14,172 research outputs found
Automated derivation of stellar atmospheric parameters and chemical abundances: the MATISSE algorithm
We present an automated procedure for the derivation of atmospheric
parameters (Teff, log g, [M/H]) and individual chemical abundances from stellar
spectra. The MATrix Inversion for Spectral SythEsis (MATISSE) algorithm
determines a basis, B_\theta(\lambda), allowing to derive a particular stellar
parameter \theta by projection of an observed spectrum. The B_\theta(\lambda)
function is determined from an optimal linear combination of theoretical
spectra and it relates, in a quantitative way, the variations in the spectrum
flux with variations in \theta. An application of this method to the GAIA/RVS
spectral range is described, together with its performances for different types
of stars of various metallicities. Blind tests with synthetic spectra of
randomly selected parameters and observed input spectra are also presented. The
method gives rapid, accurate and stable results and it can be efficiently
applied to the study of stellar populations through the analysis of large
spectral data sets, including moderate to low signal to noise spectra
A Theory of the Pseudogap State of the Cuprates
The phase diagram for a general model for Cuprates is derived in a mean-field
approximation. A phase violating time-reversal without breaking translational
symmetry is possible when both the ionic interactions and the local repulsions
are large compared to the energy difference between the Cu and O
single-particle levels. It ends at a quantum critical point as the hole or
electron doping is increased. Such a phase is necessarily accompanied by
singular forward scattering such that, in the stable phase, the density of
states at the chemical potential, projected to a particular point group
symmetry of the lattice is zero producing thereby an anisotropic gap in the
single-particle spectrum. It is suggested that this phase occupies the
"pseudogap" region of the phase diagram of the cuprates. The temperature
dependence of the single-particle spectra, the density of states, the specific
heat and the magnetic susceptibility are calculated with rather remarkable
correspondence with the experimental results. The importance of further direct
experimental verification of such a phase in resolving the principal issues in
the theory of the Cuprate phenomena is pointed out. To this end, some
predictions are provided.Comment: 41 pages, 8 figure
Theory of Superconductivity in the Cuprates
The quantum critical fluctuations of the time-reversal breaking order
parameter which is observed in the pseudogap regime of the Cuprates are shown
to couple to the lattice equivalent of the local angular momentum of the
fermions. Such a coupling favors scattering of fermions through angles close to
which is unambiguously shown to promote d-wave pairing. The right
order of magnitude of both and the normalized zero temperature gap
are calculated using the same fluctuations which give the
temperature, frequency and momentum dependence of the the anomalous normal
state properties for dopings near the quantum-critical value and with two
parameters extracted from fit to such experiments.Comment: Accepted for publication in PRB with the title "Theory of the
coupling of quantum-critical fluctuations to fermions and d-wave
superconductivity in the cuprates
The Multivalued Free-Field Maps of Liouville and Toda Gravities
Liouville and Toda gravity theories with non-vanishing interaction potentials
have spectra obtained by dividing the free-field spectra for these cases by the
Weyl group of the corresponding or Lie algebra. We study the
canonical transformations between interacting and free fields using the
technique of intertwining operators, giving explicit constructions for the
wavefunctions and showing that they are invariant under the corresponding Weyl
groups. These explicit constructions also permit a detailed analysis of the
operator-state maps and of the nature of the Seiberg bounds.Comment: 47 pages, plain Tex, 5 Postscript figures, uses epsf.tex. Repackaging
to permit Postscript generation, no changes to pape
Spectral densities of Wishart-Levy free stable random matrices: Analytical results and Monte Carlo validation
Random matrix theory is used to assess the significance of weak correlations
and is well established for Gaussian statistics. However, many complex systems,
with stock markets as a prominent example, exhibit statistics with power-law
tails, that can be modelled with Levy stable distributions. We review
comprehensively the derivation of an analytical expression for the spectra of
covariance matrices approximated by free Levy stable random variables and
validate it by Monte Carlo simulation.Comment: 10 pages, 1 figure, submitted to Eur. Phys. J.
Processing of multispectral thermal IR data for geologic applications
Multispectral thermal IR data were acquired with a 24-channel scanner flown in an aircraft over the E. Tintic Utah mining district. These digital image data required extensive computer processing in order to put the information into a format useful for a geologic photointerpreter. Simple enhancement procedures were not sufficient to reveal the total information content because the data were highly correlated in all channels. The data were shown to be dominated by temperature variations across the scene, while the much more subtle spectral variations between the different rock types were of interest. The image processing techniques employed to analyze these data are described
Cooling Rates for Relativistic Electrons Undergoing Compton Scattering in Strong Magnetic Fields
For inner magnetospheric models of hard X-ray and gamma-ray emission in
high-field pulsars and magnetars, resonant Compton upscattering is anticipated
to be the most efficient process for generating continuum radiation. This is
due in part to the proximity of a hot soft photon bath from the stellar surface
to putative radiation dissipation regions in the inner magnetosphere. Moreover,
because the scattering process becomes resonant at the cyclotron frequency, the
effective cross section exceeds the classical Thomson value by over two orders
of magnitude, thereby enhancing the efficiency of continuum production and the
cooling of relativistic electrons. This paper presents computations of the
electron cooling rates for this process, which are needed for resonant Compton
models of non-thermal radiation from such highly-magnetized pulsars. The
computed rates extend previous calculations of magnetic Thomson cooling to the
domain of relativistic quantum effects, sampled near and above the quantum
critical magnetic field of 44.13 TeraGauss. This is the first exposition of
fully relativistic, quantum magnetic Compton cooling rates for electrons, and
it employs both the traditional Johnson and Lippman cross section, and a newer
Sokolov and Ternov (ST) formulation of Compton scattering in strong magnetic
fields. Such ST formalism is formally correct for treating spin-dependent
effects that are important in the cyclotron resonance, and has not been
addressed before in the context of cooling by Compton scattering. The QED
effects are observed to profoundly lower the rates below extrapolations of the
familiar magnetic Thomson results, as expected, when recoil and Klein-Nishina
reductions become important.Comment: 33 pages, 11 figures, accepted for publication in The Astrophysical
Journa
The boundary integral method for magnetic billiards
We introduce a boundary integral method for two-dimensional quantum billiards
subjected to a constant magnetic field. It allows to calculate spectra and wave
functions, in particular at strong fields and semiclassical values of the
magnetic length. The method is presented for interior and exterior problems
with general boundary conditions. We explain why the magnetic analogues of the
field-free single and double layer equations exhibit an infinity of spurious
solutions and how these can be eliminated at the expense of dealing with
(hyper-)singular operators. The high efficiency of the method is demonstrated
by numerical calculations in the extreme semiclassical regime.Comment: 28 pages, 12 figure
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