We present numerical simulations of dynamically unstable mass transfer in a double white dwarf binary with initial mass ratio , q = 0.4 .
The binary components are approximated as polytropes of index n = 3 / 2 and the initially synchronously rotating , semi-detached equilibrium binary is evolved hydrodynamically with the gravitational potential being computed through the solution of Poisson ’ s equation .
Upon initiating deep contact in our baseline simulation , the mass transfer rate grows by more than an order of magnitude over approximately ten orbits , as would be expected for dynamically unstable mass transfer .
However , the mass transfer rate then reaches a peak value , the binary expands and the mass transfer event subsides .
The binary must therefore have crossed the critical mass ratio for stability against dynamical mass transfer .
Despite the initial loss of orbital angular momentum into the spin of the accreting star , we find that the accretor ’ s spin saturates and angular momentum is returned to the orbit more efficiently than has been previously suspected for binaries in the direct impact accretion mode .
To explore this surprising result , we directly measure the critical mass ratio for stability by imposing artificial angular momentum loss at various rates to drive the binary to an equilibrium mass transfer rate .
For one of these driven evolutions , we attain equilibrium mass transfer and deduce that effectively q _ { crit } has evolved to approximately 2 / 3 .
Despite the absence of a fully developed disk , tidal interactions appear effective in returning excess spin angular momentum to the orbit .