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COCO2CASA Report Summary

Project ID: 341157
Funded under: FP7-IDEAS-ERC
Country: Germany

Mid-Term 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 a black hole by the gravitational collapse of their degenerate core, while the rest of the star can explode as a gigantic supernova. The physical mechanism that triggers such stellar explosions is not yet well understood. Neutrinos that are emitted from the extremely hot, new-born compact remnant are hypothesized to be the main source of the energy powering the explosion. But this scenario of a "neutrino-driven mechanism" still needs more theoretical consolidation, and direct observational confirmation may only become possible when neutrinos and gravitational waves from a future Galactic supernova can be measured with high statistics and detailed time resolution.

This ERC-funded project (ERC-AdG No. 341157-COCO2CASA) has the goal to deduce constraints on the explosion mechanism from astronomical data of supernovae and their remnants, in particular to exploit the wealth of information provided by nearby, well-observed, young remnants such as Crab, Cassiopeia A, and Supernova 1987A. This shall be achieved by confronting predictions from elaborate theoretical models with observational data. Since the explosion mechanism is genericly multi-dimensional, self-consistent three-dimensional simulations are ultimately needed that follow the evolution from the final, convective shell-burning phase of the progenitor, through core collapse, explosion, and the long-time evolution of the outburst towards the remnant stage of the expanding supernova debris.

In all three branches of this project, 3D progenitor modeling, 3D explosion modeling, and 3D long-time supernova and remnant modeling, major progress could be achieved during the first half of the funding period. Some of the most remarkable successes of the research by the PI and his team concern the following points:
1) The world-wide first successful, self-consistent supernova explosions could be obtained for 9.6, 15, and 20 solar-mass stars with our neutrino-hydrodynamics code including an elaborate, energy-dependent treatment of the neutrino physics. These simulations can be considered as a major breakthrough with respect to numerical supernova modeling and lend strong support to the viability of the neutrino-driven mechanism.
2) In the course of these simulations we discovered a new neutrino-hydrodynamics instability, which leads to the growth of a large, possibly dominant, dipolar lepton-number emission asymmetry of the newly formed neutron star. This novel phenomenon, which we termed LESA for Lepton-Emission Self-sustained Asymmetry, can have important consequences for supernova nucleosynthesis, neutron-star kicks, and neutrino-flavor oscillations.
3) Simulating spherically symmetric explosion models (that account for the essence of the neutrino-driven mechanism as it is currently understood) for large sets of progenitor stars, we discovered a two-parameter criterion for predicting the "explodablity" of progenitor stars on grounds of their pre-collapse density structure. This criterion is superior to the characterization by a "compactness" value in particular because the two parameters are intimately related to the quantities that govern the physics of the neutrino-driven mechanism.
4) We have, for the first time, performed self-consistent full-sphere simulations of the convective shell burning in an 18 solar-mass star over the last five minutes prior to the gravitational collapse of the stellar iron core. The simulations demonstrate that large-scale asymmetries can develop in the convective flow of the oxygen-burning shell with potentially important consequences for the neutrino-driven explosion.
5) A Cassiopeia A like 3D supernova simulation was analyzed for its explosive production of radioactive nuclei such like titanium-44 and nickel-56. The model results show that neutrino-driven explosions and the corresponding asymmetries can explain the measured masses of these elements in Cassiopeia A and Supernova 1987A as well as the morphology of the titanium and iron distributions in Cassiopeia A as revealed by the NuSTAR map and Chandra observations, respectively.

The COCO2CASA project with the team having grown to its planned size now, is moving with full steam ahead. Several development lines of new numerical codes, involving full-3D neutrino transport, full general relativity, novel microphysics for neutrinos in dense neutron-star matter, and the physical ingredients needed for the long-time evolution of supernova ejecta towards the remnant stage, promise more exciting results at the very forefront of supernova theory during the second half of the funding period.

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