8,491 research outputs found
Ultracold, radiative charge transfer in hybrid Yb ion - Rb atom traps
Ultracold hybrid ion-atom traps offer the possibility of microscopic
manipulation of quantum coherences in the gas using the ion as a probe.
However, inelastic processes, particularly charge transfer can be a significant
process of ion loss and has been measured experimentally for the Yb ion
immersed in a Rb vapour. We use first-principles quantum chemistry codes to
obtain the potential energy curves and dipole moments for the lowest-lying
energy states of this complex. Calculations for the radiative decay processes
cross sections and rate coefficients are presented for the total decay
processes. Comparing the semi-classical Langevin approximation with the quantum
approach, we find it provides a very good estimate of the background at higher
energies. The results demonstrate that radiative decay mechanisms are important
over the energy and temperature region considered. In fact, the Langevin
process of ion-atom collisions dominates cold ion-atom collisions. For spin
dependent processes \cite{kohl13} the anisotropic magnetic dipole-dipole
interaction and the second-order spin-orbit coupling can play important roles,
inducing couplingbetween the spin and the orbital motion. They measured the
spin-relaxing collision rate to be approximately 5 orders of magnitude higher
than the charge-exchange collision rate \cite{kohl13}. Regarding the measured
radiative charge transfer collision rate, we find that our calculation is in
very good agreement with experiment and with previous calculations.
Nonetheless, we find no broad resonances features that might underly a strong
isotope effect. In conclusion, we find, in agreement with previous theory that
the isotope anomaly observed in experiment remains an open question.Comment: 7 figures, 1 table accepted for publication in J. Phys. B: At. Mol.
Opt. Phys. arXiv admin note: text overlap with arXiv:1107.114
Structural parameters for globular clusters in M31 and generalizations for the fundamental plane
The structures of globular clusters (GCs) reflect their dynamical states and
past histories. High-resolution imaging allows the exploration of morphologies
of clusters in other galaxies. Surface brightness profiles from new Hubble
Space Telescope observations of 34 globular clusters in M31 are presented,
together with fits of several different structural models to each cluster. M31
clusters appear to be adequately fit by standard King models, and do not
obviously require alternate descriptions with relatively stronger halos, such
as are needed to fit many GCs in other nearby galaxies. The derived structural
parameters are combined with corrected versions of those measured in an earlier
survey to construct a comprehensive catalog of structural and dynamical
parameters for M31 GCs with a sample size similar to that for the Milky Way.
Clusters in M31, the Milky Way, Magellanic Clouds, Fornax dwarf spheroidal and
NGC 5128 define a very tight fundamental plane with identical slopes. The
combined evidence for these widely different galaxies strongly reinforces the
view that old globular clusters have near-universal structural properties
regardless of host environment.Comment: AJ in press; 59 pages including 16 figure
IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10}: A Novel Reentrant Spin-Glass Material
A new iridium containing layered cuprate material,
IrSr_2Sm_{1.15}Ce_{0.85}Cu_{2.175}O_{10, has been synthesized by conventional
ambient-pressure solid-state techniques. The material's structure has been
fully characterized by Rietveld refinement of high resolution synchrotron X-ray
diffraction data; tilts and rotations of the IrO_6 octahedra are observed as a
result of a bond mismatch between in-plane Ir-O and Cu-O bond lengths.
DC-susceptibility measurements evidence a complex set of magnetic transitions
upon cooling that are characteristic of a reentrant spin-glass ground-state.
The glassy character of the lowest temperature, Tg=10 K, transition is further
confirmed by AC-susceptibility measurements, showing a characteristic frequency
dependence that can be well fitted by the Vogel-Fulcher law and yields a value
of \Delta_(T_f)/[T_f \Delta log({\omega})] =0.015(1), typical of dilute
magnetic systems. Electronic transport measurements show the material to be
semiconducting at all temperatures with no transition to a superconducting
state. Negative magnetoresistance is observed when the material is cooled below
25 K, and the magnitude of this magnetoresistance is seen to increase upon
cooling to a value of MR = -9 % at 8 K
A flight investigation with a STOL airplane flying curved, descending instrument approach paths
A flight investigation using a De Havilland Twin Otter airplane was conducted to determine the configurations of curved, 6 deg descending approach paths which would provide minimum airspace usage within the requirements for acceptable commercial STOL airplane operations. Path configurations with turns of 90 deg, 135 deg, and 180 deg were studied; the approach airspeed was 75 knots. The length of the segment prior to turn, the turn radius, and the length of the final approach segment were varied. The relationship of the acceptable path configurations to the proposed microwave landing system azimuth coverage requirements was examined
The Aemulus Project III: Emulation of the Galaxy Correlation Function
Using the N-body simulations of the AEMULUS Project, we construct an emulator
for the non-linear clustering of galaxies in real and redshift space. We
construct our model of galaxy bias using the halo occupation framework,
accounting for possible velocity bias. The model includes 15 parameters,
including both cosmological and galaxy bias parameters. We demonstrate that our
emulator achieves ~ 1% precision at the scales of interest, 0.1<r<10 h^{-1}
Mpc, and recovers the true cosmology when tested against independent
simulations. Our primary parameters of interest are related to the growth rate
of structure, f, and its degenerate combination fsigma_8. Using this emulator,
we show that the constraining power on these parameters monotonically increases
as smaller scales are included in the analysis, all the way down to 0.1 h^{-1}
Mpc. For a BOSS-like survey, the constraints on fsigma_8 from r<30 h^{-1} Mpc
scales alone are more than a factor of two tighter than those from the fiducial
BOSS analysis of redshift-space clustering using perturbation theory at larger
scales. The combination of real- and redshift-space clustering allows us to
break the degeneracy between f and sigma_8, yielding a 9% constraint on f alone
for a BOSS-like analysis. The current AEMULUS simulations limit this model to
surveys of massive galaxies. Future simulations will allow this framework to be
extended to all galaxy target types, including emission-line galaxies.Comment: 14 pages, 8 figures, 1 table; submitted to ApJ; the project webpage
is available at https://aemulusproject.github.io ; typo in Figure 7 and
caption updated, results unchange
The Aemulus Project I: Numerical Simulations for Precision Cosmology
The rapidly growing statistical precision of galaxy surveys has lead to a
need for ever-more precise predictions of the observables used to constrain
cosmological and galaxy formation models. The primary avenue through which such
predictions will be obtained is suites of numerical simulations. These
simulations must span the relevant model parameter spaces, be large enough to
obtain the precision demanded by upcoming data, and be thoroughly validated in
order to ensure accuracy. In this paper we present one such suite of
simulations, forming the basis for the AEMULUS Project, a collaboration devoted
to precision emulation of galaxy survey observables. We have run a set of 75
(1.05 h^-1 Gpc)^3 simulations with mass resolution and force softening of
3.51\times 10^10 (Omega_m / 0.3) ~ h^-1 M_sun and 20 ~ h^-1 kpc respectively in
47 different wCDM cosmologies spanning the range of parameter space allowed by
the combination of recent Cosmic Microwave Background, Baryon Acoustic
Oscillation and Type Ia Supernovae results. We present convergence tests of
several observables including spherical overdensity halo mass functions, galaxy
projected correlation functions, galaxy clustering in redshift space, and
matter and halo correlation functions and power spectra. We show that these
statistics are converged to 1% (2%) for halos with more than 500 (200)
particles respectively and scales of r>200 ~ h^-1 kpc in real space or k ~ 3 h
Mpc^-1 in harmonic space for z\le 1. We find that the dominant source of
uncertainty comes from varying the particle loading of the simulations. This
leads to large systematic errors for statistics using halos with fewer than 200
particles and scales smaller than k ~ 4 h^-1 Mpc. We provide the halo catalogs
and snapshots detailed in this work to the community at
https://AemulusProject.github.io.Comment: 16 pages, 12 figures, 3 Tables Project website:
https://aemulusproject.github.io
The Aemulus Project II: Emulating the Halo Mass Function
Existing models for the dependence of the halo mass function on cosmological
parameters will become a limiting source of systematic uncertainty for cluster
cosmology in the near future. We present a halo mass function emulator and
demonstrate improved accuracy relative to state-of-the-art analytic models. In
this work, mass is defined using an overdensity criteria of 200 relative to the
mean background density. Our emulator is constructed from the AEMULUS
simulations, a suite of 40 N-body simulations with snapshots from z=3 to z=0.
These simulations cover the flat wCDM parameter space allowed by recent Cosmic
Microwave Background, Baryon Acoustic Oscillation and Type Ia Supernovae
results, varying the parameters w, Omega_m, Omega_b, sigma_8, N_{eff}, n_s, and
H_0. We validate our emulator using five realizations of seven different
cosmologies, for a total of 35 test simulations. These test simulations were
not used in constructing the emulator, and were run with fully independent
initial conditions. We use our test simulations to characterize the modeling
uncertainty of the emulator, and introduce a novel way of marginalizing over
the associated systematic uncertainty. We confirm non-universality in our halo
mass function emulator as a function of both cosmological parameters and
redshift. Our emulator achieves better than 1% precision over much of the
relevant parameter space, and we demonstrate that the systematic uncertainty in
our emulator will remain a negligible source of error for cluster abundance
studies through at least the LSST Year 1 data set.Comment: https://aemulusproject.github.io
Structural parameters for globular clusters in NGC 5128. III. ACS surface-brightness profiles and model fits
We present internal surface-brightness profiles, based on HST/ACS imaging in
the F606W bandpass, for 131 globular cluster (GC) candidates with luminosities
10^4 - 3 x 10^6 solar, in the giant elliptical galaxy NGC 5128. Several
structural models are fit to the profile of each cluster and combined with
mass-to-light ratios from population-synthesis models, to derive a catalogue of
fundamental structural and dynamical parameters parallel in form to the
catalogues recently produced by McLaughlin & van der Marel and Barmby et al.
for GCs and massive young star clusters in Local Group galaxies. As part of
this, we provide corrected and extended parameter estimates for another 18
clusters in NGC 5128, which we observed previously. We show that, like GCs in
the Milky Way and some of its satellites, the majority of globulars in NGC 5128
are well fit by isotropic Wilson models, which have intrinsically more
distended envelope structures than the standard King lowered isothermal
spheres. We use our models to predict internal velocity dispersions for every
cluster in our sample. These predictions agree well in general with the
observed dispersions in a small number of clusters for which spectroscopic data
are available. In a subsequent paper, we use these results to investigate
scaling relations for GCs in NGC 5128.Comment: MNRAS, in press. 28 pages. Full data tables available at
http://www.astro.keele.ac.uk/~dem/clusters.htm
Leaf-applied sodium chloride promotes cadmium accumulation in durum wheat grain
Cadmium (Cd) accumulation in durum wheat grain is a growing concern. Among the factors affecting Cd accumulation in plants, soil chloride (Cl) concentration plays a critical role. The effect of leaf NaCl application on grain Cd was studied in greenhouse-grown durum wheat (Triticum turgidum L. durum, cv. Balcali-2000) by immersing (10 s) intact flag leaves into Cd and/or NaCl-containing solutions for 14 times during heading and dough stages. Immersing flag leaves in solutions containing increasing amount of Cd resulted in substantial increases in grain Cd concentration. Adding NaCl alone or in combination with the Cd-containing immersion solution promoted accumulation of Cd in the grains, by up to 41%. In contrast, Zn concentrations of grains were not affected or even decreased by the NaCl treatments. This is likely due to the effect of Cl complexing Cd and reducing positive charge on the metal ion, an effect that is much smaller for Zn. Charge reduction or removal (CdCl2 0 species) would increase the diffusivity/lipophilicity of Cd and enhance its capability to penetrate the leaf epidermis and across membranes. Of even more significance to human health was the ability of Cl alone to penetrate leaf tissue and mobilize and enhance shoot Cd transfer to grains, yet reducing or not affecting Zn transfer
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