Refraction of shear zones in granular materials

We study strain localization in slow shear flow focusing on layered granular materials. A heretofore unknown effect is presented here. We show that shear zones are refracted at material interfaces in analogy with refraction of light beams in optics. This phenomenon can be obtained as a consequence of a recent variational model of shear zones. The predictions of the model are tested and confirmed by 3D discrete element simulations. We found that shear zones follow Snell's law of light refraction.Comment: 4 pages, 3 figures, minor changes, jounal ref. adde

Resonant continuum in the Hartree-Fock+BCS approximation

A method for incorporating the effect of the resonant continuum into Hartree-Fock+BCS equations is proposed. The method is applied for the case of a neutron-rich nucleus calculated with a Skyrme-type force plus a zero-range pairing interaction and the results are compared with Hartree-Fock-Bogoliubov calculations. It is shown that the widths of resonant states have an important effect on the pairing properties of nuclei close to the drip line.Comment: 9 pages, 2 figures, comparison with HFB adde

A Dense Gas Trigger for OH Megamasers

HCN and CO line diagnostics provide new insight into the OH megamaser (OHM) phenomenon, suggesting a dense gas trigger for OHMs. We identify three physical properties that differentiate OHM hosts from other starburst galaxies: (1) OHMs have the highest mean molecular gas densities among starburst galaxies; nearly all OHM hosts have = 10^3-10^4 cm^-3 (OH line-emitting clouds likely have n(H2) > 10^4 cm^-3). (2) OHM hosts are a distinct population in the nonlinear part of the IR-CO relation. (3) OHM hosts have exceptionally high dense molecular gas fractions, L(HCN)/L(CO)>0.07, and comprise roughly half of this unusual population. OH absorbers and kilomasers generally follow the linear IR-CO relation and are uniformly distributed in dense gas fraction and L(HCN), demonstrating that OHMs are independent of OH abundance. The fraction of non-OHMs with high mean densities and high dense gas fractions constrains beaming to be a minor effect: OHM emission solid angle must exceed 2 pi steradians. Contrary to conventional wisdom, IR luminosity does not dictate OHM formation; both star formation and OHM activity are consequences of tidal density enhancements accompanying galaxy interactions. The OHM fraction in starbursts is likely due to the fraction of mergers experiencing a temporal spike in tidally driven density enhancement. OHMs are thus signposts marking the most intense, compact, and unusual modes of star formation in the local universe. Future high redshift OHM surveys can now be interpreted in a star formation and galaxy evolution context, indicating both the merging rate of galaxies and the burst contribution to star formation.Comment: 5 pages, 3 figures, 1 table, accepted by ApJ Letter

On the measurement of the proton-air cross section using longitudinal shower profiles

In this paper, we will discuss the prospects of deducing the proton-air cross section from fluorescence telescope measurements of extensive air showers. As it is not possible to observe the point of first interaction $X_{\rm 1}$ directly, other observables closely linked to $X_{\rm 1}$ must be inferred from the longitudinal profiles. This introduces a dependence on the models used to describe the shower development. The most straightforward candidate for a good correlation to $X_{\rm 1}$ is the depth of shower maximum $X_{\rm max}$. We will discuss the sensitivity of an $X_{\rm max}$-based analysis on $\sigma_{\rm p-air}$ and quantify the systematic uncertainties arising from the model dependence, parameters of the reconstruction method itself and a possible non-proton contamination of the selected shower sample.Comment: 4 pages, Proceedings for ISVHECRI Weihei 200

Diagnostic Ultrasound Induced Inertial Cavitation To Non-Invasively Restore Coronary And Microvascular Flow In Acute Myocardial Infarction

Ultrasound induced cavitation has been explored as a method of dissolving intravascular and microvascular thrombi in acute myocardial infarction. The purpose of this study was to determine the type of cavitation required for success, and whether longer pulse duration therapeutic impulses (sustaining the duration of cavitation) could restore both microvascular and epicardial flow with this technique. Accordingly, in 36 hyperlipidemic atherosclerotic pigs, thrombotic occlusions were induced in the mid-left anterior descending artery. Pigs were then randomized to either a) 1/2 dose tissue plasminogen activator (0.5 mg/kg) alone; or same dose plasminogen activator and an intravenous microbubble infusion with either b) guided high mechanical index short pulse (2.0 MI; 5 usec) therapeutic ultrasound impulses; or c) guided 1.0 mechanical index long pulse (20 usec) impulses. Passive cavitation detectors indicated the high mechanical index impulses (both long and short pulse duration) induced inertial cavitation within the microvasculature. Epicardial recanalization rates following randomized treatments were highest in pigs treated with the long pulse duration therapeutic impulses (83% versus 59% for short pulse, and 49% for tissue plasminogen activator alone; p \u3c 0.05). Even without epicardial recanalization, however, early microvascular recovery occurred with both short and long pulse therapeutic impulses (p \u3c 0.005 compared to tissue plasminogen activator alone), and wall thickening improved within the risk area only in pigs treated with ultrasound and microbubbles. We conclude that although short pulse duration guided therapeutic impulses from a diagnostic transducer transiently improve microvascular flow, long pulse duration therapeutic impulses produce sustained epicardial and microvascular re-flow in acute myocardial infarction

The use of permutation representations in structural computations in large finite matrix groups

We determine the minimal degree permutation representations of all finite groups with trivial soluble radical, and describe applications to structural computations in large finite matrix groups that use the output of the CompositionTree algorithm. We also describe how this output can be used to help find an effective base and strong generating set for such groups. We have implemented the resulting algorithms in Magma, and we report on their performance

Probing the Kinematics of the Narrow-Line Region in Seyfert Galaxies with Slitless Spectroscopy: Observational Results

We present slitless spectra of 10 Seyfert galaxies observed with the Space Telescope Imaging Spectrograph on the Hubble Space Telescope. The spectra cover the [OIII] 4959, 5007 emission lines at a spectral resolving power of ~9000 and a spatial resolution of 0.1". We compare the slitless spectra with previous HST narrow-band images to determine the velocity shifts and dispersions of the bright emission-line knots in the narrow-line regions (NLRs) of these Seyferts. Many knots are spatially resolved with sizes of tenths of arcsecs, corresponding to tens of pcs, and yet they appear to move coherently with radial velocities between zero and +/- 1200 km/s with respect to the systemic velocities of their hostgalaxies. The knots also show a broad range in velocity dispersion, ranging from ~30 km/s (the velocity resolution) to ~1000 km/s FWHM. Most of the Seyfert galaxies in this sample show an organized flow pattern, with radial velocities near zero at the nucleus (defined by the optical continuum peak) and increasing to maximum blueshifts and redshifts within ~1'' of the nucleus, followed by a decline to the systemic velocity. The emission-line knots also follow a general trend of decreasing velocity dispersion with increasing distance. In the Seyfert 2 galaxies, the presence of blueshifts and redshifts on either side of the nucleus indicates that rotation alone cannot explain the observed radial velocities, and that radial outflow plays an important role. Each of the Seyfert galaxies in this sample (with the exception of Mrk 3) shows a bright, compact (FWHM < 0.5") [O III] knot at the position of its optical nucleus. These nuclear emission-line knots have radial-velocity centroids near zero, but they typically have the highest velocity dispersions.Comment: 28 pages, 5 figures (on 9 pages), accepted for A