0 Ju n 20 05 Multifragmentation reactions and properties of hot stellar matter at sub - nuclear densities

Multifragmentation reactions and properties of hot stellar matter at sub-nuclear densities. Abstract We point out similarity of thermodynamic conditions reached in intermediate-energy nuclear collisions and in supernova explosions. We show that a statistical approach, which has been previously applied for nuclear multifragmentation reactions , can be very useful for description of the electro-neutral stellar matter. Then properties of hot unstable nuclei extracted from analysis of multifragmentation data can be used for construction of a realistic equation of state of supernova matter. A type II supernova explosion is one of the most spectacular events in astrophysics, with huge energy release of about 10 53 erg or several tens of MeV per nucleon [1]. When the core of a massive star collapses, it reaches densities several times larger than the normal nuclear density ρ 0 = 0.15 fm −3. The repulsive nucleon-nucleon interaction gives rise to a bounce-off and creation of a shock wave propagating through the in-falling stellar material. This shock wave is responsible for the ejection of a star envelope that is observed as a supernova explosion. During the collapse and subsequent explosion the temperatures T ≈ (0.5 ÷ 10) MeV and baryon densities ρ B ≈ (10 −5 ÷ 2)ρ 0 can be reached. As shown by many theoretical studies, a liquid-gas phase transition is expected in nuclear matter under these conditions. It is remarkable that similar conditions can be obtained in energetic nuclear collisions studied in terrestrial laboratories. This fact gives a ground to use well established models of nuclear reactions, after certain modifications, for describing matter states in the course of supernova explosions. On the other hand, the supernova physics stimulates investigations of specific reaction mechanisms and exotic nuclei. As demonstrated by several authors (see e.g. [2, 3]), present hydrodynamical simulations of the core collapse do not produce successful explosions, even when neutrino heating and convection effects are included. On the other hand, it is known that nuclear composition is extremely important for understanding the physics of supernova explosions. In particular, the weak reaction rates and energy spectra of emitted neutrinos are very 1