From Common Envelope Evolution to Luminous Red Novae I: A One-dimensional Radiation Hydrodynamic Model

The acceleration and unbinding of the common envelope during the plunge-in phase are governed by complex physical processes that often manifest observationally as luminous red novae. We investigate the dynamics of this phase using one-dimensional radiation hydrodynamic simulations evolved with the code {\tt Guangqi}. We perform a parameter survey to quantify the impact of key physical conditions on the unbound mass fraction, $η$, and the resulting light curves. Our survey spans a range of radiation-to-gas internal energy ratios ($\mathcal{E}/e_{\text{g}}\in[0.2,3.2]$), ratios of total envelope energy to gravitational binding energy ($ζ\in[0.54,2.87]$), and mass injection rates ($\dot{M}\in[2.5,10]M_{\odot}/\rm{yr}$), while covering both subsonic and supersonic expansion regimes ($v_{\rm ej}/v_{\rm esc}\in[0.3,0.6]$). We demonstrate that: (1) radiation pressure becomes the dominant driver of mass ejection in the high-opacity, high-luminosity region immediately below the recombination front; (2) $η$ exhibits a nonlinear dependence on $ζ$, which is modulated by the mass injection rate and gravitational potential; and (3) the recombination of atomic to molecular hydrogen ($\ce{H}\to\ce{H2}$) releases latent heat that sustains a secondary plateau in the late-time light curve. These findings are substantiated by detailed error analysis and convergence testing presented in the Appendices.

💡 Research Summary

This paper presents a comprehensive study of the plunge‑in phase of common‑envelope evolution (CEE) using one‑dimensional radiation‑hydrodynamic (RHD) simulations performed with the newly developed code “Guangqi.” The authors aim to elucidate how radiation pressure, recombination energy, and mass‑injection properties control the unbinding of the envelope and the observable signatures that appear as luminous red novae (LRNe).

The governing equations include mass, momentum, total‑energy, and radiation‑energy conservation, coupled through a flux‑limited diffusion (FLD) approximation for radiative transfer. The radiation flux and pressure tensor are expressed in terms of the flux‑limiter λ(R) and the Eddington factor f(R), ensuring a smooth transition between the optically thick diffusion limit and the optically thin free‑streaming limit. Gas–radiation energy exchange is modeled by a Planck‑mean opacity term G₀ = κ_P ρ c (E − a T_g⁴). The code solves the hydrodynamics with an HLLC Riemann solver and MUSCL reconstruction, while the stiff radiation–gas coupling is treated implicitly using a GMRES Krylov solver (PETSc) with a relative tolerance of 10⁻¹⁰.

The computational domain is spherical, extending from an inner radius r_in = 450 R_⊙ to an outer radius r_out = 1.5 × 10⁴ R_⊙. A base grid of Nₓ = 2048 cells is refined once over the inner region