A Numerical Method for Studying Super-Eddington Mass Transfer in Double White Dwarf Binaries

TL;DR

This paper introduces a high-accuracy numerical method to simulate super-Eddington mass transfer in double white dwarf binaries, revealing that radiation pressure minimally impacts accretion flow in highly super-Eddington regimes.

Contribution

The paper presents a new numerical approach that conserves energy more accurately and models super-Eddington mass transfer in DWD systems, including radiation transport and hydrodynamics.

Findings

Mass transfer rate exceeds Eddington limit by many orders of magnitude.

Radiation pressure has little effect in highly super-Eddington flows.

A common envelope forms rapidly but contains little gravitationally bound material.

Abstract

We present a numerical method for the study of double white dwarf (DWD) binary systems at the onset of super-Eddington mass transfer. We incorporate the physics of ideal inviscid hydrodynamical flow, Newtonian self-gravity, and radiation transport on a three-dimensional uniformly rotating cylindrical Eulerian grid. Care has been taken to conserve the key physical quantities such as angular momentum and energy. Our new method conserves total energy to a higher degree of accuracy than other codes that are presently being used to model mass-transfer in DWD systems. We present the results of verification tests and we simulate the first 20+ orbits of a binary system of mass ratio q = 0.7 at the onset of dynamically unstable direct impact mass transfer. The mass transfer rate quickly exceeds the critical Eddington limit by many orders of magnitude, and thus we are unable to model a trans-Eddington phase. It appears that radiation pressure does not significantly effect the accretion flow in the highly super-Eddington regime. An optically thick common envelope forms around the binary within a few orbits. Although this envelope quickly exceeds the spatial domain of the computational grid, the fraction of the common envelope that exceeds zero gravitational binding energy is extremely small, suggesting that radiation-driven mass loss is insignificant in this regime. It remains to be seen whether simulations that capture the trans-Eddington phase of such flows will lead to the same conclusion or show that substantial material gets expelled.