Hydrodynamics of Coalescing White Dwarfs
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**Session 41 -- Computational Astrophysics II**
*Display presentation, Wednesday, 1, 1994, 9:20-6:30*

## [41.12] Hydrodynamics of Coalescing White Dwarfs

*F. A. Rasio (Inst. for Advanced Study)*

The results of a new numerical study of the equilibrium and stability
properties of close, degenerate binaries will be presented.
The smoothed-particle hydrodynamics (SPH) technique is used both
to construct accurate equilibrium configurations in three dimensions,
and to follow their hydrodynamic evolution.
The equation of state is that of a polytrope with $\Gamma=5/3$, applicable
to low-mass white dwarfs.
Along a sequence of binary equilibrium configurations for two identical stars,
the SPH calculations demonstrate the existence of both secular and dynamical
instabilities, confirming directly the results of recent analytic work.
The SPH method can be used to calculate the nonlinear development of
a dynamical instability and to determine the final fate of the system:
the two stars merge together into a single, rapidly rotating
object in just a few orbital periods. Degenerate binary configurations
with mass ratio $q\ne1$ remain hydrodynamically stable
all the way to the Roche limit.
However, unstable mass transfer can occur beyond the Roche
limit, and this is indeed observed in the SPH simulations.
Dynamically unstable mass transfer also leads to the rapid
coalescence of the binary system, although the details of the
hydrodynamic evolution are quite different.

**Wednesday
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