Scientific background
Stellar evolution models predict that stars up to 10 - 12 Msun undergo only a handful of nuclear burning stages: H-burning, He-burning, and possibly, C-burning. These hydrostatic processes induce dramatic changes in the structure of a star, leading to a gentle ejection of their outermost layers, while the innermost regions are reshaped into a compact object of planetary size known as a white dwarf (WD).
More massive stars face a more violent fate, a (core-collapse) supernova explosion, leaving either a neutron star (NS) or a black hole as remnants. Many stars form binary or multiple systems, with a fraction hosting one or two degenerate objects (WD and/or NS) in short-period orbits, such that mass transfer episodes (accretion) onto the degenerate component ensue. This scenario is the framework for a suite of violent stellar events, such as type Ia supernovae (SNIa), classical novae (CNe), type I X-ray bursts (XRBs), or eventually, stellar mergers (WD+WD, WD+NS, NS+NS). The expected nucleosynthesis accompanying these cataclysmic events is very rich: CNe are driven by proton-capture reactions in competition with β+-decays, proceeding close to the valley of stability, up to Ca. XRBs are powered by a suite of nuclear processes, including the rp-process (rapid p-captures and β+-decays), the 3α-reaction, and the αp-process (a sequence of (α,p) and (p,γ) reactions); here, the nuclear flow proceeds far away from the valley of stability, merging with the proton drip-line beyond A = 38, and reaching eventually the SnSbTe-mass region, or beyond. In SNIa, the detailed abundances of the freshly synthesized elements depend on the peak temperature reached and on the excess of neutrons and protons (which depend in turn on the metallicity of the white dwarf progenitor as well as on the density at which the thermonuclear runaway occurs); they constitute the major factory of Fe-peak elements in the Galaxy, and roughly speaking, the abundance pattern of their ejecta is the result of four different burning regimes: nuclear statistical equilibrium, and incomplete Si-, O-, and C-Ne-burning. Stellar mergers certainly contribute as well to the metallicity enrichment of the Galaxy. Moreover, mergers of NS+NS have been suggested as a possible site for the r-process.
Despite the astrophysical relevance of the abovementioned scenarios many key issues remain yet to be clarified.
Scientific Objectives
The main objective of EXNUC is to provide a detailed description of all these explosive events (CNe, XRBs, SNIa, and stellar mergers), addressing these uncertain aspects of their evolution that are relevant for their accompanying nucleosynthesis. To accomplish this, a truly multidisciplinary approach is clearly required, combining experts from different disciplines: theoretical astrophysicists, that will perform state-of-the-art numerical simulations of these stellar explosions; observational astronomers, that will provide atomic abundances inferred spectroscopically, using space-borne and/or ground-based facilities; cosmochemists, that will determine isotopic abundances through laboratory measurements of meteoritic grains (and will also provide clues on the way matter condenses in the ejecta to form solids); and finally, nuclear physicists that will provide information on nuclear processes in stars relying on theoretical and experimental grounds.
Specifically, the milestones to be achieved during this 3-year long project are:
1st year
- Detailed analysis of the sensitivity of nucleosynthesis in explosive scenarios to uncertainties in nuclear processes; identification of key uncertain reactions, particularly for SNIa
- Multidimensional simulations of the impact between the ejecta of CNe and SNIa with the companion star
- Condensation in the ejecta of CNe and SNIa
- Microscopic calculations of alpha-nucleus optical potentials for nuclei near magic numbers
2nd year
- Observations (optical, X-rays) of SN remnants
- Nucleosynthesis in WD+WD, WD+NS, and NS+NS mergers
- Presolar grain analysis: signatures of explosive events in the SiC, oxide, and A+B populations
- Setup of experimental equipment and measurements of key reactions
- Extension of microscopic a-nucleus optical potentials to non-magic numbers; evaluation of α-involving reactions relevant in explosive scenarios
3rd year
- Nucleosynthetic yields in multidimensional SNIa simulations as a function of the explosion mechanism
- Observations (optical, high-energy) of CNe and SNIa in low metallicity environments (globular clusters, SMC…)
- Setup of experimental equipment and measurements of key reactions
- Simulations of radiation-driven winds in XRBs and their role in mass-ejection from accreting NS
