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 ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mi class="MJX-tex-calligraphic" mathvariant="script">E</mml:mi> <mml:mo>/</mml:mo> <mml:msub> <mml:mrow> <mml:mi>e</mml:mi> </mml:mrow> <mml:mrow> <mml:mspace width="0.1em"/> <mml:mtext>g</mml:mtext> <mml:mspace width="0.1em"/> </mml:mrow> </mml:msub> <mml:mo>∈</mml:mo> <mml:mo stretchy="false">[</mml:mo> <mml:mn>0.2</mml:mn> <mml:mo>,</mml:mo> <mml:mn>3.2</mml:mn> <mml:mo stretchy="false">]</mml:mo> </mml:math> ), ratios of total envelope energy to gravitational binding energy ( ζ ∈ [0.54, 2.87]), and mass injection rates ( <mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" overflow="scroll"> <mml:mover accent="true"> <mml:mi>M</mml:mi> <mml:mo>̇</mml:mo> </mml:mover> <mml:mo>∈</mml:mo> <mml:mo stretchy="false">[</mml:mo> <mml:mn>2.5</mml:mn> <mml:mo>,</mml:mo> <mml:mn>10</mml:mn> <mml:mo stretchy="false">]</mml:mo> <mml:msub> <mml:mi>M</mml:mi> <mml:mo>⊙</mml:mo> </mml:msub> <mml:mspace width="0.25em"/> <mml:msup> <mml:mi>yr</mml:mi> <mml:mrow> <mml:mo>-</mml:mo> <mml:mn>1</mml:mn> </mml:mrow> </mml:msup> </mml:math> ), 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.