Session J14: Minisymposium on From Thermonuclear to Pyconuclear Burning

Five regimes of fusion reactions in dense stellar matter

Five regimes of nuclear burning of matter in dense stellar plasma are discussed. They are weakly screening thermonuclear, strongly screening thermonuclear, pycnonuclear, thermally excited pycnonuclear, and intermediate pycno-thermonuclear regimes. The uncertainties of nuclear reaction rates in these regimes are analyzed; astrophysical applications are outlined, with the emphasis on nuclear reactions under extreme conditions in white dwarfs and neutron stars.   

Molecular dynamics simulations of the crust of accreting neutron stars

We model the crust of accreting neutron stars with molecular dynamics simulations involving complex compositions with many different impurities as predicted by Gupta el al. electron capture calculations. We present results for the phase structure [1], thermal conductivity, and screening factors for nuclear reactions [2]. We find a lattice structure with a high thermal conductivity, instead of an amorphous solid, and we discuss the distribution of impurities. These thermal conductivity results agree with X-Ray observations of crust cooling for neutron stars after extended outbursts. We find that screening factors for the enhancement of thermonuclear reactions may be insensitive to the large scale distribution of impurities in the solid. Fusion of neutron rich oxygen isotopes such as $^{24}$O + $^{24}$O may be an important heat source at densities near ten to the eleventh g/cm$^{3}$. Indeed these and similar fusion reactions may be important to heat the crust to carbon ignition temperatures in superbursts. [1] C. J. Horowitz, D. K. Berry, and E. F. Brown, PRE75 (2007) 066101. [2] C. J. Horowitz, H. Dussan, and D. K. Berry, arXiv:0710.5714.   

From thermo- to pycno-nuclear reactions in multi-component dense matter

We analyze thermonuclear and pycnonuclear reaction rates in multi-component dense stellar plasma. First we describe calculations of the astrophysical $S$-factor at low energies using the S\~ao Paulo potential on the basis of the barrier penetration model. Then we present a simple phenomenological expression for a reaction rate. The expression contains several fit parameters which we adjust to reproduce the best microscopic calculations available in the literature. For illustration, we applied the results to study nuclear burning in a $^{12}$C-$^{16}$O mixture in a wide range of densities and temperature.   

Recent progress in the study of heavy-ion fusion hindrance at extreme sub-barrier energies.

A new phenomenon, heavy-ion fusion hindrance at extreme sub-barrier energies, has been discovered at ANL several years ago [1]. It was first observed in medium-mass systems, but later measurements and analyses showed that this might be a general behavior of heavy-ion fusion at extreme sub-barrier energies. As a result, it could also have an effect on fusion reactions of importance in nuclear astrophysics, such as $^{12}$C + $^{12}$C etc. [1]. The main difference between the systems studied earlier and astrophysically important fusion reactions is in the reaction Q-values, which are positive for lighter nuclei and negative for medium-mass systems. We have, therefore, recently studied fusion in the system $^{28}$Si + $^{30}$Si with a Q-value of 14.3 MeV, which is comparable to that of $^{12}$C + $^{12}$C (Q=13.9 MeV). Results from this experiment as well as from other studies will be discussed. [1] C.L. Jiang \textit{et al}., Phys. Rev. Lett. 87, 052701 (2002); Phys. Rev. C73, 014603 (2006)~; Phys. Rev. C75, 015803 (2007).   

Coupled-channels Calculations of Heavy-ion Fusion at Extreme Sub-barrier Energies.

The hindrance of heavy-ion fusion at extreme sub-barrier energies is now a well established phenomenon, which has been observed experimentally in many medium-heavy systems [1]. The low-energy data can be explained fairly accurately by coupled-channels calculations that use a shallow potential in the entrance channel. This will be illustrated by comparing to the fusion data for $^{64}$Ni+$^{64}$Ni and $^{16}$O+$^{208}$Pb (see [2,3].) Good agreement with the low-energy data can only be achieved by determining the fusion from ingoing wave boundary conditions that are imposed at the minimum of the pocket. The shallow potential we use is generated by correcting the M3Y double-folding potential for the effect of the nuclear incompressibility, which we simulate by a repulsive, effective $NN$ interaction. The shallow potential also helps explaining the suppression of fusion data that has been observed at energies far above the Coulomb barrier. [1] C. L. Jiang et al., Phys. Rev. C 73, 014613 (2006). [2] \c S. Mi\c sicu and H. Esbensen, Phys. Rev. Lett. 96, 112701 (2006). [3] H. Esbensen and \c S. Mi\c sicu, Phys. Rev. C 76, 054609 (2007).   

Aspects and Reaction Rates for Pycnonuclear Fusion at High Densities

Pycnonuclear reactions are of great importance in the nuclear astrophysics of high density conditions such as the centers of white dwarf starts, and the deep layers of accreting neutron stars. We present here a single phenomenological expression for the calculation of pycnonuclear reaction rates which is not only valid in mixed component plasma, but can also be extended to cover the five stellar burning regimes   

The JINA Reaclib Project

Nuclear astrophysics is a rich and vital field of study, using experimental/theoretical input for calculations of processes that create the elements we are made from. In order to facilitate this research further, the Joint Institute for Nuclear Astrophysics (JINA) has created a public, web-based database for thermonuclear reaction rates. Data are stored in the standard Reaclib format and are continually updated as new data or new compilations become available. A versioning system has been adopted to keep track of new rates. Recommended rate libraries representing ``snap shots'' of the live database are stored for users wanting a fixed/unchanging set of rates. The database and its use will be presented with emphasis on its role in nuclear astrophysics calculations. For more information, see the JINA Reaclib website: http://www.nscl.msu.edu/$\sim$nero/db.