216 research outputs found
Validating Continuum Lowering Models via Multi-Wavelength Measurements of Integrated X-ray Emission
X-ray emission spectroscopy is a well-established technique used to study
continuum lowering in dense plasmas. It relies on accurate atomic physics
models to robustly reproduce high-resolution emission spectra, and depends on
our ability to identify spectroscopic signatures such as emission lines or
ionization edges of individual charge states within the plasma. Here we
describe a method that forgoes these requirements, enabling the validation of
different continuum lowering models based solely on the total intensity of
plasma emission in systems driven by narrow-bandwidth x-ray pulses across a
range of wavelengths. The method is tested on published Al spectroscopy data
and applied to the new case of solid-density partially-ionized Fe plasmas,
where extracting ionization edges directly is precluded by the significant
overlap of emission from a wide range of charge states
XUV Opacity of Aluminum between the Cold-Solid to Warm-Plasma Transition
We present calculations of the free-free XUV opacity of warm, solid-density
aluminum at photon energies between the plasma frequency at 15 eV and the
L-edge at 73 eV, using both density functional theory combined with molecular
dynamics and a semi-analytical model in the RPA framework with the inclusion of
local field corrections. As the temperature is increased from room temperature
to 10 eV, with the ion and electron temperatures equal, we calculate an
increase in the opacity in the range over which the degree of ionization is
constant. The effect is less pronounced if only the electron temperature is
allowed to increase. The physical significance of these increases is discussed
in terms of intense XUV-laser matter interactions on both femtosecond and
picosecond time-scales.Comment: 4 pages, 3 figure
Correlation energy of the paramagnetic electron gas at the thermodynamic limit
The variational and diffusion quantum Monte Carlo methods are used to
calculate the correlation energy of the paramagnetic three-dimensional
homogeneous electron gas at intermediate to high density. Ground state energies
in finite cells are determined using Slater-Jastrow-backflow trial wave
functions, and finite-size errors are removed using twist-averaged boundary
conditions and extrapolation of the energy per particle to the thermodynamic
limit of infinite system size. Our correlation energies in the thermodynamic
limit are lower (i.e., more negative, and therefore more accurate according to
the variational principle) than previous results, and can be used for the
parameterization of density functionals to be applied to high-density systems
The Exceptionally Luminous Type Ia Supernova 2007If
SN 2007if was the third over-luminous Type Ia supernova (SN Ia) detected after 2003fg and 2006gz. We present the photometric and spectroscopic observations of the SN and its host by ROTSE-III, HET, and Keck. From the H a line identified in the host spectra, we determine a redshift of 0.0736. At this distance, the SN reached an absolute magnitude of -20.4, brighter than any other SNe Ia ever observed. If the source of luminosity is radioactive decay, a large amount of radioactive nickel (similar to 1.5 M(circle dot)) is required to power the peak luminosity, more than can be produced realistically in a Chandrasekhar mass progenitor. Low expansion velocity, similar to that of 2003fg, is also measured around the maximum light. The observations may suggest that SN 2007if was from a massive white dwarf progenitor, plausibly exploding with mass well beyond 1.4 M(circle dot). Alternatively, we investigate circumstellar interaction that may contribute to the excess luminosity.NASA NNX-08AN25G, NNX-08AV63GNSF AST-0707769, PHY-0801007Australian Research CouncilUniversity of New South WalesUniversity of TexasUniversity of MichiganAstronom
Random hypergraphs and their applications
In the last few years we have witnessed the emergence, primarily in on-line
communities, of new types of social networks that require for their
representation more complex graph structures than have been employed in the
past. One example is the folksonomy, a tripartite structure of users,
resources, and tags -- labels collaboratively applied by the users to the
resources in order to impart meaningful structure on an otherwise
undifferentiated database. Here we propose a mathematical model of such
tripartite structures which represents them as random hypergraphs. We show that
it is possible to calculate many properties of this model exactly in the limit
of large network size and we compare the results against observations of a real
folksonomy, that of the on-line photography web site Flickr. We show that in
some cases the model matches the properties of the observed network well, while
in others there are significant differences, which we find to be attributable
to the practice of multiple tagging, i.e., the application by a single user of
many tags to one resource, or one tag to many resources.Comment: 11 pages, 7 figure
Correlation energy of the spin-polarized electron liquid studied using quantum Monte Carlo simulations
Variational and diffusion quantum Monte Carlo (VMC and DMC) methods with Slater-Jastrow-backflow trial wave functions are used to study the spin-polarized three-dimensional uniform electron fluid. We report ground state VMC and DMC energies in the density range 0.5 ≤ r s ≤ 20 . Finite-size errors are corrected using canonical-ensemble twist-averaged boundary conditions and extrapolation of the twist-averaged energy per particle calculated at three system sizes ( N = 113 , 259 , and 387 ) to the thermodynamic limit of infinite system size. The DMC energies in the thermodynamic limit are used to parametrize a local spin density approximation correlation function for inhomogeneous electron systems
Hypergraph topological quantities for tagged social networks
Recent years have witnessed the emergence of a new class of social networks,
that require us to move beyond previously employed representations of complex
graph structures. A notable example is that of the folksonomy, an online
process where users collaboratively employ tags to resources to impart
structure to an otherwise undifferentiated database. In a recent paper[1] we
proposed a mathematical model that represents these structures as tripartite
hypergraphs and defined basic topological quantities of interest. In this paper
we extend our model by defining additional quantities such as edge
distributions, vertex similarity and correlations as well as clustering. We
then empirically measure these quantities on two real life folksonomies, the
popular online photo sharing site Flickr and the bookmarking site CiteULike. We
find that these systems share similar qualitative features with the majority of
complex networks that have been previously studied. We propose that the
quantities and methodology described here can be used as a standard tool in
measuring the structure of tagged networks.Comment: 8 pages, 9 figures, revte
The Young, Massive, Star Cluster Sandage-96 After the Explosion of SN 2004dj in NGC 2403
The bright Type II-plateau supernova (SN) 2004dj occurred within the young,
massive stellar cluster Sandage-96 in a spiral arm of NGC 2403. New
multi-wavelength observations obtained with several ground-based and
space-based telescopes are combined to study the radiation from Sandage-96
after SN 2004dj faded away. Sandage-96 started to dominate the flux in the
optical bands starting September 2006 (~800 d after explosion). The optical
fluxes are equal to the pre-explosion ones within the observational
uncertainties. An optical Keck spectrum obtained ~900 d after explosion shows
the dominant blue continuum from the cluster stars shortward of 6000 \AA as
well as strong SN nebular emission lines redward. The integrated spectral
energy distribution (SED) of the cluster has been extended into the ultraviolet
region by archival XMM-Newton and new Swift observations, and compared with
theoretical models. The outer parts of the cluster have been resolved by the
Hubble Space Telescope, allowing the construction of a color-magnitude diagram.
The fitting of the cluster SED with theoretical isochrones results in cluster
ages between 10--40 Myr, depending on metallicity and the model family. The
isochrone fitting indicates that the resolved part of the cluster has a bimodal
age distribution: a younger population at ~10--16 Myr, and an older one at
~32--100 Myr which is similar to the age distribution of the nearby field
stars. These stars may have been captured from the field during the cluster
formation. The young age of Sandage-96 suggest 12 < M_prog < 20 M_\odot as the
most probable mass range for the progenitor of SN 2004dj. This is consistent
with, but perhaps slightly higher than, most of the other Type II-plateau SN
progenitor masses determined so far.Comment: accepted in Ap
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