The Formation of Short Period Binary Star Systems From Stable, Self-Gravitating, Gaseous Bars.

Abstract

Although we have a general understanding of how stars form, and there. are accepted theories that explain the formation of long-period binaries, we do not yet understand how short period binaries form. Here we present simulations that clearly demonstrate how such systems may form naturally from dense interstellar gas clouds. First, we present two models of compressible, self-gravitating fluid configurations with bar-like structures and supersonic internal motions. Both models have been constructed via dynamical simulations that have started from initially axisymmetric, rapidly rotating polytropes that were known to be dynamically unstable toward the development of a bar-like or two-armed spiral structure. The two initial models differed mainly in their initial angular momentum distributions. In each case, the nonlinear development of the dynamical instability results in the formation of a bar-like configuration that is spinning with a well-defined pattern speed. By all accounts, these models appear to be compressible analogs of Riemann ellipsoids. Our final "steady-state" configurations appear to be dynamically stable and include a mild standing shock front. We have allowed one of these dynamically stable, triaxial configurations to cool slowly and have continually followed its dynamical evolution. A "binary instability," results after reducing the mean pressure of the configuration to ∼50% of its original value. The instability appears as an oscillation between two configurations: One that resembles a common envelope binary system with circulation around the two local density maxima, and the other that appears to be an ellipsoidal configuration with density maxima near the center. Unfortunately, as the model cools, it continues to contract and becomes less well resolved in our numerical grid. Hence, we have not been able to follow this instability to its ultimate fate. However, the strength and nature of the instability lead us to conclude that fission will be the outcome. This work provides the strongest evidence, to date, that short period binary stars form in a very natural way through a fission instability, as proposed by Lebovitz (1987), that fission is the only possible outcome.