Explosions in binary systems  Astrophysical Sites  Stellar evolution

Type II supernovae

The Fe-core, emerging after Si-burning in massive stars, undergoes a core collapse to neutron star densities. This leads either to type II supernovae (which display hydrogen lines in their spectra from the outer unburned envelope) and/or to black hole formation, if the core collapse produces an object beyond the neutron star mass limit. The discovery of 24 neutrinos with the KAMIOKANDE II, IMB, and BAKSAN underground laboratories for supernova SN 1987A confirmed this theoretical picture. These neutrinos, originating from the extremely dense and hot supernova core, could be utilised to derive constraints on neutrino properties, particle physics models, and nuclear physics [10]. Aspects which strongly enter the outcome are the (nuclear) equation of state and neutrino interactions with matter (nucleons, nuclei, electrons), together with the treatment of the hydrodynamics of the explosion, affecting strongly the mass cut between the central neutron star and the supernova ejecta. The observationally indicated mixing of radioactive elements must be interpreted as very strong indication of turbulent processes. Possibly such multi-dimensional processes can be of importance for the explosion mechanism itself [11]. Unfortunately, the few SN 1987A neutrino events did neither enable us to constrain the total energy release of the forming neutron star accurately, nor did they provide sufficiently detailed temporal and spectral resolution for a full understanding of the explosion of the star. Our hope in this respect rests on a future Galactic supernova to be detected by the next generation of neutrino experiments, e.g. SUPERKAMIOKANDE and SNO. They should be a probe of the physical events preceding the disruption of the star and contain a wealth of information about particle and nuclear physics at supernova conditions.  

Figure: A 2D simulation of a type II supernovae explosion [11]. The matter is heated by neutrinos from the hot proto-neutron star. Entropies are given in units of k_B/nucleon.

 

The explosive processing and nucleosynthesis in the ejecta gives rise to a large fraction of the present day element abundances. Explosive nucleosynthesis calculations require the knowledge of nuclear reaction rates at high temperatures, to a large extent for unstable nuclei, based on theoretical or experimental efforts. The comparison with abundances from specific supernova observations can probe the correctness of the stellar evolution treatment and the ¹²C( $\alpha,\gamma)^{16}$O rate. SN 1987A showed reasonable agreement with C, O, Si, Cl, and Ar abundance observations. Supernova remnants make it possible to compare with the observational results from optical, UV, and X-ray observations. The delay time between collapse and explosion via neutrino heating determines the amount of accreted matter onto the proto-neutron star. Combined observations of the ejected ⁵⁶Ni, as deduced from the ⁵⁶Co powered supernova light curve observations, the ⁵⁷Co/⁵⁶Co ratio from gamma-ray observations (possible in SN 1987A) and the amount of ⁴⁴Ti (possible in Cas A) would give unique information on the position of the mass cut, the energy of the shock wave and the neutron/proton ratio in the innermost ejecta [12]. The very innermost layers of the ejecta, driven off the surface of the nascent, cooling neutron star and heated by a strong neutrino flux, might also include r-process nucleosynthesis, based on the built-up of heavy elements up to Th and U via rapid neutron captures. Provided that r-process conditions can be obtained [13], neutron densities and temperatures well in excess of $n_{\rm n}$>10²⁰cm^-3 and T>10⁹K result, which cause reaction timescales as short as $\approx 10^{-4}$s, while the beta-decay half-lives of the involved nuclei are longer, roughly of the order of 10^-1 to a few 10^-3s. This approaches an $(n,\gamma)$- $(\gamma,n)$ equilibrium and a capture path on contour lines of constant neutron separation energy in the nuclear chart. Only a few nuclei along the magic neutron numbers 50 and 82 are know in the regime of neutron separation energies between 2 and 4 MeV. A full understanding requires a highly increased amount of data and nuclear structure knowledge far from stability, i.e. masses, half-lives, possibly neutrino interaction cross sections and level densities and giant resonance properties to predict gamma widths [14] for the reaction rate calculations. The question whether shell closures are quenched for nuclei far from stability is here highly relevant and enters in an important way in the abundance features. Astrophysical uncertainties include the major question about the obtained entropies and n/p ratios in this matter and whether this material is finally ejected or hindered by fall back, due to reverse shocks in the ejected envelope.