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Modeling Stellar Collapse and Explosion: Evolving Progenitor Stars to Supernova Remnants

Final Report Summary - COCO2CASA (Modeling Stellar Collapse and Explosion: Evolving Progenitor Stars to Supernova Remnants)

At the end of their lives, stars more massive than about nine solar masses form a neutron star or black hole by the gravitational collapse of their degenerate core, while the rest of the star can explode in a gigantic outburst called core-collapse supernova. The physical mechanism that triggers such stellar explosions had not been well understood until a few years ago. But one of the grand breakthroughs of this ERC-AdG project COCO2CASA was a direct confirmation of the long-standing paradigm of the "delayed neutrino-driven mechanism" by self-consistent, ab initio three-dimensional (3D) computer simulations. It could be demonstrated that neutrinos, which are emitted by the extremely hot, new-born neutron star in huge numbers, are indeed the main source of the energy powering the explosion in the far majority of supernovae. Besides performing the worldwide most detailed and most complete numerical simulations of the physics of the mechanism, another primary goal of the COCO2CASA project was to collect further evidence for the validity of this neutrino-driven explosion scenario from observed supernovae.
COCO2CASA successfully achieved this goal by a comparison of 3D explosion models, based on the neutrino mechanism, with observational information of supernovae and of their compact as well as gaseous remnants, focussing, in particular, on the well-observed, young remnants of Supernova 1987A, Cassiopeia A, and Crab. To reach its goal, COCO2CASA contained three research branches: (1) '3D Neutrino Transport' for modeling the detailed physics of the explosion mechanism, (2) '3D Progenitor Models' for determining the initial conditions defined by convective oxygen and silicon shell burning prior to stellar core collapse, and (3) '3D Supernova Remnant Formation' for continuing the computer simulations from the onset of the supernova explosion to late stages, where observable properties can be predicted and compared to astronomical data.
All three branches of this project achieved major breakthroughs into new scientific territory. Some of the most remarkable successes are:
(1) Aside of demonstrating the viability of the neutrino-driven supernova mechanism by the most sophisticated 3D models, the PI and collaborators also discovered a novel phenomenon in the neutrino emission that was termed LESA (Lepton-Emission Self-sustained Asymmetry). LESA shows up as a large-amplitude dipolar or quadrupolar asymmetry of the emission of electron neutrinos and antineutrinos, with more electron neutrinos radiated in one hemisphere and relatively more antineutrinos in the opposite hemisphere. It is connected to a corresponding hemispheric difference of convective activity inside the new-born neutron star and causes neutron star recoil kicks of several 10 km/s. It also has an impact on the detectable neutrino signal from supernovae and on the formation of trans-iron elements in the innermost supernova ejecta.
(2) For the first time, it was achieved to simulate the very last phase of convective oxygen shell burning prior to stellar collapse for several minutes in 3D, thus overcoming the shortcomings of traditionally used spherically symmetric stellar progenitor models as initial conditions for supernova simulations. It was demonstrated that massive progenitors stars (18 to 19 solar masses) explode successfully only when the density and velocity perturbations associated with the turbulent convection in the oxygen layer are taken into account. These perturbations boost the growth of hydrodynamic instabilities (convective overturn, SASI, turbulence) behind the stagnant supernova shock and thus facilitate the onset of the supernova blast.
(3) Long-time 3D supernova simulations of the COCO2CASA project predicted a spatial anti-correlation between the kick direction of the neutron star and the intermediate-mass and iron-group elements produced in the explosion. This prediction could be confirmed splendidly by observations of young supernova remnants and by a detailed mapping of the 3D distribution of radioactive 44-titanium in the Cassiopeia A remnant by NuSTAR X-ray measurements. 3D simulations of the project were also able to explain the morphology of this highly asymmetric remnant, and they guided the ultimate discovery of the new-born neutron star in Supernova 1987A by high angular resolution ALMA sub-millimeter imaging of dust and molecules in the ejecta.