Hot spot abundance, ridge subduction and the evolution of greenstone belts

A number of plate tectonic hypotheses have been proposed to explain the origin of Archaean and Phanerozoic greenstone/ophiolite terranes. In these models, ophiolites or greenstone belts represent the remnants of one or more of the following: island arcs, rifted continental margins, oceanic crustal sections, and hot spot volcanic products. If plate tectonics has been active since the creation of the Earth, it is logical to suppose that the same types of tectonic processes which form present day ophiolites also formed Archaean greenstone belts. However, the relative importance of the various tectonic processes may well have been different and are discussed

Rocket instrumentation for the measurement of D-region electron density and collision frequencies Scientific report no. 244

Black Brant II sounding rocket instrumentation for measurement of D layer electron density and collision rat

The Production of Ti44 and Co60 in Supernova

The production of the radioactive isotopes $^{44}$Ti and $^{60}$Co in all types of supernovae is examined and compared to observational constraints including Galactic $\gamma$--ray surveys, measurements of the diffuse 511 keV radiation, $\gamma$--ray observations of Cas A, the late time light curve of SN 1987A, and isotopic anomalies found in silicon carbide grains in meteorites. The (revised) line flux from $^{44}$Ti decay in the Cas A supernova remnant reported by COMPTEL on the Compton Gamma-Ray Observatory is near the upper bound expected from our models. The necessary concurrent ejection of $^{56}$Ni would also imply that Cas A was a brighter supernova than previously thought unless extinction in the intervening matter was very large. Thus, if confirmed, the reported amount of $^{44}$Ti in Cas A provides very interesting constraints on both the supernova environment and its mechanism. The abundances of $^{44}$Ti and $^{60}$Co ejected by Type II supernovae are such that gamma-radiation from $^{44}$Ti decay SN 1987A could be detected by a future generation of gamma-ray telescopes and that the decay of $^{60}$Co might provide an interesting contribution to the late time light curve of SN 1987A and other Type II supernovae. To produce the solar $^{44}$Ca abundance and satisfy all the observational constraints, nature may prefer at least the occasional explosion of sub-Chandrasekhar mass white dwarfs as Type Ia supernovae. Depending on the escape fraction of positrons due to $^{56}$Co made in all kinds of Type Ia supernovae, a significant fraction of the steady state diffuse 511 keV emission may arise from the annihilation of positrons produced during the decay of $^{44}$Ti to $^{44}$Ca. The Ca and Ti isotopic anomalies in pre-solar grains confirm the production of $^{44}$Ti in supernovae and thatComment: 27 pages including 7 figures. uuencoded, compressed, postscript. in press Ap

An analytical and experimental investigation of resistojet plumes

As a part of the electrothermal propulsion plume research program at the NASA Lewis Research Center, efforts have been initiated to analytically and experimentally investigate the plumes of resistojet thrusters. The method of G.A. Simons for the prediction of rocket exhaust plumes is developed for the resistojet. Modifications are made to the source flow equations to account for the increased effects of the relatively large nozzle boundary layer. Additionally, preliminary mass flux measurements of a laboratory resistojet using CO2 propellant at 298 K have been obtained with a cryogenically cooled quartz crystal microbalance (QCM). There is qualitative agreement between analysis and experiment, at least in terms of the overall number density shape functions in the forward flux region

Evolution of Phase-Space Density in Dark Matter Halos

The evolution of the phase-space density profile in dark matter (DM) halos is investigated by means of constrained simulations, designed to control the merging history of a given DM halo. Halos evolve through a series of quiescent phases of a slow accretion intermitted by violent events of major mergers. In the quiescent phases the density of the halo closely follows the NFW profile and the phase-space density profile, Q(r), is given by the Taylor & Navarro power law, r^{-beta}, where beta ~ 1.9 and stays remarkably stable over the Hubble time. Expressing the phase-space density by the NFW parameters, Q(r)=Qs (r/Rs)^{-beta}, the evolution of Q is determined by Qs. We have found that the effective mass surface density within Rs, Sigma_s = rhos Rs, remains constant throughout the evolution of a given DM halo along the main branch of its merging tree. This invariance entails that Qs ~ Rs^{-5/2} and Q(r) ~ Sigma_s^{-1/2} Rs^{-5/2} (r/ Rs)^{-beta}. It follows that the phase-space density remains constant, in the sense of Qs=const., in the quiescent phases and it decreases as Rs^{-5/2} in the violent ones. The physical origin of the NFW density profile and the phase-space density power law is still unknown. Yet, the numerical experiments show that halos recover these relations after the violent phases. The major mergers drive Rs to increase and Qs to decrease discontinuously while keeping Qs Rs^{5/2} = const. The virial equilibrium in the quiescent phases implies that a DM halos evolves along a sequence of NFW profiles with constant energy per unit volume (i.e., pressure) within Rs.Comment: 7 pages, 5 figures, accepted by the Astrophysical Journal. Revised, 2 figures adde

Intermodal Energy Transfer in a Tapered Optical Fiber: Optimizing Transmission

We present an experimental and theoretical study of the energy transfer between modes during the tapering process of an optical nanofiber through spectrogram analysis. The results allow optimization of the tapering process, and we measure transmission in excess of 99.95% for the fundamental mode. We quantify the adiabaticity condition through calculations and place an upper bound on the amount of energy transferred to other modes at each step of the tapering, giving practical limits to the tapering angle.Comment: 29 pages, 17 figure

Ferro-lattice-distortions and charge fluctuations in superconducting LaO1−x_{1-x}Fx_{x}BiS2_{2}

Competing ferroelectric and charge density wave phases have been proposed to be present in the electron-phonon coupled LaO$_{1-x}$F$_{x}$BiS$_{2}$ superconductor. The lattice instability arises from unstable phonon modes that can break the crystal symmetry. Upon examination of the crystal structure using single crystal diffraction, we find a superlattice pattern arising from coherent in-plane displacements of the sulfur atoms in the BiS$_{2}$ superconducting planes. The distortions morph into coordinated ferro-distortive patterns, challenging previous symmetry suggestions including the possible presence of unstable antiferro-distortive patterns. The ferro-distortive pattern remains in the superconducting state, but with the displacements diminished in magnitude. Moreover, the sulfur displacements can exist in several polytypes stacked along the c-axis. Charge carriers can get trapped in the lattice deformations reducing the effective number of carriers available for pairing

Nuclear Aspects of Nucleosynthesis in Massive Stars

Preliminary results of a new set of stellar evolution and nucleosynthesis calculations for massive stars are presented. These results were obtained with an extended reaction network up to Bi. The discussion focuses on the importance of nuclear rates in pre- and post-explosive nucleosynthesis. The need for further experiments to study specific reactions and nuclear properties (optical alpha+nucleus potentials) is emphasized.Comment: 6 pages, 2 figures; invited talk, to appear in the Proceedings of the Int. Conf. "Structure of the Nucleus at the Dawn of the Century", May 2000, Bologna, Ital

Nucleosynthesis in massive stars revisited

We have performed the first calculations to follow the evolution of all stable nuclei and their radioactive progenitors in a finely-zoned stellar model computed from the onset of central hydrogen burning through explosion as a Type II supernova. Calculations were done for 15, 20, and 25 solar masses Pop I stars using the most recently available set of experimental and theoretical nuclear data, revised opacity tables, and taking into account mass loss due to stellar winds. Here results are presented for one 15 solar masses model.Comment: 4 pages, 1 figure; needs espcrc1.sty; talk at "Nuclei in the Cosmos 2000", Aarhus, Denmark, June 2000; will appear in Nucl. Phys.