Phyllobates terribilis

Number of Pages: 5Integrative BiologyGeological Science

Dendrobates leucomelas

Number of Pages: 6Integrative BiologyGeological Science

Lifetimes of 26Al and 34Cl in an astrophysical plasma

We study here the onset of thermal equilibrium affecting the lifetimes of 26Al and 34Cl nuclei within a hot astrophysical photon gas. The 26Al isotope is of prime interest for gamma ray astronomy with the observation of its delayed (t_1\2=0.74 My) 1.809MeV gamma-ray line. Its nucleosynthesis is complicated by the presence of a short lived (t_1\2=6.34s) spin isomer. A similar configuration is found in 34Cl where the decay of its isomer (34mCl, t_1\2=32m) is followed by delayed gamma-ray emission with characteristic energies. The lifetimes of such nuclei are reduced at high temperature by the thermal population of shorter lived levels. However, thermal equilibrium within 26Al and 34Cl levels is delayed by the presence of the isomer. We study here the transition to thermal equilibrium where branching ratios for radiative transitions are needed in order to calculate lifetimes. Since some of these very small branching ratios are not known experimentally, we use results of shell model calculations.Comment: 11 pages, 5 figures, Latex, accepted for publication in Phys. Rev.

Nilsson-SU3 selfconsistency in heavy N=Z nuclei

It is argued that there exist natural shell model spaces optimally adapted to the operation of two variants of Elliott' SU3 symmetry that provide accurate predictions of quadrupole moments of deformed states. A selfconsistent Nilsson-like calculation describes the competition between the realistic quadrupole force and the central field, indicating a {\em remarkable stability of the quadruplole moments}---which remain close to their quasi and pseudo SU3 values---as the single particle splittings increase. A detailed study of the $N=Z$ even nuclei from $^{56}$Ni to $^{96}$Cd reveals that the region of prolate deformation is bounded by a pair of transitional nuclei $^{72}$Kr and $^{84}$Mo in which prolate ground state bands are predicted to dominate, though coexisting with oblate ones,Comment: Replacement I) Title simplified. II) Major revision: structure of paper kept but two thirds totally rewritten (same number of pages); 20 references adde

Deformation of the lowermost mantle from seismic anisotropy

Understanding the lowermost part of the Earth’s mantle—known as D''—can help us investigate whole-mantle dynamics, core-mantle interactions and processes such as slab deformation in the deep Earth. D'' shows significant seismic anisotropy, the variation of seismic wave speed with direction. This is likely due to deformation- induced alignment of MgSiO3-post-perovskite (ppv), believed to be the main mineral phase present in the region; however if this is the case, then previous measurements of D'' anisotropy, which are generally made in one direction only, are insufficient to distinguish candidate mechanisms of slip in ppv because the mineral is orthorhombic. Here we measure anisotropy in D'' beneath North and Central America, where slab material impinges6 on the core-mantle boundary (CMB), using shallow as well as deep earthquakes to increase the azimuthal coverage in D!. We make >700 individual measurements of shear wave splitting in D'' in three regions from two different azimuths in each case, and we show that the previously-assumed case of vertical transverse isotropy (VTI, where wave speed shows no azimuthal variation) is not possible; more complicated mechanisms must be involved. We test the fit of different MgSiO3-ppv deformation mechanisms to our results and find that shear on (001) is most consistent with observations and expected shear above the CMB beneath subduction zones. With new models of mantle flow, or improved experimental evidence of which ppv slip systems dominate, this method will allow us to map deformation at the CMB and link processes in D'', such as plume initiation, to the rest of the mantle

Backbending in 50Cr

The collective yrast band and the high spin states of the nucleus 50Cr are studied using the spherical shell model and the HFB method. The two descriptions lead to nearly the same values for the relevant observables. A first backbending is predicted at I=10\hbar corresponding to a collective to non-collective transition. At I=16\hbar a second backbending occurs, associated to a configuration change that can also be interpreted as an spherical to triaxial transition.Comment: ReVTeX v 3.0 epsf.sty, 5 pages, 5 figures included. Full Postscript version available at http://www.ft.uam.es/~gabriel/Cr50art.ps.g

Dynamical systems analysis of spike-adding mechanisms in transient bursts

Transient bursting behaviour of excitable cells, such as neurons, is a common feature observed experimentally, but theoretically, it is not well understood. We analyse a five-dimensional simplified model of after-depolarisation that exhibits transient bursting behaviour when perturbed with a short current injection. Using one-parameter continuation of the perturbed orbit segment formulated as a well-posed boundary value problem, we show that the spike-adding mechanism is a canard-like transition that has a different character from known mechanisms for periodic burst solutions. The biophysical basis of the model gives a natural time-scale separation, which allows us to explain the spike-adding mechanism using geometric singular perturbation theory, but it does not involve actual bifurcations as for periodic bursts. We show that unstable sheets of the critical manifold, formed by saddle equilibria of the system that only exist in a singular limit, are responsible for the spike-adding transition; the transition is organised by the slow flow on the critical manifold near folds of this manifold. Our analysis shows that the orbit segment during the spike-adding transition includes a fast transition between two unstable sheets of the slow manifold that are of saddle type. We also discuss a different parameter regime where the presence of additional saddle equilibria of the full system alters the spike-adding mechanism