High energy astrophysics  Astrophysical Sites  Type II supernovae

Explosions in binary systems

Many events involving stellar binary systems are characterised by the revival of dormant, degenerate objects via mass overflow from the binary companion or binary mergers via gravitational radiation energy loss. Degenerate conditions prevent controlled burning and causes a thermonuclear runaway because a temperature increase does not lead to a pressure increase. Low accretion rates lead to a pile-up of unburned hydrogen, causing the ignition via pp-chains with pycnonuclear reaction enhancements (screening) when a critical mass layer is attained. This triggers nova events on white dwarfs and X-ray burst on neutron stars. The nuclear energy source is explosive hydrogen burning, ranging from hot CNO-cycles to long sequences of proton captures and beta-decays (the rp-process [15]). The main energy source in novae is the hot CNO-cycle, characterized by proton-induced reactions on radioactive nuclei like ¹³N($p,\gamma$)¹⁴O, ¹⁷F($p,\gamma$)¹⁸Ne, ¹⁸F($p,\alpha$)¹⁵O, and possibly ¹⁸F($p,\gamma$)¹⁹Ne. Present reaction rate information seems to indicate that the gap between the hot CNO-cycle and nuclei beyond Ne can only be overcome by alpha-capture reactions for temperatures above $4\times 10^8$K, unlikely to be attained in novae. The observed high Si, S, and Ar abundances in some novae ask for a class of white dwarfs which was formed after core C-burning, containing O, Ne, and Mg. This permits nucleosynthesis up to A$\sim$40 (including ²⁶Al) due to proton capture reaction sequences on the initial high Ne and Mg abundances. To understand and interpret these observations, detailed measurements of capture reactions on radioactive and stable isotopes in the Ne-Ca range are required. The nucleosynthesis and energy generation in X-ray bursts and black hole accretion disks is dominated by combined H- and He-burning at extreme temperatures and densities [15]. The key nuclear properties for the energy generation and abundance predictions are break-out reactions from the hot CNO, e.g. ¹⁵O( $\alpha,\gamma)^{19}$Ne and proton captures on even-Z T_z=-1/2 nuclei like ²³Mg, ²⁷Si, ³¹S, ³⁵Ar, and ³⁹Ca in competition with beta-decays and the connecting $(\alpha,p)$-reactions. Heavier and more proton-rich nuclei are only populated for conditions when a $(p,\gamma)$- $(\gamma,p)$-equilibrium is approached, requiring essentially beta-decay half-lives and nuclear masses like in the r-process. Beyond Se, the details of the proton drip-line are not that well known yet, but can influence the endpoint of the rp-process. The even-even N=Z nuclei between A=68 and 100 play a dominant role. First calculations which include 2p-captures (coming to a $(p,\gamma)$- $(\gamma,p)$-equilibrium with intermediate p-unstable nuclei) indicate that it seems possible to produce nuclei with A=90-100 in X-ray bursts, including some hard to explain p-process nuclei. Accretion rates above a critical limit cause stable H-burning or only weak flashes. Supersoft X-ray sources, observed by ROSAT, have been identified as white dwarfs with such accretion rates, subsequently experiencing He-accretion above the critical value for stable He-burning and producing a growing C/O white dwarf, which exceeds the Chandrasekhar mass and ignites carbon fusion degenerately with screening enhancements. A burning front propagates through the whole star, causing complete disruption without a remnant. Such objects are candidates for type I(a) supernovae, which observationally do not exhibit hydrogen lines in their spectra due to the absence of extended H-envelopes. Possible alternatives correspond to the ignition of a freshly accreted helium layer (He-detonations) and/or the merging of two white dwarfs in a binary system. In all of these cases a complete disruption of a white dwarf occurs with a large amount of ⁵⁶Ni formation [16]. The propagation of the burning front occurs initially via heat conduction in the degenerate electron gas (requiring a spatial resolution in hydrodynamic calculations of the order 10^-4-10^-5cm). Convective instabilities, leading to burning front propagation via convection, cause a detonation if they accelerate to supersonic speed. This behaviour is generally understood, but cannot be numerically resolved, yet. Observations favour the Chandrasekhar mass models and/or white dwarf mergers which follow this burning pattern. Type Ia supernovae are the main producers of Fe-peak elements in the Galaxy. Electron captures on the incinerated material and the neutron-excess due to the He-burning product ²²Ne, originating from the prior CNO metallicity, lead to the production of neutron-rich Fe-group nuclei which constrain critically the burning front propagation. Another outcome of binary system evolution is a pair of neutron stars. The famous binary pulsar discovered by Hulse and Taylor is expected to lead to a merged system in about 10⁸y. From a purely statistical point of view the number of gamma-ray bursts observed with the Compton Gamma Ray Observatory (CGRO) [17] agrees with the expected number of neutron star mergers in the cosmos, and the isotropic behaviour agrees with homogeneously distributed roughly standard candles out to redshifts of z$\approx$1. It has to be verified whether such events can really reproduce the gamma-ray burst observations. A merger of two neutron stars may also lead to the ejection of neutron-rich material as an alternative site for the production of r-process elements [13]. A hydrodynamical calculation coupled with r-process calculations has still to be undertaken.