2026ApJ...999..217C

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 (${ \mathcal E }/{e}_{\,\rm{g}\,}\in [0.2,3.2]$), ratios of total envelope energy to gravitational binding energy (ζ ∈ [0.54, 2.87]), and mass injection rates ($\dot{M}\in [2.5,10]{M}_{\odot }\,{\mathrm{yr}}^{-1}$), 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 → 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.