38,891 research outputs found
Rosenblatt distribution subordinated to gaussian random fields with long-range dependence
The Karhunen-Lo\`eve expansion and the Fredholm determinant formula are used
to derive an asymptotic Rosenblatt-type distribution of a sequence of integrals
of quadratic functions of Gaussian stationary random fields on R^d displaying
long-range dependence. This distribution reduces to the usual Rosenblatt
distribution when d=1. Several properties of this new distribution are
obtained. Specifically, its series representation in terms of independent
chi-squared random variables is given, the asymptotic behavior of the
eigenvalues, its L\`evy-Khintchine representation, as well as its membership to
the Thorin subclass of self-decomposable distributions. The existence and
boundedness of its probability density is then a direct consequence.Comment: This paper has 40 pages and it has already been submitte
Completely dark galaxies: their existence, properties, and strategies for finding them
There are a number of theoretical and observational hints that large numbers
of low-mass galaxies composed entirely of dark matter exist in the field. The
theoretical considerations follow from the prediction of cold dark matter
theory that there exist many low-mass galaxies for every massive one. The
observational considerations follow from the observed paucity of these low-mass
galaxies in the field but not in dense clusters of galaxies; this suggests that
the lack of small galaxies in the field is due to the inhibition of star
formation in the galaxies as opposed to the fact that their small dark matter
halos do not exist. In this work we outline the likely properties of low-mass
dark galaxies, and describe observational strategies for finding them, and
where in the sky to search. The results are presented as a function of the
global properties of dark matter, in particular the presence or absence of a
substantial baryonic dark matter component. If the dark matter is purely cold
and has a Navarro, Frenk and White density profile, directly detecting dark
galaxies will only be feasible with present technology if the galaxy has a
maximum velocity dispersion in excess of 70 km/s, in which case the dark
galaxies could strongly lens background objects. This is much higher than the
maximum velocity dispersions in most dwarf galaxies. If the dark matter in
galaxy halos has a baryonic component close to the cosmic ratio, the
possibility of directly detecting dark galaxies is much more realistic; the
optimal method of detection will depend on the nature of the dark matter. A
number of more indirect methods are also discussed.Comment: 12 pages, 4 figures, MNRAS in pres
On a class of minimum contrast estimators for Gegenbauer random fields
The article introduces spatial long-range dependent models based on the
fractional difference operators associated with the Gegenbauer polynomials. The
results on consistency and asymptotic normality of a class of minimum contrast
estimators of long-range dependence parameters of the models are obtained. A
methodology to verify assumptions for consistency and asymptotic normality of
minimum contrast estimators is developed. Numerical results are presented to
confirm the theoretical findings.Comment: 23 pages, 8 figure
Anode Fabrication for Solid Oxide Fuel Cells: Electroless and Electrodeposition of Nickel and Silver into Doped Ceria Scaffolds
© 2016 The Authors.A novel fabrication method using electroless and electrodeposited Ni/Ag/GDC for SOFC anodes is presented. First a porous Ce0.9Gd0.1O2-x (GDC) scaffold was deposited on a YSZ electrolyte by screen printing and sintering. The scaffold was then metallized with silver using Tollens reaction, followed by electrodeposition of nickel from a Watts bath. The electrodes (Ni/Ag/GDC) were tested in both symmetrical and fuel cell configurations. The microstructures of the Ni/Ag/GDC anodes were analyzed using scanning electron microscopy (SEM) and energy dispersive x-ray spectroscopy (EDX). Nano-particles of Ni formed in the porous GDC scaffold provided triple phase boundaries (TPB). The electronic conductivity of the Ni/Ag/GDC (3.5/24.7/71.8 vol%) electrode was good even at relatively low Ni volume fractions. The electrochemical performance was examined in different concentrations of humidified hydrogen (3% H2O) and over a range of temperatures (600-750 °C). The total area specific resistance (ASR) of the anode at 750 °C in humidified 97 vol% H2 was 1.12 Ω cm2, with low-frequency polarization (R-l) as the largest contributor. The electrodes were successfully integrated into a fuel cell and operated in both H2 and syngas
Study of the derivative expansions for the nuclear structure functions
We study the convergence of the series expansions sometimes used in the
analysis of the nuclear effects in Deep Inelastic Scattering (DIS) proccesses
induced by leptons. The recent advances in statistics and quality of the data,
in particular for neutrinos calls for a good control of the theoretical
uncertainties of the models used in the analysis. Using realistic nuclear
spectral functions which include nucleon correlations, we find that the
convergence of the derivative expansions to the full results is poor except at
very low values of
The Stars in M15 Were Born with the r-process
High-resolution spectroscopy of stars on the red giant branch (RGB) of the globular cluster M15 has revealed a large (~1 dex) dispersion in the abundances of r-process elements such as Ba and Eu. Neutron star mergers (NSMs) have been proposed as a major source of the r-process. However, most NSM models predict a delay time longer than the timescale for cluster formation. One possibility is that a NSM polluted the surfaces of stars in M15 long after the cluster finished forming. In this case, the abundances of the polluting elements would decrease in the first dredge-up as stars turn on to the RGB. We present Keck/DEIMOS abundances of Ba in 66 stars along the entire RGB and the top of the main sequence. The Ba abundances have no trend with stellar luminosity (evolutionary phase). Therefore, the stars were born with the Ba that they have today, and Ba did not originate in a source with a delay time longer than the timescale for cluster formation. In particular, if the source of Ba was a NSM, it would have had a very short delay time. Alternatively, if Ba enrichment took place before the formation of the cluster, an inhomogeneity of a factor of 30 in Ba abundance needs to be able to persist over the length scale of the gas cloud that formed M15, which is unlikely
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