Formation of Pillars at the Boundaries between H II Regions and Molecular Clouds

We investigate numerically the hydrodynamic instability of an ionization front (IF) accelerating into a molecular cloud, with imposed initial perturbations of different amplitudes. When the initial amplitude is small, the imposed perturbation is completely stabilized and does not grow. When the initial perturbation amplitude is large enough, roughly the ratio of the initial amplitude to wavelength is greater than 0.02, portions of the IF temporarily separate from the molecular cloud surface, locally decreasing the ablation pressure. This causes the appearance of a large, warm HI region and triggers nonlinear dynamics of the IF. The local difference of the ablation pressure and acceleration enhances the appearance and growth of a multimode perturbation. The stabilization usually seen at the IF in the linear regimes does not work due to the mismatch of the modes of the perturbations at the cloud surface and in density in HII region above the cloud surface. Molecular pillars are observed in the late stages of the large amplitude perturbation case. The velocity gradient in the pillars is in reasonably good agreement with that observed in the Eagle Nebula. The initial perturbation is imposed in three different ways: in density, in incident photon number flux, and in the surface shape. All cases show both stabilization for a small initial perturbation and large growth of the second harmonic by increasing amplitude of the initial perturbation above a critical value.Comment: 21 pages, 8 figures, accepted for publication in ApJ. high resolution figures available upon reques

Plastic Deformation in Laser-Induced Shock Compression of Monocrystalline Copper

Copper monocrystals were subjected to shock compression at pressures of 10–60 GPa by a short (3 ns initial) duration laser pulse. Transmission electron microscopy revealed features consistent with previous observations of shock-compressed copper, albeit at pulse durations in the µs regime. The results suggest that the defect structure is generated at the shock front. A mechanism for dislocation generation is presented, providing a realistic prediction of dislocation density as a function of pressure. The threshold stress for deformation twinning in shock compression is calculated from the constitutive equations for slip, twinning, and the Swegle-Grady relationship

Observation of magnetic field generation via the Weibel instability in interpenetrating plasma flows

Collisionless shocks can be produced as a result of strong magnetic fields in a plasma flow, and therefore are common in many astrophysical systems. The Weibel instability is one candidate mechanism for the generation of sufficiently strong fields to create a collisionless shock. Despite their crucial role in astrophysical systems, observation of the magnetic fields produced by Weibel instabilities in experiments has been challenging. Using a proton probe to directly image electromagnetic fields, we present evidence of Weibel-generated magnetic fields that grow in opposing, initially unmagnetized plasma flows from laser-driven laboratory experiments. Three-dimensional particle-in-cell simulations reveal that the instability efficiently extracts energy from the plasma flows, and that the self-generated magnetic energy reaches a few percent of the total energy in the system. This result demonstrates an experimental platform suitable for the investigation of a wide range of astrophysical phenomena, including collisionless shock formation in supernova remnants, large-scale magnetic field amplification, and the radiation signature from gamma-ray bursts

X-ray Astronomy in the Laboratory with a Miniature Compact Object Produced by Laser-Driven Implosion

Laboratory spectroscopy of non-thermal equilibrium plasmas photoionized by intense radiation is a key to understanding compact objects, such as black holes, based on astronomical observations. This paper describes an experiment to study photoionizing plasmas in laboratory under well-defined and genuine conditions. Photoionized plasma is here generated using a 0.5-keV Planckian x-ray source created by means of a laser-driven implosion. The measured x-ray spectrum from the photoionized silicon plasma resembles those observed from the binary stars Cygnus X-3 and Vela X-1 with the Chandra x-ray satellite. This demonstrates that an extreme radiation field was produced in the laboratory, however, the theoretical interpretation of the laboratory spectrum significantly contradicts the generally accepted explanations in x-ray astronomy. This model experiment offers a novel test bed for validation and verification of computational codes used in x-ray astronomy.Comment: 5 pages, 4 figures are included. This is the original submitted version of the manuscript to be published in Nature Physic

An exact solution of the moving boundary problem for the relativistic plasma expansion in a dipole magnetic field

An exact analytic solution is obtained for a uniformly expanding, neutral, highly conducting plasma sphere in an ambient dipole magnetic field with an arbitrary orientation of the dipole moment in the space. Based on this solution the electrodynamical aspects related to the emission and transformation of energy have been considered. In order to highlight the effect of the orientation of the dipole moment in the space we compare our results obtained for parallel orientation with those for transversal orientation. The results obtained can be used to treat qualitatively experimental and simulation data, and several phenomena of astrophysical and laboratory significance.Comment: 7 pages, 2 figures. arXiv admin note: substantial text overlap with arXiv:physics/060323

A comparative study of Rayleigh-Taylor and Richtmyer-Meshkov instabilities in 2D and 3D in tantalum

Driving a shock wave through the interface between two materials with different densities can result in the Richtmyer-Meshkov or Rayleigh-Taylor instability and initial perturbations at the interface will grow. If the shock wave is sufficiently strong, the instability will lead to plastic flow at the interface. Material strength will reduce the amount of plastic flow and suppress growth. While such instabilities have been investigated in 2D, no studies of this phenomena have been performed in 3D on materials with strength. Initial perturbations to seed the hydrodynamic instability were coined into tantalum recovery targets. Two types of perturbations were used, two dimensional (2D) perturbations (hill and valley) and three-dimensional (3D) perturbations (egg crate pattern). The targets were subjected to dynamic loading using the Janus laser at the Jupiter Laser Facility. Shock pressures ranged from 50 GPa up to 150 GPa and were calibrated using VISAR drive targets

Correlation property of length sequences based on global structure of complete genome

This paper considers three kinds of length sequences of the complete genome. Detrended fluctuation analysis, spectral analysis, and the mean distance spanned within time $L$ are used to discuss the correlation property of these sequences. The values of the exponents from these methods of these three kinds of length sequences of bacteria indicate that the long-range correlations exist in most of these sequences. The correlation have a rich variety of behaviours including the presence of anti-correlations. Further more, using the exponent $\gamma$, it is found that these correlations are all linear ($\gamma=1.0\pm 0.03$). It is also found that these sequences exhibit $1/f$ noise in some interval of frequency ($f>1$). The length of this interval of frequency depends on the length of the sequence. The shape of the periodogram in $f>1$ exhibits some periodicity. The period seems to depend on the length and the complexity of the length sequence.Comment: RevTex, 9 pages with 5 figures and 3 tables. Phys. Rev. E Jan. 1,2001 (to appear

Striatal dopamine D2 receptor binding of risperidone in schizophrenic patients as assessed by 123I-iodobenzamide SPECT: a comparative study with olanzapine

The aim of this investigation was to compare the degree of striatal dopamine-(D2) receptor blockade by two atypical antipsychotic drugs, risperidone and olanzapine. The percentage of D2 receptor occupancy during treatment was calculated by comparing the results of 123I-iodobenzamide SPECT with those from healthy control subjects. Twenty inpatients suffering from schizophrenia or schizoaffective psychosis according to DSM IV/ICD-10 criteria were treated with clinically recommended doses of risperidone and compared with 13 inpatients treated with up to 20 mg olanzapine. Neuroleptic dose and D2 receptor blockade correlated strongly for both risperidone (Pearson r = –0.86, p = 0.0001) and olanzapine (Pearson r = –0.77, p = 0.002). There was no significant difference between the D2 receptor occupancy of the two substances when given in the clinically recommended dose range (unpaired t-test, t= –0.112, p=0.911)

On Validating an Astrophysical Simulation Code

We present a case study of validating an astrophysical simulation code. Our study focuses on validating FLASH, a parallel, adaptive-mesh hydrodynamics code for studying the compressible, reactive flows found in many astrophysical environments. We describe the astrophysics problems of interest and the challenges associated with simulating these problems. We describe methodology and discuss solutions to difficulties encountered in verification and validation. We describe verification tests regularly administered to the code, present the results of new verification tests, and outline a method for testing general equations of state. We present the results of two validation tests in which we compared simulations to experimental data. The first is of a laser-driven shock propagating through a multi-layer target, a configuration subject to both Rayleigh-Taylor and Richtmyer-Meshkov instabilities. The second test is a classic Rayleigh-Taylor instability, where a heavy fluid is supported against the force of gravity by a light fluid. Our simulations of the multi-layer target experiments showed good agreement with the experimental results, but our simulations of the Rayleigh-Taylor instability did not agree well with the experimental results. We discuss our findings and present results of additional simulations undertaken to further investigate the Rayleigh-Taylor instability.Comment: 76 pages, 26 figures (3 color), Accepted for publication in the ApJ

Seed systems smallholder farmers use

Seed can be an important entry point for promoting productivity, nutrition and resilience among smallholder farmers. While investments have primarily focused on strengthening the formal sector, this article documents the degree to which the informal sector remains the core for seed acquisition, especially in Africa. Conclusions drawn from a uniquely comprehensive data set, 9660 observations across six countries and covering 40 crops, show that farmers access 90.2 % of their seed from informal systems with 50.9 % of that deriving from local markets. Further, 55 % of seed is paid for by cash, indicating that smallholders are already making important investments in this arena. Targeted interventions are proposed for rendering formal and informal seed sector more smallholder-responsive and for scaling up positive impacts