*Magnetorotational collapse of very massive stars to black holes: Simulations in full general relativity*

** Yuk Tung Liu
Stuart L. Shapiro
Branson C. Stephens**

**ABSTRACT**

We perform axisymmetric simulations of the magnetorotational collapse of very massive stars in full general relativity. Our simulations
are applicable to the collapse of supermassive stars with masses M ≥ 10^{3} M_{solar} and to very massive Population
III stars. We model our initial configurations by n = 3 polytropes, uniformly rotating near the mass-shedding limit and at the onset of
radial instability to collapse. The ratio of magnetic to rotational kinetic energy in these configurations is chosen to be small (1% and
10%). We find that such magnetic fields do not affect the initial collapse significantly. The core collapses to a black hole, after which
black hole excision is employed to continue the evolution long enough for the hole to reach a quasi-stationary state. We find that the
black hole mass is M_{h} = 0.95M and its spin parameter is J_{h}/M_{h}^{2} = 0.7, with the remaining
matter forming a torus around the black hole. The subsequent evolution of the torus depends on the strength of the magnetic field. We
freeze the spacetime metric ("Cowling approximation") and continue to follow the evolution of the torus after the black hole has relaxed
to quasi-stationary equilibrium. In the absence of magnetic fields, the torus settles down following ejection of a small amount of matter
due to shock heating. When magnetic fields are present, the field lines gradually collimate along the hole's rotation axis. MHD shocks
and the magnetorotational instability (MRI) generate MHD turbulence in the torus and stochastic accretion onto the central black hole.
When the magnetic field is strong, a wind is generated in the torus, and the torus undergoes radial oscillations that drive episodic
accretion onto the hole. These oscillations produce long-wavelength gravitational waces potentially detectable by the Laser
Interferometer Space Antenna (LISA). The final state of the magnetorotational collapse always consists of a central black holes
surrounded by a collimated magnetic field and a hot, thick accretion torus. The system is a viable candidate for the central engine of a
long-soft gamma-ray burst.

Phys. Rev. D76 (2007) 084017, astroph/0706.2360

Star S0 (J/M^{2} = 0.96, R_{eq}/M = 640, P_{mag}/P = 0)

Star S1 (J/M^{2} = 0.96, R_{eq}/M = 640, P_{mag}/P = 2.7x10^{-4})

Star S2 (J/M^{2} = 0.96, R_{eq}/M = 640, P_{mag}/P = 2.7x10^{-3})

*Scientific visualization by*

Last Updated 5 Nov 14 by SEC