42,144 research outputs found

    Observationally-Motivated Analysis of Simulated Galaxies

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    The spatial and temporal relationships between stellar age, kinematics, and chemistry are a fundamental tool for uncovering the physics driving galaxy formation and evolution. Observationally, these trends are derived using carefully selected samples isolated via the application of appropriate magnitude, colour, and gravity selection functions of individual stars; conversely, the analysis of chemodynamical simulations of galaxies has traditionally been restricted to the age, metallicity, and kinematics of `composite' stellar particles comprised of open cluster-mass simple stellar populations. As we enter the Gaia era, it is crucial that this approach changes, with simulations confronting data in a manner which better mimics the methodology employed by observers. Here, we use the \textsc{SynCMD} synthetic stellar populations tool to analyse the metallicity distribution function of a Milky Way-like simulated galaxy, employing an apparent magnitude plus gravity selection function similar to that employed by the RAdial Velocity Experiment (RAVE); we compare such an observationally-motivated approach with that traditionally adopted - i.e., spatial cuts alone - in order to illustrate the point that how one analyses a simulation can be, in some cases, just as important as the underlying sub-grid physics employed.Comment: Accepted for publication in PoS (Proceedings of Science): Nuclei in the Cosmos XIII (Debrecen, Jul 2014); 6 pages; 3 figure

    A Gaussian process framework for modelling instrumental systematics: application to transmission spectroscopy

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    Transmission spectroscopy, which consists of measuring the wavelength-dependent absorption of starlight by a planet's atmosphere during a transit, is a powerful probe of atmospheric composition. However, the expected signal is typically orders of magnitude smaller than instrumental systematics, and the results are crucially dependent on the treatment of the latter. In this paper, we propose a new method to infer transit parameters in the presence of systematic noise using Gaussian processes, a technique widely used in the machine learning community for Bayesian regression and classification problems. Our method makes use of auxiliary information about the state of the instrument, but does so in a non-parametric manner, without imposing a specific dependence of the systematics on the instrumental parameters, and naturally allows for the correlated nature of the noise. We give an example application of the method to archival NICMOS transmission spectroscopy of the hot Jupiter HD 189733, which goes some way towards reconciling the controversy surrounding this dataset in the literature. Finally, we provide an appendix giving a general introduction to Gaussian processes for regression, in order to encourage their application to a wider range of problems.Comment: 6 figures, 1 table, accepted for publication in MNRA

    Single-level resonance parameters fit nuclear cross-sections

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    Least squares analyses of experimental differential cross-section data for the U-235 nucleus have yielded single level Breit-Wigner resonance parameters that fit, simultaneously, three nuclear cross sections of capture, fission, and total

    Laser microprobe study of cosmic dust (IDPs) and potential source materials

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    The study of cosmic dust or interplanetary dust particles (IDP) can provide vital information about primitive materials derived primarily from comets and asteroids along with a small unknown fraction from the nearby interstellar medium. The study of these particles can enhance our understanding of comets along with the decoding of the history of the early solar system. In addition the study of the cosmic dust for IDP particles can assist in the elucidation of the cosmic history of the organogenic elements which are vital to life processes. Studies to date on these particles have shown that they are complex, heterogeneous assemblages of both amorphous and crystalline components. In order to understand the nature of these particles, any analytical measurements must be able to distinguish between the possible sources of these particles. A study was undertaken using a laser microprobe interfaced to a quadrupole mass spectrometer for the analysis of the volatile components present in cosmic dust particles, terrestrial contaminants present in the upper atmosphere, and primitive carbonaceous chondrites. From the study of the volatiles released from the carbonaceous materials it is hoped that one could distinguish between components and sources in the IDP particles analyzed. The technique is briefly described and results for the CI, CM, and CV chondrites and cosmic dust particle W7027B8 are presented

    Gravitational hydrodynamics of large scale structure formation

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    The gravitational hydrodynamics of the primordial plasma with neutrino hot dark matter is considered as a challenge to the bottom-up cold dark matter paradigm. Viscosity and turbulence induce a top-down fragmentation scenario before and at decoupling. The first step is the creation of voids in the plasma, which expand to 37 Mpc on the average now. The remaining matter clumps turn into galaxy clusters. Turbulence produced at expanding void boundaries causes a linear morphology of 3 kpc fragmenting protogalaxies along vortex lines. At decoupling galaxies and proto-globular star clusters arise; the latter constitute the galactic dark matter halos and consist themselves of earth-mass H-He planets. Frozen planets are observed in microlensing and white-dwarf-heated ones in planetary nebulae. The approach also explains the Tully-Fisher and Faber-Jackson relations, and cosmic microwave temperature fluctuations of micro-Kelvins.Comment: 6 pages, no figure

    Increasing trap stiffness with position clamping in holographic optical tweezers

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    We present a holographic optical tweezers system capable of position clamping multiple particles. Moving an optical trap in response to the trapped object's motion is a powerful technique for optical control and force measurement. We have now realised this experimentally using a Boulder Nonlinear Systems Spatial Light Modulator (SLM) with a refresh rate of 203Hz. We obtain a reduction of 44% in the variance of the bead's position, corresponding to an increase in effective trap stiffness of 77%. This reduction relies on the generation of holograms at high speed. We present software capable of calculating holograms in under 1ms using a graphics processor unit. © 2009 Optical Society of America

    Metastable liquid-liquid coexistence and density anomalies in a core-softened fluid

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    Linearly-sloped or `ramp' potentials belong to a class of core-softened models which possess a liquid-liquid critical point (LLCP) in addition to the usual liquid-gas critical point. Furthermore they exhibit thermodynamic anomalies in the density and compressibility, the nature of which may be akin to those occurring in water. Previous simulation studies of ramp potentials have focused on just one functional form, for which the LLCP is thermodynamically stable. In this work we construct a series of ramp potentials, which interpolate between this previously studied form and a ramp-based approximation to the Lennard-Jones (LJ) potential. By means of Monte Carlo simulation, we locate the LLCP, the first order high density liquid (HDL)-low density liquid (LDL) coexistence line, and the line of density maxima for a selection of potentials in the series. We observe that as the LJ limit is approached, the LLCP becomes metastable with respect to freezing into a hexagonal close packed crystalline solid. The qualitative nature of the phase behaviour in this regime shows a remarkable resemblance to that seen in simulation studies of accurate water models. Specifically, the density of the liquid phase exceeds that of the solid; the gradient of the metastable LDL-HDL line is negative in the pressure (p)-temperature (T) plane; while the line of density maxima in the p-T plane has a shape similar to that seen in water and extends well into the {\em stable} liquid region of the phase diagram. As such, our results lend weight to the `second critical point' hypothesis as an explanation for the anomalous behaviour of water.Comment: 7 pages, 8 figure
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