Radiation pressure and equation of state are important in the envelope unbinding process in common envelope evolution

TL;DR

The paper addresses envelope unbinding in common envelope evolution by performing a 1D radiation hydrodynamics study that varies the radiative-to-gas energy ratio $\mathcal{E}/e_{\text{g}}$, ejecta velocity, and equation of state. It demonstrates that radiation pressure, especially in a layer below the recombination front with high opacity and luminosity, can dominate acceleration and unbind material, while a realistic EoS enhances early pressure gradients and aids ejection. The results show substantial impact on light curves, with higher $\mathcal{E}/e_{\text{g}}}$ producing brighter peaks and earlier fading, and hydrogen recombination contributing to late-time plateaus. The study provides observational diagnostics for CEE dynamics, emphasizes the necessity of radiation transport and EoS treatment in predictive models, and notes limitations of the 1D approach, suggesting future multi-D investigations with more realistic inner boundary conditions.

Abstract

In common envelope evolution, the ultimate unbinding of the envelope during the plunge-in phase involves complex and poorly understood physical processes that may give rise to luminous red novae. In this work, we investigate the roles of radiation and gas pressures in envelope unbinding. We perform a parameter space survey using a one-dimensional radiation hydrodynamic model that is solved by {\tt Guangqi} to study the impact of key parameters on the mass unbound fraction and resulting light curves. The parameters include the radiation to gas energy ratio $\mathcal{E}/e_{\text{g}}\in[0.2,3.2]$, speed of the ejecta, ranging from 70\% to 85\% of the escape velocity, and equation of state (EoS). For comparison, we also perform simulations with pure hydrodynamic or no radiation pressure effect conditions. Our simulations demonstrate that the radiation pressure is crucial for the envelope unbinding. Specifically, the radiation pressure may dominate in a high opacity and high luminosity layer just below the recombination front, where it can accelerate the sub-escape material to escape velocities. A realistic EoS further enhances the pressure gradient, especially during the early phase of ejection at small radii, promoting additional envelope ejection. Both $\mathcal{E}/e_{\text{g}}$ and EoS significantly alter light curve shapes, and provide observable diagnostics for these processes. We show that the relative energy error of all the simulations is no more than 1.4\%, and all the simulations are close to convergence.

Figures (7)

• Figure 1: The evolution of model E10 ($\mathcal{E}/e_{\text{g}}=0.8,\bar{v}_{\rm{ej}}=0.75$) from the baseline group. In each panel, the x-axis is time in days, and the y-axis is the $r$ coordinate in $1000R_{\odot}$. From panel 0 to 7, they are the evolution of the density in g$\cdot$cm$^{-3}$, luminosity in $10^{37}\rm{erg}\cdot\rm{s}^{-1}$, Rosseland mean opacity in cm$^{2}\cdot$g$^{-1}$, radiation pressure acceleration compared to the gravitational force, gas temperature in K, mean atomic weight, conservative escape measure defined Equation \ref{['eqn:xi']}, and negative pressure gradient compared to the gravitational force.
• Figure 2: The unbound mass fraction $\eta$ with different $\mathcal{E}/e_{\text{g}}$ and $\bar{v}_{\rm{ej}}$. The baseline, pure hydrodynamic and no radiation pressure effect group are results from groups 1, 4 and 5.
• Figure 3: The evolution of $a_{\rm{rad}}$ of models E2, E6, E10, E14, and E18 from the baseline group. They all have $\bar{v}_{\rm{ej}}=0.75$, but their $\mathcal{E}/e_{\text{g}}\in\{0.2,0.4,0.8,1.6,3.2\}$, respectively. The grey and red color region indicates our newly discovered radiation pressure dominated layer below the recombination front.
• Figure 4: The fraction of the unbound mass $\eta$ v.s. different $\mathcal{E}/e_{\text{g}}$, $\bar{v}_{\rm{ej}}$, and EoSs. The baseline, $\gamma_{1}=1.4$, and $\gamma_{2}=1.1$ are results from groups 1, 2, and 3.
• Figure 5: First row: the combined acceleration rate $[a_{\rm{rad}}-\pdv{p}{r}]/g$ of the model E2 from groups 1, 2, and 3. Second row: $-(\pdv{p}{r})/g$ of the model E2 from groups 1, 2, and 3. Third row: the break down of $-(\pdv{p}{r})/g$ of model E2 into $-\nabla_{\rho}p/g$, $-\nabla_{T}p/g$, $-\nabla_{\mu}p/g$, defined in Equation \ref{['eqn:breakdown']}. To enhance the contrast, the colormap range of the first row is $[0,2]$, different from the other two.