Nucleosynthesis in O-Ne-Mg Supernovae

We have studied detailed nucleosynthesis in the shocked surface layers of an Oxygen-Neon-Magnesium core collapse supernova with an eye to determining if the conditions are suitable for r process nucleosynthesis. We find no such conditions in an unmodified model, but do find overproduction of N=50 nuclei (previously seen in early neutron-rich neutrino winds) in amounts that, if ejected, would pose serious problems for galactic chemical evolution.Comment: 12 pages, 1 figure, to be published in Astrophysical Journal Letter

Extracting the rp-process from X-ray burst light curves

The light curves of type I X-ray bursts (XRBs) result from energy released from the atmosphere of a neutron star when accreted hydrogen and helium ignite and burn explosively via the rp-process. Since charged particle reaction rates are both density and very temperature dependent, a simulation model must provide accurate values of these variables to predict the reaction flow. This paper uses a self-consistent one-dimensional model calculation with a constant accretion rate of dM/dt=5e16g/s (0.045 Eddington) and reports on the detailed rp-process reaction flow of a given burst.Comment: 4 pages, submitted to Nucl. Phys. A as part of the Nuclei in Cosmos 8 proceeding

Measurement of direct neutron capture by neutron-rich sulfur isotopes

Thermal neutron capture cross sections for $^{34}$S(n,$\gamma$)$^{35}$S and $^{36}$S(n,$\gamma$)$^{37}$S have been measured and spectroscopic factors of the final states have been extracted. The calculated direct-capture cross sections reproduce the experimental data.Comment: 4 pages (uses espcrc1.sty), 1 postscript figure (uses psfig), accepted for publication in Nucl. Phys. A (Suppl.), uuencoded tex-files and postscript-files available at ftp://is1.kph.tuwien.ac.at/pub/ohu/Stherm.u

The Intermediate r-process in Core-collapse Supernovae Driven by the Magneto-rotational Instability

We investigated r-process nucleosynthesis in magneto-rotational supernovae, based on a new explosion mechanism induced by the magneto-rotational instability (MRI). A series of axisymmetric magneto-hydrodynamical simulations with detailed microphysics including neutrino heating is performed, numerically resolving the MRI. Neutrino-heating dominated explosions, enhanced by magnetic fields, showed mildly neutronrich ejecta producing nuclei up to A similar to 130 (i. e., the weak r-process), while explosion models with stronger magnetic fields reproduce a solar-like r-process pattern. More commonly seen abundance patterns in our models are in between the weak and regular r-process, producing lighter and intermediate-mass nuclei. These intermediate r-processes exhibit a variety of abundance distributions, compatible with several abundance patterns in r-process-enhanced metal-poor stars. The amount of Eu ejecta similar to 10(-5) M circle dot in magnetically driven jets agrees with predicted values in the chemical evolution of early galaxies. In contrast, neutrino-heating dominated explosions have a significant amount of Fe (Ni-56) and Zn, comparable to regular supernovae and hypernovae, respectively. These results indicate magneto-rotational supernovae can produce a wide range of heavy nuclei from iron-group to r-process elements, depending on the explosion dynamics

Spinon localization in the heat transport of the spin-1/2 ladder compound (C5_5H12_{12}N)2_2CuBr4_4

We present experiments on the magnetic field-dependent thermal transport in the spin-1/2 ladder system (C$_5$H$_{12}$N)$_2$CuBr$_4$. The thermal conductivity $\kappa(B)$ is only weakly affected by the field-induced transitions between the gapless Luttinger-liquid state realized for $B_{c1}< B < B_{c2}$ and the gapped states, suggesting the absence of a direct contribution of the spin excitations to the heat transport. We observe, however, that the thermal conductivity is strongly suppressed by the magnetic field deeply within the Luttinger-liquid state. These surprising observations are discussed in terms of localization of spinons within finite ladder segments and spinon-phonon umklapp scattering of the predominantly phononic heat transport.Comment: 4 pages, 3 figure

Uncertainties In Direct Neutron Capture Calculations Due To Nuclear Structure Models

The prediction of cross sections for nuclei far off stability is crucial in the field of nuclear astrophysics. For spherical nuclei close to the dripline the statistical model (Hauser-Feshbach) approach is not applicable and direct contributions may dominate the cross sections. For neutron-rich, even-even Sn targets, we compare the resulting neutron capture cross sections when consistently taking the input for the direct capture calculations from three different microscopic models. The results underline the sensitivity of cross sections calculated in the direct model to nuclear structure models which can lead to high uncertainties when lacking experimental information.Comment: 4 pages, using espcrc1.sty, Proc. Intl. Conf. "Nuclei in the Cosmos IV", Univ. Notre Dame 1996, Nucl. Phys. A, in press. A postscript version can also be obtained from http://quasar.physik.unibas.ch/research.htm

Spectral Modeling of SNe Ia Near Maximum Light: Probing the Characteristics of Hydro Models

We have performed detailed NLTE spectral synthesis modeling of 2 types of 1-D hydro models: the very highly parameterized deflagration model W7, and two delayed detonation models. We find that overall both models do about equally well at fitting well observed SNe Ia near to maximum light. However, the Si II 6150 feature of W7 is systematically too fast, whereas for the delayed detonation models it is also somewhat too fast, but significantly better than that of W7. We find that a parameterized mixed model does the best job of reproducing the Si II 6150 line near maximum light and we study the differences in the models that lead to better fits to normal SNe Ia. We discuss what is required of a hydro model to fit the spectra of observed SNe Ia near maximum light.Comment: 29 pages, 14 figures, ApJ, in pres