793 research outputs found
Ellipsoidal primary of the RS CVn binary zeta And: Investigation using high-resolution spectroscopy and optical interferometry
We have obtained high-resolution spectroscopy, optical interferometry, and
long-term broad band photometry of the ellipsoidal primary of the RS CVn-type
binary system zeta And. Based on the optical interferometry the apparent limb
darkened diameter of zeta And is 2.55 +/- 0.09 mas using a uniform disk fit.
The Hipparcos distance and the limb-darkened diameter obtained with a uniform
disk fit give stellar radius of 15.9 +/- 0.8 Rsolar, and combined with
bolometric luminosity, it implies an effective temperature of 4665 +/- 140 K.
The temperature maps obtained from high resolution spectra using Doppler
imaging show a strong belt of equatorial spots and hints of a cool polar cap.
The equatorial spots show a concentration around the phase 0.75. This spot
configuration is reminiscent of the one seen in the earlier published
temperature maps of zeta And. Investigation of the Halpha line reveals both
prominences and cool clouds in the chromosphere. Long-term photometry spanning
12 years shows hints of a spot activity cycle, which is also implied by the
Doppler images, but the cycle length cannot be reliably determined from the
current data.Comment: 9 pages, 9 figures, accepted for A&
Characterization of the Benchmark Binary NLTT 33370
We report the confirmation of the binary nature of the nearby, very low-mass
system NLTT 33370 with adaptive optics imaging and present resolved
near-infrared photometry and integrated light optical and near-infrared
spectroscopy to characterize the system. VLT-NaCo and LBTI-LMIRCam images show
significant orbital motion between 2013 February and 2013 April. Optical
spectra reveal weak, gravity sensitive alkali lines and strong lithium 6708
Angstrom absorption that indicate the system is younger than field age.
VLT-SINFONI near-IR spectra also show weak, gravity sensitive features and
spectral morphology that is consistent with other young, very low-mass dwarfs.
We combine the constraints from all age diagnostics to estimate a system age of
~30-200 Myr. The 1.2-4.7 micron spectral energy distribution of the components
point toward T_eff=3200 +/- 500 K and T_eff=3100 +/- 500 K for NLTT 33370 A and
B, respectively. The observed spectra, derived temperatures, and estimated age
combine to constrain the component spectral types to the range M6-M8.
Evolutionary models predict masses of 113 +/- 8 M_Jup and 106 +/- 7 M_Jup from
the estimated luminosities of the components. KPNO-Phoenix spectra allow us to
estimate the systemic radial velocity of the binary. The Galactic kinematics of
NLTT 33370AB are broadly consistent with other young stars in the Solar
neighborhood. However, definitive membership in a young, kinematic group cannot
be assigned at this time and further follow-up observations are necessary to
fully constrain the system's kinematics. The proximity, age, and late-spectral
type of this binary make it very novel and an ideal target for rapid, complete
orbit determination. The system is one of only a few model calibration
benchmarks at young ages and very low-masses.Comment: 25 pages, 3 tables, 13 figures, accepted for publication in The
Astrophysical Journa
Phonon anomalies due to strong electronic correlations in layered organic metals
We show how the coupling between the phonons and electrons in a strongly
correlated metal can result in phonon frequencies which have a non-monotonic
temperature dependence. Dynamical mean-field theory is used to study the
Hubbard-Holstein model that describes the \kappa-(BEDT-TTF)_2 X family of
superconducting molecular crystals. The crossover with increasing temperature
from a Fermi liquid to a bad metal produces phonon anomalies that are
consistent with recent Raman scattering and acoustic experiments.Comment: 6 pages, 3 eps figure
In-plane polarized collective modes in detwinned YBaCuO observed by spectral ellipsometry
The in-plane dielectric response of detwinned YBaCuO has
been studied by far-infared ellipsometry. A surprisingly lare number of
in-plane polarized modes are observed. Some of them correspond to pure phonon
modes. Others posses a large electronic contribution which strongly increases
in the superconducting state. The free carrier response and the collective
modes exhibit a pronounced a-b anisotropy. We discuss our results in terms of a
CDW state in the 1-d CuO chains and induced charge density fluctuations within
the 2-d CuO planes
Multi-Orbital Hubbard Model in Infinite Dimensions: Quantum Monte Carlo Calculation
Using Quantum Monte Carlo we compute thermodynamics and spectra for the
orbitally degenerate Hubbard model in infinite spatial dimensions. With
increasing orbital degeneracy we find in the one-particle spectra: broader
Hubbard bands (consistent with increased kinetic energy), a narrowing Mott gap,
and increasing quasi-particle spectral weight. In opposition, Hund's rule
exchange coupling decreases the critical on-site Coulomb energy for the Mott
transition. The metallic regime resistivity for two-fold degeneracy is
quadratic-in-temperature at low temperatures.Comment: 4 pages, 4 figures, to be published in PR
Correlator Bank Detection of GW chirps. False-Alarm Probability, Template Density and Thresholds: Behind and Beyond the Minimal-Match Issue
The general problem of computing the false-alarm rate vs. detection-threshold
relationship for a bank of correlators is addressed, in the context of
maximum-likelihood detection of gravitational waves, with specific reference to
chirps from coalescing binary systems. Accurate (lower-bound) approximants for
the cumulative distribution of the whole-bank supremum are deduced from a class
of Bonferroni-type inequalities. The asymptotic properties of the cumulative
distribution are obtained, in the limit where the number of correlators goes to
infinity. The validity of numerical simulations made on small-size banks is
extended to banks of any size, via a gaussian-correlation inequality. The
result is used to estimate the optimum template density, yielding the best
tradeoff between computational cost and detection efficiency, in terms of
undetected potentially observable sources at a prescribed false-alarm level,
for the simplest case of Newtonian chirps.Comment: submitted to Phys. Rev.
The low-energy scale of the periodic Anderson model
Wilson's Numerical Renormalization Group method is used to study the
paramagnetic ground state of the periodic Anderson model within the dynamical
mean-field approach. For the particle-hole symmetric model, which is a Kondo
insulator, we find that the lattice Kondo scale T_0 is strongly enhanced over
the impurity scale T_K; T_0/T_K ~ exp(1/3I), where I is the Schrieffer-Wolff
exchange coupling. In the metallic regime, where the conduction band filling is
reduced from one, we find characteristic signatures of Nozi\`eres exhaustion
scenario, including a strongly reduced lattice Kondo scale, a significant
suppression of the states available to screen the f-electron moment, and a
Kondo resonance with a strongly enhanced height. However, in contrast to the
quantitative predictions of Nozi\`eres, we find that the T_0 ~ T_K with a
coefficient which depends strongly on conduction band filling.Comment: 11 pages, 9 figures, submitted to Phys. Rev.
Stability of a metallic state in the two-orbital Hubbard model
Electron correlations in the two-orbital Hubbard model at half-filling are
investigated by combining dynamical mean field theory with the exact
diagonalization method. We systematically study how the interplay of the intra-
and inter-band Coulomb interactions, together with the Hund coupling, affects
the metal-insulator transition. It is found that if the intra- and inter-band
Coulomb interactions are nearly equal, the Fermi-liquid state is stabilized due
to orbital fluctuations up to fairly large interactions, while the system is
immediately driven to the Mott insulating phase away from this condition. The
effects of the isotropic and anisotropic Hund coupling are also addressed.Comment: 7 pages, 9 figure
Survival of the mm-cm size grain population observed in protoplanetary disks
Millimeter interferometry provides evidence for the presence of mm to cm size
"pebbles" in the outer parts of disks around pre-main-sequence stars. The
observations suggest that large grains are produced relatively early in disk
evolution (< 1 Myr) and remain at large radii for longer periods of time (5 to
10 Myr). Simple theoretical estimates of the radial drift time of solid
particles, however, imply that they would drift inward over a time scale of
less than 0.1 Myr. In this paper, we address this conflict between theory and
observation, using more detailed theoretical models, including the effects of
sedimentation, collective drag forces and turbulent viscosity. We find that,
although these effects slow down the radial drift of the dust particles, this
reduction is not sufficient to explain the observationally determined long
survival time of mm/cm-sized grains in protoplanetary disks. However, if for
some reason the gas to dust ratio in the disk is reduced by at least a factor
of 20 from the canonical value of 100 (for instance through photoevaporation of
the gas), then the radial drift time scales become sufficiently large to be in
agreement with observations.Comment: Accepted for publication in Astronomy and Astrophysic
CDW, Superconductivity and Anomalous Metallic Behavior in 2D Transition Metal Dichalcogenides
We propose a theory for quasi-two-dimensional transition metal
dichalcogenides that provides a unified microscopic picture of the charge
density wave (CDW) and superconducting phases. We show, based on the
electron-phonon coupling and Fermi surface topology, that a CDW order parameter
with six-fold symmetry and nodes (f-wave) gives a consistent description of the
available experimental data. The elementary excitations in the CDW phase are
Dirac electrons. The superconducting state has its origin on the attractive
interaction mediated by phonons. The theory predicts strong deviations from
Fermi liquid theory in the CDW phase.Comment: 4 pages, 3 figure
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