Past exam of the astrophysics course of the University of Oxford / 2018 / 6 / a / Solution

In a massive star undergoing core-collapse supernova formation, an iron-rich core cannot gain further support from exothermic fusion. Electron captures and nuclear photodisintegration promote collapse. The inner core stiffens near nuclear density and bounces, launching a shock into the infalling outer core. Dissociation and neutrino losses drain its energy, so the prompt shock generally stalls while matter continues to accrete. Neutrino absorption behind the shock, aided by multidimensional flow, can revive it and eject the envelope; sufficiently rapid rotation and strong magnetic fields offer additional possibilities. The remnant may be a neutron star or black hole. Major uncertainties include progenitor structure, nuclear equation of state, neutrino transport and interactions, turbulent flow, rotation and magnetic fields.

The standing accretion shock instability is a nonradial instability of this stalled shock and accretion flow. Feedback between advected disturbances and acoustic response can amplify large-scale sloshing or spiral motions, producing an asymmetric shock even without an initially unstable convective gradient.^[ref]

An asymmetric explosion can impart an opposite recoil to the remnant by momentum conservation. In neutrino-driven calculations, long-lasting gravitational attraction by asymmetric, slower inner ejecta can dominate the acceleration: the gravitational tug-boat mechanism. SASI and convection can seed the asymmetries, but the kick is not necessarily an instantaneous hydrodynamic push or uniquely attributable to SASI.^[ref]