Silicon and magnesium in planetary nebulae

The IUE satellite spectra of some planetary nebulae show features due to silicon and magnesium: Si III wavelengths 1883, 1892; Si IV wavelengths 1394, 1403; Mg II wavelengths 2796, 2804 and Mg V wavelengths 2784, 2929. With the aid of modeling techniques, the corresponding elemental abundances are found. In addition to previous observations of NGC 7662 and IC 418, data were found for NGC 2440, Hu 1-2, IC 2003 and IC 2165. Silicon appears depleted by up to an order of magnitude relative to the sun. Large variations of magnesium abundance are found, which are likely to reflect differing degrees of depletion due to grain formation

Elemental abundances in high-excitation planetary nebulae

The IUE satellite was used to obtain low dispersion spectra of the high excitation planetary nebulae IC 351, IC 2003, NGC 2022, IC 2165, NGC 2440, Hu 1-2, and IC 5217. Numerical modeling was undertaken to determine the chemical composition of these objects with particular emphasis on obtaining elemental carbon and nitrogen abundances. Large variations in the C/N ratio from object to object are suggested

Multi-wavelength Signatures of Cosmic Rays in the Milky Way

Cosmic rays (CRs) propagate in the Milky Way and interact with the interstellar medium and magnetic fields. These interactions produce emissions that span the electromagnetic spectrum, and are an invaluable tool for understanding the intensities and spectra of CRs in distant regions, far beyond those probed by direct CR measurements. We present updates on the study of CR properties by combining multi-frequency observations of the interstellar emission and latest CR direct measurements with propagation models.Comment: 8 pages, 4 figures. Proceedings of the 35th International Cosmic Ray Conference, ICRC201

Scattered Lyman-alpha Radiation Around Sources Before Cosmological Reionization

The spectra of the first galaxies and quasars in the Universe should be strongly absorbed shortward of their rest-frame Lyman-alpha wavelength by neutral hydrogen (HI) in the intervening intergalactic medium. However, the Lyman-alpha line photons emitted by these sources are not eliminated but rather scatter until they redshift out of resonance and escape due to the Hubble expansion of the surrounding intergalactic HI. We calculate the resulting brightness distribution and the spectral shape of the diffuse Lyman-alpha line emission around high redshift sources, before the intergalactic medium was reionized. Typically, the Lyman-alpha photons emitted by a source at z=10 scatter over a characteristic angular radius of order 15 arcseconds around the source and compose a line which is broadened and redshifted by about a thousand km/s relative to the source. The scattered photons are highly polarized. Detection of the diffuse Lyman-alpha halos around high redshift sources would provide a unique tool for probing the neutral intergalactic medium before the epoch of reionization. On sufficiently large scales where the Hubble flow is smooth and the gas is neutral, the Lyman-alpha brightness distribution can be used to determine the cosmological mass densities of baryons and matter.Comment: 21 pages, 5 Postscript figures, accepted by ApJ; figures 1--3 corrected; new section added on the detectability of Lyman alpha halos; conclusions update

Monte Carlo Simulation of Lyman Alpha Scattering and Application to Damped Lyman Alpha Systems

A Monte Carlo code to solve the transfer of Lyman alpha (Lya) photons is developed, which can predict the Lya image and two-dimensional Lya spectra of a hydrogen cloud with any given geometry, Lya emissivity, neutral hydrogen density distribution, and bulk velocity field. We apply the code to several simple cases of a uniform cloud to show how the Lya image and emitted line spectrum are affected by the column density, internal velocity gradients, and emissivity distribution. We then apply the code to two models for damped Lya absorption systems: a spherical, static, isothermal cloud, and a flattened, axially symmetric, rotating cloud. If the emission is due to fluorescence of the external background radiation, the Lya image should have a core corresponding to the region where hydrogen is self-shielded. The emission line profile has the characteristic double peak with a deep central trough. We show how rotation of the cloud causes the two peaks to shift in wavelength as the slit is perpendicular to the rotation axis, and how the relative amplitude of the two peaks is changed. In reality, damped Lya systems are likely to have a clumpy gas distribution with turbulent velocity fields, which should smooth the line emission profile, but should still leave the rotation signature of the wavelength shift across the system.Comment: 19 pages, 17 eps figures. One panel is added in Fig.1 to show the recoil effect. Revisions are made in response to the referee's comments. Accepted for publication in Ap

T Pyxidis: The First Cataclysmic Variable with a Collimated Jet

We present the first observational evidence for a collimated jet in a cataclysmic variable system; the recurrent nova T Pyxidis. Optical spectra show bipolar components of H$\alpha$ with velocities $\sim 1400 km/s$, very similar to those observed in the supersoft X-ray sources and in SS 433. We argue that a key ingredient of the formation of jets in the supersoft X-ray sources and T Pyx (in addition to an accretion disk threaded by a vertical magnetic field), is the presence of nuclear burning on the surface of the white dwarf.Comment: 10 pages 2 figures to appear in ApJ Letter

Fault-tolerant quantum computation with high threshold in two dimensions

We present a scheme of fault-tolerant quantum computation for a local architecture in two spatial dimensions. The error threshold is 0.75% for each source in an error model with preparation, gate, storage and measurement errors.Comment: 4 pages, 4 figures; v2: A single 2D layer of qubits (simple square lattice) with nearest-neighbor translation-invariant Ising interaction suffices. Slightly improved threshol

The interpretation of the field angle dependence of the critical current in defect-engineered superconductors

We apply the vortex path model of critical currents to a comprehensive analysis of contemporary data on defect-engineered superconductors, showing that it provides a consistent and detailed interpretation of the experimental data for a diverse range of materials. We address the question of whether electron mass anisotropy plays a role of any consequence in determining the form of this data and conclude that it does not. By abandoning this false interpretation of the data, we are able to make significant progress in understanding the real origin of the observed behavior. In particular, we are able to explain a number of common features in the data including shoulders at intermediate angles, a uniform response over a wide angular range and the greater discrimination between individual defect populations at higher fields. We also correct several misconceptions including the idea that a peak in the angular dependence of the critical current is a necessary signature of strong correlated pinning, and conversely that the existence of such a peak implies the existence of correlated pinning aligned to the particular direction. The consistency of the vortex path model with the principle of maximum entropy is introduced.Comment: 14 pages, 7 figure