The Anisoplanatic Point Spread Function in Adaptive Optics

The effects of anisoplanatism on the adaptive optics point spread function are investigated. A model is derived that combines observations of the guide star with an analytic formulation of anisoplanatism to generate predictions for the adaptive optics point spread function at arbitrary locations within the field of view. The analytic formulation captures the dependencies of anisoplanatism on aperture diameter, observing wavelength, angular offset, zenith angle and turbulence profile. The predictions of this model are compared to narrowband 2.12 um and 1.65 um images of a 21 arcsec binary (mV=7.3, 7.6) acquired with the Palomar Adaptive Optics System on the Hale 5 meter telescope. Contemporaneous measurements of the turbulence profile made with a DIMM/MASS unit are used together with images of the primary to predict the point spread function of the binary companion. Predicted companion Strehl ratios are shown to match measurements to within a few percent, whereas predictions based on the isoplanatic angle approximation are highly discrepant. The predicted companion point spread functions are shown to agree with observations to 10%. These predictions are used to measure the differential photometry between binary members to an accuracy of 1 part in 10^{3}, and the differential astrometry to an accuracy of 1 mas. Errors in the differential astrometry are shown to be dominated by differential atmospheric tilt jitter. These results are compared to other techniques that have been employed for photometry, astrometry, and high contrast imaging.Comment: 26 pages, 7 figure

Diversity in parasitic nematode genomes: the microRNAs of Brugia pahangi and Haemonchus contortus are largely novel

<b>BACKGROUND:</b> MicroRNAs (miRNAs) play key roles in regulating post-transcriptional gene expression and are essential for development in the free-living nematode Caenorhabditis elegans and in higher organisms. Whether microRNAs are involved in regulating developmental programs of parasitic nematodes is currently unknown. Here we describe the the miRNA repertoire of two important parasitic nematodes as an essential first step in addressing this question. <b>RESULTS:</b> The small RNAs from larval and adult stages of two parasitic species, Brugia pahangi and Haemonchus contortus, were identified using deep-sequencing and bioinformatic approaches. Comparative analysis to known miRNA sequences reveals that the majority of these miRNAs are novel. Some novel miRNAs are abundantly expressed and display developmental regulation, suggesting important functional roles. Despite the lack of conservation in the miRNA repertoire, genomic positioning of certain miRNAs within or close to specific coding genes is remarkably conserved across diverse species, indicating selection for these associations. Endogenous small-interfering RNAs and Piwi-interacting (pi)RNAs, which regulate gene and transposon expression, were also identified. piRNAs are expressed in adult stage H. contortus, supporting a conserved role in germline maintenance in some parasitic nematodes. <b>CONCLUSIONS:</b> This in-depth comparative analysis of nematode miRNAs reveals the high level of divergence across species and identifies novel sequences potentially involved in development. Expression of novel miRNAs may reflect adaptations to different environments and lifestyles. Our findings provide a detailed foundation for further study of the evolution and function of miRNAs within nematodes and for identifying potential targets for intervention

Electrostatics of Gapped and Finite Surface Electrodes

We present approximate methods for calculating the three-dimensional electric potentials of finite surface electrodes including gaps between electrodes, and estimate the effects of finite electrode thickness and an underlying dielectric substrate. As an example we optimize a radio-frequency surface-electrode ring ion trap, and find that each of these factors reduces the trapping secular frequencies by less than 5% in realistic situations. This small magnitude validates the usual assumption of neglecting the influences of gaps between electrodes and finite electrode extent.Comment: 9 pages, 9 figures (minor changes

Radio Astronomical Polarimetry and the Lorentz Group

In radio astronomy the polarimetric properties of radiation are often modified during propagation and reception. Effects such as Faraday rotation, receiver cross-talk, and differential amplification act to change the state of polarized radiation. A general description of such transformations is useful for the investigation of these effects and for the interpretation and calibration of polarimetric observations. Such a description is provided by the Lorentz group, which is intimately related to the transformation properties of polarized radiation. In this paper the transformations that commonly arise in radio astronomy are analyzed in the context of this group. This analysis is then used to construct a model for the propagation and reception of radio waves. The implications of this model for radio astronomical polarimetry are discussed.Comment: 10 pages, accepted for publication in Astrophysical Journa

Metal Cooling in Simulations of Cosmic Structure Formation

The addition of metals to any gas can significantly alter its evolution by increasing the rate of radiative cooling. In star-forming environments, enhanced cooling can potentially lead to fragmentation and the formation of low-mass stars, where metal-free gas-clouds have been shown not to fragment. Adding metal cooling to numerical simulations has traditionally required a choice between speed and accuracy. We introduce a method that uses the sophisticated chemical network of the photoionization software, Cloudy, to include radiative cooling from a complete set of metals up to atomic number 30 (Zn) that can be used with large-scale three-dimensional hydrodynamic simulations. Our method is valid over an extremely large temperature range (10 K < T < 10^8 K), up to hydrogen number densities of 10^12 cm^-3. At this density, a sphere of 1 Msun has a radius of roughly 40 AU. We implement our method in the adaptive mesh refinement (AMR) hydrodynamic/N-body code, Enzo. Using cooling rates generated with this method, we study the physical conditions that led to the transition from Population III to Population II star formation. While C, O, Fe, and Si have been previously shown to make the strongest contribution to the cooling in low-metallicity gas, we find that up to 40% of the metal cooling comes from fine-structure emission by S, when solar abundance patterns are present. At metallicities, Z > 10^-4 Zsun, regions of density and temperature exist where gas is both thermally unstable and has a cooling time less than its dynamical time. We identify these doubly unstable regions as the most inducive to fragmentation. At high redshifts, the CMB inhibits efficient cooling at low temperatures and, thus, reduces the size of the doubly unstable regions, making fragmentation more difficult.Comment: 19 pages, 12 figures, significant revision, including new figure

High resolution characterisation of microstructural evolution in Rbx_{x}Fe2−y_{2-y}Se2_{2} crystals on annealing

The superconducting and magnetic properties of phase-separated A$_x$Fe$_{2-y}$Se$_2$ compounds are known to depend on post-growth heat treatments and cooling profiles. This paper focusses on the evolution of microstructure on annealing, and how this influences the superconducting properties of Rb$_x$Fe$_2-y$Se$_2$ crystals. We find that the minority phase in the as-grown crystal has increased unit cell anisotropy (c/a ratio), reduced Rb content and increased Fe content compared to the matrix. The microstructure is rather complex, with two-phase mesoscopic plate-shaped features aligned along {113} habit planes. The minority phase are strongly facetted on the {113} planes, which we have shown to be driven by minimising the volume strain energy introduced as a result of the phase transformation. Annealing at 488K results in coarsening of the mesoscopic plate-shaped features and the formation of a third distinct phase. The subtle differences in structure and chemistry of the minority phase(s) in the crystals are thought to be responsible for changes in the superconducting transition temperature. In addition, scanning photoemission microscopy has clearly shown that the electronic structure of the minority phase has a higher occupied density of states of the low binding energy Fe3d orbitals, characteristic of crystals that exhibit superconductivity. This demonstrates a clear correlation between the Fe-vacancy-free phase with high c/a ratio and the electronic structure characteristics of the superconducting phase.Comment: 6 figures v2 is exactly the same as v1. The typesetting errors in the abstract have been correcte

Experimental demonstration of a technique to generate arbitrary quantum superposition states

Using a single, harmonically trapped $^9$Be$^+$ ion, we experimentally demonstrate a technique for generation of arbitrary states of a two-level particle confined by a harmonic potential. Rather than engineering a single Hamiltonian that evolves the system to a desired final sate, we implement a technique that applies a sequence of simple operations to synthesize the state