The effect of magnetic dipolar interactions on the interchain spin wave dispersion in CsNiF_3

Inelastic neutron scattering measurements were performed on the ferromagnetic chain system CsNiF_3 in the collinear antiferromagnetic ordered state below T_N = 2.67K. The measured spin wave dispersion was found to be in good agreement with linear spin wave theory including dipolar interactions. The additional dipole tensor in the Hamiltonian was essential to explain some striking phenomena in the measured spin wave spectrum: a peculiar feature of the dispersion relation is a jump at the zone center, caused by strong dipolar interactions in this system. The interchain exchange coupling constant and the planar anisotropy energy were determined within the present model to be J'/k_B = -0.0247(12)K and A/k_B = 3.3(1)K. This gives a ratio J/J' \approx 500, using the previously determined intrachain coupling constant J/k_B = 11.8$. The small exchange energy J' is of the same order as the dipolar energy, which implies a strong competition between the both interactions.Comment: 18 pages, TeX type, 7 Postscript figures included. To be published in Phys. Rev.

Constraints on Neutron Star Crusts From Oscillations in Giant Flares

We show that the fundamental seismic shear mode, observed as a quasi-periodic oscillation in giant flares emitted by highly-magnetized neutron stars, is particularly sensitive to the nuclear physics of the crust. The identification of an oscillation at ~ 30 Hz as the fundamental crustal shear mode requires a nuclear symmetry energy that depends very weakly on density near saturation. If the nuclear symmetry energy varies more strongly with density, then lower frequency oscillations, previously identified as torsional Alfven modes of the fluid core, could instead be associated with the crust. If this is the case, then future observations of giant flares should detect oscillations at around 18 Hz. An accurate measurement of the neutron skin thickness of lead will also constrain the frequencies predicted by the model.Comment: 5 pages, 3 figures; Version to appear in Phys. Rev. Let

An annotated checklist of the Coleoptera (Insecta) of the Cayman Islands, West Indies

A faunal list of 605 species of Coleoptera in 396 genera in 63 families is presented for the Cayman Islands. For most species, island and locality within island collecting information is provided

The Political Economy of Exchange Rate Policy in Colombia

This paper looks into the political economy forces that helped shape exchange rate policy in Colombia since the early 1960s. As witnessed by the remarkable longevity of the managed crawl, Colombian exchange rate policies since 1967 did not take the form of major regime changes. Policy shifts came about through variations in the rate of crawl, varying degrees of administrative restrictions on capital flows and, on occasions, ad-hoc measures to generate multiple exchange rates.

Revealing the nature of magnetic shadows with numerical 3D-MHD simulations

We investigate the interaction of magneto-acoustic waves with magnetic network elements with the aim of finding possible signatures of the magnetic shadow phenomenon in the vicinity of network elements. We carried out three-dimensional numerical simulations of magneto-acoustic wave propagation in a model solar atmosphere that is threaded by a complexly structured magnetic field, resembling that of a typical magnetic network element and of internetwork regions. High-frequency waves of 10 mHz are excited at the bottom of the simulation domain. On their way through the upper convection zone and through the photosphere and the chromosphere they become perturbed, refracted, and converted into different mode types. We applied a standard Fourier analysis to produce oscillatory power-maps of the line-of-sight velocity. In the power maps of the upper photosphere and the lower chromosphere, we clearly see the magnetic shadow: a seam of suppressed power surrounding the magnetic network elements. We demonstrate that this shadow is linked to the mode conversion process and that power maps at these height levels show the signature of three different magneto-acoustic wave modes.Comment: Astronomy & Astrophysics Letters, in print 4 pages, 4 figure

Transient jets in V617 Sagittarii

Some of the luminous Compact Binary Supersoft X-Ray sources (CBSS) have shown indications of jets, also called satellites due to their appearance in the spectra. In V Sagittae (V Sge) stars, the galactic counterparts of the CBSS, such features have been reported only for WX Cen. If V Sge stars are indeed the analogs of CBSS, one may expect transient jet emission in other objects of this class. Spectroscopic observations of the V Sge star V617 Sgr have been made, both at high photometric state and at decline. We show that V617 Sgr presents Halpha satellites at high photometric state with velocities of +/-780 km/s. This feature confirms, once more, the CBSS nature of the V Sge stars, however the details of the spectral characteristics also suggest that the two groups of stars display some intrinsic spectroscopic differences, which are likely to be due to a selection effect related to chemical abundance.Comment: Four pages, accepted to be published as a Letter in A&