Non-Axisymmetric Instabilities in Self-Gravitating Star-Disk Systems: Bifurcation of Rapidly Rotating Protostars
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Date
2021-11-23
Authors
Tumblin, Rebecka
Journal Title
Journal ISSN
Volume Title
Publisher
University of Oregon
Abstract
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.
Description
Keywords
Accretion Disks, Binaries, Circumbinary, Hydrodynamics, Instability, Protostellar Disks