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

Abstract 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 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 ( E / e g ∈ [ 0.2 , 3.2 ] ), ratios of total envelope energy to gravitational binding energy ( ζ ∈ [0.54, 2.87]), and mass injection rates ( M ̇ ∈ [ 2.5 , 10 ] M ⊙ yr - 1 ), while covering both subsonic and supersonic expansion regimes ( v ej / v esc ∈ [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 (H → H 2 ) 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.