Non-Axisymmetric Instabilities in Self-Gravitating Star-Disk Systems: Bifurcation of Rapidly Rotating Protostars

The nonlinear evolution of three-dimensional protostars surrounded by circumstellar disks are numerically investigated to understand the conditions under which short-period binary star systems could form and to probe models of circumbinary Jovian planet formation. This study considers the fission hypothesis for binary star formation, namely, that binary star systems can originate from the bifurcation of one star into two due to rotational instability during the early stages of star formation. The stellar structures investigated are modeled as differentially rotating, compressible fluids and have a specific angular momentum distribution that scales with cylindrical mass. The protoplanetary disks are assumed to rotate with power law angular velocity distributions. The equilibrium star and disk structure is determined from a modified form of the Hachisu self-consistent field method. The density distributions are then perturbed away from equilibrium with low amplitude random noise and evolved forward in time using both linear and nonlinear computational methods. Nonlinear simulations are performed using the radiation-hydrodynamics code CHYMERA. Previous studies of the fission hypothesis did not investigate the affectscircumstellar material would have on stellar evolution. We find that circumstellar material tends to reduce the inward force of gravity which lowers the threshold where dynamic bar-like nonaxisymmetric instabilities develop compared to systems without circumstellar disk material. In the system tested, a bar-like instability develops in the central star. Once instability reaches saturation amplitudes, the star interacts with the disk and the system evolves into two stellar objects which rotate on independent axes and orbit a common center of mass. Further studies of this system include radiative losses using a constant cooling function with a constant local cooling timescale. For weak cooling, the m=2 Fourier component of the density perturbation reaches maximal amplitude and a short-period, equal mass binary star system forms. For the system with a moderate cooling rate, an m=3 components grows in tandem with the m=2 components and the system develops into an unequal mass pair of central stellar objects. For the fastest cooling rates tested, the disk undergoes fragmentation and within several dynamical timescales an unequal mass binary pair orbited by two smaller over-densities is produced