584 research outputs found
Low temperature properties of the infinite-dimensional attractive Hubbard model
We investigate the attractive Hubbard model in infinite spatial dimensions by
combining dynamical mean-field theory with a strong-coupling continuous-time
quantum Monte Carlo method. By calculating the superfluid order parameter and
the density of states, we discuss the stability of the superfluid state. In the
intermediate coupling region above the critical temperature, the density of
states exhibits a heavy fermion behavior with a quasi-particle peak in the
dense system, while a dip structure appears in the dilute system. The formation
of the superfluid gap is also addressed.Comment: 8 pages, 9 figure
Superbubbles
Individual massive stars with M sub bol -6 have huge stellar winds that create interstellar bubbles. Stars with masses greater than 8 solar mass are considered supernova progenitors. These massive stars are numerous in OB associations where few supernova remnants are detected. Model calculations describing the evolution of an association show: that large, hot cavities are formed by pushing the ambient gas into neutral shells; that the shell radii change with galactocentric radius; that only thirty percent of the interstellar medium is in the form of supercavities; and that a consequence is that only a small fraction of supernovae form supernova remnants
Revisited abundance diagnostics in quasars: Fe II/Mg II ratios
Both the Fe II UV emission in the 2000- 3000 A region [Fe II (UV)] and
resonance emission line complex of Mg II at 2800 A are prominent features in
quasar spectra. The observed Fe II UV/ Mg II emission ratios have been proposed
as means to measure the buildup of the Fe abundance relative to that of the
alpha-elements C, N, O, Ne and Mg as a function of redshift. The current
observed ratios show large scatter and no obvious dependence on redshift. Thus,
it remains unresolved whether a dependence on redshift exists and whether the
observed Fe II UV/ Mg II ratios represent a real nucleosynthesis diagnostic. We
have used our new 830-level model atom for Fe+ in photoionization calculations,
reproducing the physical conditions in the broad line regions of quasars. This
modeling reveals that interpretations of high values of Fe II UV/ Mg II are
sensitive not only to Fe and Mg abundance, but also to other factors such as
microturbulence, density, and properties of the radiation field. We find that
the Fe II UV/ Mg II ratio combined with Fe II (UV)/ Fe II (Optical) emission
ratio, where Fe II (Optical) denotes Fe II emission in 4000 - 6000 A can be
used as a reliable nucleosynthesis diagnostic for the Fe/Mg abundance ratios
for the physical conditions relevant to the broad-line regions (BLRs) of
quasars. This has extreme importance for quasar observations with the Hubble
Space Telescope and also with the future James Webb Space Telescope.Comment: kverner.gzip, 9 pages, f1-5.eps; aastex.cls; aastexug.sty, ApJL in
pres
Reconstruction of Calmodulin Single-Molecule FRET States, Dye-Interactions, and CaMKII Peptide Binding by MultiNest and Classic Maximum Entropy
We analyze single molecule FRET burst measurements using Bayesian nested sampling. The MultiNest algorithm produces accurate FRET efficiency distributions from single-molecule data. FRET efficiency distributions recovered by MultiNest and classic maximum entropy are compared for simulated data and for calmodulin labeled at residues 44 and 117. MultiNest compares
favorably with maximum entropy analysis for simulated data, judged by the Bayesian evidence. FRET efficiency distributions recovered for calmodulin labeled with two different FRET dye pairs depended on the dye pair and changed upon Ca2+ binding. We also looked at the FRET efficiency distributions of calmodulin bound to the calcium/calmodulin dependent protein kinase II
(CaMKII) binding domain. For both dye pairs, the FRET efficiency distribution collapsed to a single peak in the case of calmodulin bound to the CaMKII peptide. These measurements strongly suggest that consideration of dye-protein interactions is crucial in forming an accurate picture of protein conformations from FRET data
Classic Maximum Entropy Recovery of the Average Joint Distribution of Apparent FRET Efficiency and Fluorescence Photons for Single-molecule Burst Measurements
We describe a method for analysis of single-molecule Förster resonance energy transfer (FRET) burst measurements using classic maximum entropy. Classic maximum entropy determines the Bayesian inference for the joint probability describing the total fluorescence photons and the apparent FRET efficiency. The method was tested with simulated data and then with DNA labeled with fluorescent dyes. The most probable joint distribution can be marginalized to obtain both the overall distribution of fluorescence photons and the apparent FRET efficiency distribution. This method proves to be ideal for determining the distance distribution of FRET-labeled biomolecules, and it successfully predicts the shape of the recovered distributions
Analytic Continuation of Quantum Monte Carlo Data by Stochastic Analytical Inference
We present an algorithm for the analytic continuation of imaginary-time
quantum Monte Carlo data which is strictly based on principles of Bayesian
statistical inference. Within this framework we are able to obtain an explicit
expression for the calculation of a weighted average over possible energy
spectra, which can be evaluated by standard Monte Carlo simulations, yielding
as by-product also the distribution function as function of the regularization
parameter. Our algorithm thus avoids the usual ad-hoc assumptions introduced in
similar algortihms to fix the regularization parameter. We apply the algorithm
to imaginary-time quantum Monte Carlo data and compare the resulting energy
spectra with those from a standard maximum entropy calculation
Thermal X-ray emission from shocked ejecta in Type Ia Supernova Remnants. Prospects for explosion mechanism identification
The explosion mechanism behind Type Ia supernovae is a matter of continuing
debate. The diverse attempts to identify or at least constrain the physical
processes involved in the explosion have been only partially successful so far.
In this paper we propose to use the thermal X-ray emission from young supernova
remnants originated in Type Ia events to extract relevant information
concerning the explosions themselves. We have produced a grid of thermonuclear
supernova models representative of the paradigms currently under debate: pure
deflagrations, delayed detonations, pulsating delayed detonations and
sub-Chandrasekhar explosions, using their density and chemical composition
profiles to simulate the interaction with the surrounding ambient medium and
the ensuing plasma heating, non-equilibrium ionization and thermal X-ray
emission of the ejecta. Key observational parameters such as electron
temperatures, emission measures and ionization time scales are presented and
discussed. We find that not only is it possible to identify the explosion
mechanism from the spectra of young Type Ia Supernova Remnants, it is in fact
necessary to take the detailed ejecta structure into account if such spectra
are to be modeled in a self-consistent way. Neither element line flux ratios
nor element emission measures are good estimates of the true ratios of ejected
masses, with differences of as much as two or three orders of magnitude for a
given model. Comparison with observations of the Tycho SNR suggests a delayed
detonation as the most probable explosion mechanism. Line strengths, line
ratios, and the centroid of the Fe Kalpha line are reasonably well reproduced
by a model of this kind.Comment: 11 pages, 8 figures (5 of them color), accepted for publication by
the Ap
The Radio Recovery of SN 1970G: The Continuing Radio Evolution of SN 1970G
Using the Very Large Array, we have detected radio emission from the site of
SN 1970G in the Sc galaxy M101. These observations are 31 years after the
supernova event, making SN 1970G the longest monitored radio supernova. With
flux densities of 0.12 +/- 0.020 mJy at 6 cm and 0.16 +/- 0.015 mJy at 20 cm,
the spectral index of -0.24 +/- 0.20 appears to have flattened somewhat when
compared with the previously reported value of -0.56 +/- 0.11, taken in 1990.
The radio emission at 20 cm has decayed since the 1990 observations with a
power-law index of beta_20cm = -0.28 +/- 0.13. We discuss the radio properties
of this source and compare them to those of other Type II radio supernovae.Comment: 11 pages, 1 table and 2 figures; To appear in Astrophysical Journal
Letter
Diagrammatic Monte Carlo for Correlated Fermions
We show that Monte Carlo sampling of the Feynman diagrammatic series (DiagMC)
can be used for tackling hard fermionic quantum many-body problems in the
thermodynamic limit by presenting accurate results for the repulsive Hubbard
model in the correlated Fermi liquid regime. Sampling Feynman's diagrammatic
series for the single-particle self-energy we can study moderate values of the
on-site repulsion () and temperatures down to . We
compare our results with high temperature series expansion and with single-site
and cluster dynamical mean-field theory.Comment: 4 pages, 5 figures, stylistic change
The Fading Radio Emission from SN 1961V: Evidence for a Type II Peculiar Supernova?
Using the Very Large Array (VLA), we have detected radio emission from the
site of SN 1961V in the Sc galaxy NGC 1058. With a peak flux density of 0.063
+/- 0.008 mJy/beam at 6 cm and 0.147 +/- 0.026 mJy/beam at 18 cm, the source is
non-thermal, with a spectral index of -0.79 +/- 0.23. Within errors, this
spectral index is the same value reported for previous VLA observations taken
in 1984 and 1986. The radio emission at both wavelengths has decayed since the
mid 1980's observations with power-law indices of beta(20cm) = -0.69 +/- 0.23
and beta(6cm) = -1.75 +/- 0.16. We discuss the radio properties of this source
and compare them with those of Type II radio supernovae and luminous blue
variables.Comment: 19 pages, 3 figures; To appear in the Astronomical Journa
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