STABILITY OF COALESCING BINARY STARS AGAINST GRAVITATIONAL COLLAPSE -HYDRODYNAMICAL SIMULATIONS - ART. NO. 023002

We perform simulations of relativistic binary stars in post-Newtonian gravity to investigate their dynamical stability prior to merger again st gravitational collapse in a tidal field. In general, our equations are only strictly accurate to first post-Newtonian order, but they rec over full general relativity for spherical, static stars. We study bot h corotational and irrotational binary configurations of identical sta rs in circular orbits. We adopt a soft, adiabatic equation of state wi th Gamma=1.4, for which the onset of instability occurs at a sufficien tly small value of the compaction M/R that a post-Newtonian approximat ion is quite accurate. For such a soft equation of state there is no i nnermost stable circular orbit, so that we can study arbitrarily close binaries. This choice still allows us to study all the qualitative fe atures exhibited by any adiabatic equation of state regarding stabilit y against gravitational collapse. We demonstrate that, independent of the internal stellar velocity profile, the tidal field from a binary c ompanion stabilizes a star against gravitational collapse. [S0556-2821 (98)07114-8].