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Stellar HYdrodynamics Nucleosynthesis and Evolution

Final Report Summary - SHYNE (Stellar HYdrodynamics Nucleosynthesis and Evolution)

Stars, massive stars in particular, play a key role through the light they shine, all the chemical elements they produce and the supernova explosions that mark their death. They thus have a strong impact in many fields of astronomy and astrophysics. Stars are complex objects that would ideally require 3-dimensional (3D) models including all the relevant physics: nuclear reactions, convection, rotation, magnetic fields, ... . However 3D simulations will never be able to simulate the entire evolution of stars due to over 10 orders of magnitude difference between convection time scale and the lifetime of stars. The best way to model their evolution is thus to develop synergy between 3D and 1D (spherical symmetry) models by improving and establishing new 1D prescriptions using 3D simulations and incorporate these prescriptions into 1D models. Stars and their modelling thus require a multi-disciplinary approach involving nuclear physics, astrophysics and hydrodynamics. This programme is thus called SHYNE, which stands for Stellar HYdrodynamics Nucleosynthesis and Evolution.

Complex models require the latest computer technology and we have been collaborating with the Norwegian company, Numascale (developing shared-memory systems), in order to harness the full potential of present and future computing facilities, giving this programme an inter-sectoral dimension. We have fully integrated this computer hardware and our tools and simulations have greatly benefited from this collaboration. We have also provided important feedback to Numascale for their future developments. We have also presented the fruits of this collaboration at the international high performance computing conference in Frankfurt in July 2015.

This programme tackled multi-disciplinary challenges, which are out of reach of individual research groups. For this purpose, the SHYNE team collaborates with a network of world leading experts in their respective disciplines, and we have integrated innovating approaches using and extending techniques from other disciplines such as Monte Carlo simulations (regularly used in other disciplines of physics) to develop a unique suite of software tools able to (1) simulate the most complex processes in stars, (2) apply these models to stars of all masses and (3) use stars as virtual nuclear physics laboratories.

Some of the key achievements to this date are the following. We have established a priority list of test cases to be studied with 3D hydrodynamics simulations. Following this priority list, we have in particular completed the first series of 3D simulations of carbon shells burning and have integrated the results of 3D simulations back into 1D stellar evolution codes. The first multi-D models of shear mixing in massive rotating stars have also been completed.

We completed studies for several key nucleosynthesis processes (weak s, main s, p and r processes). These helped us produce priority lists for future experimental work and will provide crucial guidance for nuclear physics experiments such as the Facility for Anti-proton and Ion Research (FAIR) facility currently being built at GSI in Darmstadt, Germany. Our priority list has already led to the successful proposal of a new experiment at CERN nTOF. This experiment, which ran at CERN this autumn re-measured the neutron-capture rate on selenium 77 & 78 and shows how the SHYNE project is already guiding new nuclear physics experiment and boosting the return on these huge investments. Our datasets and software tools will also provide a theoretical framework in order to analyse and explain new findings at those costly facilities, thus boosting their innovation cycle and return on investments.

Improvements brought to the 1D stellar evolution codes now enable simulations of intermediate-mass super-AGB stars up to the electron-capture induced collapse and also to include the effects of rotation in AGB star simulations. This work led to the discovery of a new evolutionary path between massive stars and super-AGB stars, that we named “failed” massive stars. In parallel, we studied the evolution and explosion of the most massive stars known to date and predicted their fate. The models of stars of all masses will provide important theoretical information for the newly-opened field of gravitational wave astronomy. Thus the SHYNE project has made great contributions to the fields of stellar hydrodynamics, nucleosynthesis and stellar evolution.