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Register a new userA New Mechanism for Core-Collapse Supernova Explosions
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We present a new mechanism for core-collapse supernova explosions that relies upon acoustic power generated in the inner core as the driver. In our simulation using an 11-solar-mass progenitor, a strong advective-acoustic oscillation a la Foglizzo with a period of ~25-30 milliseconds (ms) arises ~200 ms after bounce. Its growth saturates due to the generation of secondary shocks, and kinks in the resulting shock structure funnel and regulate subsequent accretion onto the inner core. However, this instability is not the primary agent of explosion. Rather, it is the acoustic power generated in the inner turbulent region and most importantly by the excitation and sonic damping of core g-mode oscillations. An l=1 mode with a period of ~3 ms grows to be prominent around ~500 ms after bounce. The accreting protoneutron star is a self-excited oscillator. The associated acoustic power seen in our 11-solar-mass simulation is sufficient to drive the explosion. The angular distribution of the emitted sound is fundamentally aspherical. The sound pulses radiated from the core steepen into shock waves that merge as they propagate into the outer mantle and deposit their energy and momentum with high efficiency. The core oscillation acts like a transducer to convert accretion energy into sound. An advantage of the acoustic mechanism is that acoustic power does not abate until accretion subsides, so that it is available as long as it may be needed to explode the star. [abridged]
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We investigate the criteria for successful core-collapse supernova explosions by the neutrino mechanism. We find that a critical-luminosity/mass-accretion-rate condition distinguishes non-exploding from exploding models in hydrodynamic one-dimensional (1D) and two-dimensional (2D) simulations. We present 95 such simulations that parametrically explore the dependence on neutrino luminosity, mass accretion rate, resolution, and dimensionality. While radial oscillations mediate the transition between 1D accretion (non-exploding) and exploding simulations, the non-radial standing accretion shock instability characterizes 2D simulations. We find that it is useful to compare the average dwell time of matter in the gain region with the corresponding heating timescale, but that tracking the residence time distribution function of tracer particles better describes the complex flows in multi-dimensional simulations. Integral quantities such as the net heating rate, heating efficiency, and mass in the gain region decrease with time in non-exploding models, but for 2D exploding models, increase before, during, and after explosion. At the onset of explosion in 2D, the heating efficiency is $sim$2% to $sim$5% and the mass in the gain region is $sim$0.005 M$_{sun}$ to $sim$0.01 M$_{sun}$. Importantly, we find that the critical luminosity for explosions in 2D is $sim$70% of the critical luminosity required in 1D. This result is not sensitive to resolution or whether the 2D computational domain is a quadrant or the full 180$^{circ}$. We suggest that the relaxation of the explosion condition in going from 1D to 2D (and to, perhaps, 3D) is of a general character and is not limited by the parametric nature of this study.
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We investigate the action of the magnetorotational instability (MRI) in the context of iron-core collapse. Exponential growth of the field on the rotation time scale by the MRI will dominate the linear growth process of field line wrapping with the same characteristic time. We examine a variety of initial rotation states, with solid body rotation or a gradient in rotational velocity, that correspond to models in the literature. A relatively modest value of the initial rotation, a period of ~ 10 s, will give a very rapidly rotating PNS and hence strong differential rotation with respect to the infalling matter. We assume conservation of angular momentum on spherical shells. Results are discussed for two examples of saturation fields, a fiducial field that corresponds to Alfven velocity = rotational velocity and a field that corresponds to the maximum growing mode of the MRI. Modest initial rotation velocities of the iron core result in sub-Keplerian rotation and a sub-equipartition magnetic field that nevertheless produce substantial MHD luminosity and hoop stresses: saturation fields of order 10^{15} - 10^{16} G develop within 300 msec after bounce with an associated MHD luminosity of about 10^{52} erg/s. Bi-polar flows driven by this MHD power can affect or even cause the explosions associated with core-collapse supernovae.
We investigate the possibility of boiling instability of nuclear liquid in the inner core of the proto-neutron star formed in the core collapse of a type II supernova. We derive a simple criterion for boiling to occur. Using this criterion for one of best described equations of state of supernova matter, we find that boiling is quite possible under the conditions realized inside the proto-neutron star. We discuss consequences of this process such as the increase of heat transfer rate and pressure in the boiling region. We expect that taking this effect into account in the conventional neutrino-driven delayed-shock mechanism of type II supernova explosions can increase the explosion energy and reduce the mass of the neutron-star remnant.
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