3,547 research outputs found
Markov chain Monte Carlo analysis of Bianchi VII_h models
We have extended the analysis of Jaffe et al. to a complete Markov chain
Monte Carlo (MCMC) study of the Bianchi type models including a
dark energy density, using 1-year and 3-year Wilkinson Microwave Anisotropy
Probe (WMAP) cosmic microwave background (CMB) data. Since we perform the
analysis in a Bayesian framework our entire inference is contained in the
multidimensional posterior distribution from which we can extract marginalised
parameter constraints and the comparative Bayesian evidence. Treating the
left-handed Bianchi CMB anisotropy as a template centred upon the `cold-spot'
in the southern hemisphere, the parameter estimates derived for the total
energy density, `tightness' and vorticity from 3-year data are found to be:
, , with orientation ). This template is preferred by a factor of roughly
unity in log-evidence over a concordance cosmology alone. A Bianchi type
template is supported by the data only if its position on the sky is heavily
restricted. The low total energy density of the preferred template, implies a
geometry that is incompatible with cosmologies inferred from recent CMB
observations. Jaffe et al. found that extending the Bianchi model to include a
term in creates a degeneracy in the plane. We explore this region fully by MCMC and find that the
degenerate likelihood contours do not intersect areas of parameter space that 1
or 3 year WMAP data would prefer at any significance above . Thus we
can confirm that a physical Bianchi model is not responsible for
this signature.Comment: 8 pages, 10 figures, significant update to include more accurate
results and conclusions to match version accepted by MNRA
Experimental studies on the impact properties of water ice
Experimental studies on the impact of ice particles at very low velocity were continued. These measurements have applications in the dynamics of Saturn's rings. Initially data were obtained on the coefficient of restitution for ice spheres of one radius of curvature. The type of measurements were expanded to include restitution data for balls with a variety of surfaces as well as sticking forces between ice particles. Significant improvements were made to this experiment, the most important being the construction of a new apparatus. The new apparatus consists of a smaller version of the disk pendulum and a stainless steel, double-walled cryostat. The apparatus has proved to be a significant improvement over the old one. Measurements can now be made at temperatures near 90 K, comparable to the temperature of the environment of Saturn's rings, and with much greater temperature stability. It was found that a roughened contact surface or the presence of frost can cause a much larger change in the restitution measure than the geometrical effect of the radius of curvature
Ion laser plasmas
The typical noble gas ion laser plasma consists of a high-current-density glow discharge in a noble gas, in the presence of a magnetic field. Typical CW plasma conditions are current densities of 100 to 2000 A/cm^2, tube diameters of 1 to 10 mm, filling pressures of 0.1 to 1.0 torr, and an axial magnetic field of the order of 1000 G. Under these conditions the typical fractional ionization is about 2 percent and the electron temperature between 2 and 4 eV. Pulsed ion lasers typically use higher current densities and lower operating pressures.
This paper discusses the properties of ion laser plasmas, in terms of both their external discharge parameters and their internal ion and excited state densities. The effect these properties have on laser operation is explained. Many interesting plasma effects, which are important in ion lasers, are given attention. Among these are discharge nonuniformity near tube constrictions, extremely high ion radial drift velocities, wall losses intermediate between ambipolar diffusion and free fall, gas pumping effects, and radiation trapping. The current status of ion laser technology is briefly reviewed
Earth-like sand fluxes on Mars
Strong and sustained winds on Mars have been considered rare, on the basis of surface meteorology measurements and global circulation models, raising the question of whether the abundant dunes and evidence for wind erosion seen on the planet are a current process. Recent studies showed sand activity, but could not determine whether entire dunes were moving—implying large sand fluxes—or whether more localized and surficial changes had occurred. Here we present measurements of the migration rate of sand ripples and dune lee fronts at the Nili Patera dune field. We show that the dunes are near steady state, with their entire volumes composed of mobile sand. The dunes have unexpectedly high sand fluxes, similar, for example, to those in Victoria Valley, Antarctica, implying that rates of landscape modification on Mars and Earth are similar
ACS photometry of extended, luminous globular clusters in the outskirts of M31
A new population of extended, luminous globular clusters has recently been
discovered in the outskirts of M31. These objects have luminosities typical of
classical globular clusters, but much larger half-light radii. We report the
first results from deep ACS imaging of four such clusters, one of which is a
newly-discovered example lying at a projected distance of ~60 kpc from M31. Our
F606W, F814W colour-magnitude diagrams extend ~3 magnitudes below the
horizontal branch level, and clearly demonstrate, for the first time, that all
four clusters are composed of >10 Gyr old, metal-poor stellar populations. No
evidence for multiple populations is observed. From a comparison with Galactic
globular cluster fiducials we estimate metallicities in the range -2.2 < [Fe/H]
< -1.8. The observed horizontal branch morphologies show a clear second
parameter effect between the clusters. Preliminary radial luminosity profiles
suggest integrated magnitudes in the range -6.6 < M_V < -7.7, near the median
value of the globular cluster luminosity function. Our results confirm that
these four objects are bona fide old, metal-poor globular clusters, albeit with
combined structures and luminosities unlike those observed for any other
globular clusters in the Local Group or beyond.Comment: 12 pages, 3 figures, 1 table; accepted for publication in ApJ Letter
A Bayesian study of the primordial power spectrum from a novel closed universe model
We constrain the shape of the primordial power spectrum using recent
measurements of the cosmic microwave background (CMB) from the Wilkinson
Microwave Anisotropy Probe (WMAP) 7-year data and other high-resolution CMB
experiments. We also include observations of the matter power spectrum from the
luminous red galaxy (LRG) subset DR7 of the Sloan Digital Sky Survey (SDSS). We
consider two different models of the primordial power spectrum. The first is
the standard nearly scale-invariant spectrum in the form of a generalised
power-law parameterised in terms of the spectral amplitude , the
spectral index and (possibly) the running parameter .
The second spectrum is derived from the Lasenby and Doran (LD) model. The LD
model is based on the restriction of the total conformal time available in a
closed Universe and the predicted primordial power spectrum depends upon just
two parameters. An important feature of the LD spectrum is that it naturally
incorporates an exponential fall-off on large scales, which might provide a
possible explanation for the lower-than-expected power observed at low
multipoles in the CMB. In addition to parameter estimation, we compare both
models using Bayesian model selection. We find there is a significant
preference for the LD model over a simple power-law spectrum for a CMB-only
dataset, and over models with an equal number of parameters for all the
datasets considered.Comment: minor corrections to match accepted version to MNRA
High-power CW ultraviolet ion lasers
Over 1 watt of continuous ultraviolet output has been obtained from an argon ion laser in the 3500-3700Å range. This paper describes the discharge tube structure used and the operating characteristics of high-power ultraviolet laser transitions in argon and other noble gas ions. Operation at new wavelengths as well as CW operation of transitions previously observed only in pulsed operation are reported. Measurements of the dependence of spontaneous emission intensity from different ionization states on current and pressure are related to the excitation mechanisms for visible and uv ion lasers. The technique of population modulation by intracavity interruption has been used to study processes in the throat and uniform bore regions of ion laser discharges. The results of these studies are described. Problems encountered in the CW operation of ion laser discharge tubes at current densities above 1000Å/cm^2 are described
Relationship between macroscopic physical properties and local distortions of low doping La{1-x}Ca{x}MnO3: an EXAFS study
A temperature-dependent EXAFS investigation of La{1-x}Ca{x}MnO3 is presented
for the concentration range that spans the ferromagnetic-insulator (FMI) to
ferromagnetic-metal (FMM) transition region, x = 0.16-0.22. The samples are
insulating for x = 0.16-0.2 and show a metal/insulator transition for x = 0.22.
All samples are ferromagnetic although the saturation magnetization for the 16%
Ca sample is only ~ 70% of the expected value at 0.4T. We find that the FMI
samples have similar correlations between changes in the local Mn-O distortions
and the magnetization as observed previously for the colossal magnetoresistance
(CMR) samples (0.2 < x < 0.5) - except that the FMI samples never become fully
magnetized. The data show that there are at least two distinct types of
distortions. The initial distortions removed as the insulating sample becomes
magnetized are small and provides direct evidence that roughly 50% of the Mn
sites have a small distortion/site and are magnetized first. The large
remaining Mn-O distortions at low T are attributed to a small fraction of
Jahn-Teller-distorted Mn sites that are either antiferromagnetically ordered or
unmagnetized. Thus the insulating samples are very similar to the behavior of
the CMR samples up to the point at which the M/I transition occurs for the CMR
materials. The lack of metallic conductivity for x <= 0.2, when 50% or more of
the sample is magnetic, implies that there must be preferred magnetized Mn
sites and that such sites do not percolate at these concentrations.Comment: 27 pages, 8 figures, to be submitted to Phys. Rev.
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