.
   • Including viscous damping further reduces this to $\sim$22 Myr.
   • Overall, the companion's inspiral is accelerated by roughly 25–110 Myr relative to classical models.
3. Wave Propagation and Dissipation: Analysis of the tidal forcing frequency relative to the Brunt–Väisälä frequency reveals that the system operates in a regime where IGWs are excited at the radiative-convective boundary (RCB) and propagate inward. These waves encounter steep gradients near the hydrogen-burning shell, where radiative damping becomes highly efficient.
4. Evolutionary Regimes: The early inspiral is driven by the non-resonant dissipation of IGWs. As the system evolves and the orbital frequency increases, the system transitions into a regime dominated by resonance crossings as it approaches RLOF.
5. Angular Momentum Transport: The study maps the radial profile of the cumulative tidal torque. It finds that while gravitational torque is similar across models in the deep interior, the inclusion of viscous damping creates localized angular momentum deposition in the convective zone, leading to non-uniform redistribution and zonal flows.

Significance and Claims
The authors claim that their work necessitates a fundamental reframing of how tidal interactions are categorized. They argue against the historical dichotomy between "equilibrium tides" (convective) and "dynamical tides" (radiative), noting that this separation is often misapplied in population synthesis codes. Instead, they propose categorizing tidal interactions based on their dissipation mechanisms: radiatively damped tides and viscously damped tides.

The paper asserts that their MESA–GYRE-tides framework provides a computationally feasible, self-consistent method to model these mechanisms simultaneously. By capturing the full linear tidal response, the framework corrects the underestimation of tidal decay in post-main-sequence systems. The authors suggest this approach is broadly applicable to a wide class of star-companion systems, including binary stars and hot Jupiters, offering a more accurate physical basis for understanding orbital decay, the survival of companions, and the formation of compact remnants.