53,533 research outputs found
Carbon burning in intermediate mass primordial stars
The evolution of a zero metallicity 9 M_s star is computed, analyzed and
compared with that of a solar metallicity star of identical ZAMS mass. Our
computations range from the main sequence until the formation of a massive
oxygen-neon white dwarf. Special attention has been payed to carbon burning in
conditions of partial degeneracy as well as to the subsequent thermally pulsing
Super-AGB phase. The latter develops in a fashion very similar to that of a
solar metallicity 9 M_s star, as a consequence of the significant enrichment in
metals of the stellar envelope that ensues due to the so-called third dredge-up
episode. The abundances in mass of the main isotopes in the final ONe core
resulting from the evolution are X(^{16}O) approx 0.59, X(^{20}Ne) approx 0.28
and X(^{24}Mg) approx 0.05. This core is surrounded by a 0.05 M_s buffer mainly
composed of carbon and oxygen, and on top of it a He envelope of mass 10^{-4}
M_sComment: 11 pages, 11 figures, accepted for publication in A&
Validation Through Simulations of a Cn2 Profiler for the ESO/VLT Adaptive Optics Facility
The Adaptive Optics Facility (AOF) project envisages transforming one of the
VLT units into an adaptive telescope and providing its ESO (European Southern
Observatory) second generation instruments with turbulence corrected
wavefronts. For MUSE and HAWK-I this correction will be achieved through the
GALACSI and GRAAL AO modules working in conjunction with a 1170 actuators
Deformable Secondary Mirror (DSM) and the new Laser Guide Star Facility
(4LGSF). Multiple wavefront sensors will enable GLAO and LTAO capabilities,
whose performance can greatly benefit from a knowledge about the stratification
of the turbulence in the atmosphere. This work, totally based on end-to-end
simulations, describes the validation tests conducted on a Cn2 profiler adapted
for the AOF specifications. Because an absolute profile calibration is strongly
dependent on a reliable knowledge of turbulence parameters r0 and L0, the tests
presented here refer only to normalized output profiles. Uncertainties in the
input parameters inherent to the code are tested as well as the profiler
response to different turbulence distributions. It adopts a correction for the
unseen turbulence, critical for the GRAAL mode, and highlights the effects of
masking out parts of the corrected wavefront on the results. Simulations of
data with typical turbulence profiles from Paranal were input to the profiler,
showing that it is possible to identify reliably the input features for all the
AOF modes.Comment: 15 pages, 12 figures, accepted for publication in the MNRAS Accepted
2015 January 22. Received 2015 January 21; in original form 2014 December
Exploring the structure of the quenched QCD vacuum with overlap fermions
Overlap fermions have an exact chiral symmetry on the lattice and are thus an
appropriate tool for investigating the chiral and topological structure of the
QCD vacuum. We study various chiral and topological aspects of quenched gauge
field configurations. This includes the localization and chiral properties of
the eigenmodes, the local structure of the ultraviolet filtered field strength
tensor, as well as the structure of topological charge fluctuations. We
conclude that the vacuum has a multifractal structure.Comment: 68 pages, 31 figures, file size: 1.7 MB (PDF
What Produced the Ultraluminous Supernova Remnant in NGC 6946?
The ultraluminous supernova remnant (SNR) in NGC 6946 is the brightest known
SNR in X-rays, ~1000 times brighter than Cas A. To probe the nature of this
remnant and its progenitor, we have obtained high-dispersion optical echelle
spectra. The echelle spectra detect H-alpha, [N II], and [O III] lines, and
resolve these lines into a narrow (FWHM ~20--40 km/s) component from un-shocked
material and a broad (FWHM ~250 km/s) component from shocked material. Both
narrow and broad components have unusually high [N II]/H-alpha ratios, ~1.
Using the echelle observation, archival HST images, and archival ROSAT X-ray
observations, we conclude that the SNR was produced by a normal supernova,
whose progenitor was a massive star, either a WN star or a luminous blue
variable. The high luminosity of the remnant is caused by the supernova ejecta
expanding into a dense, nitrogen-rich circumstellar nebula created by the
progenitor.Comment: 20 pages, 5 figures. To be published in The Astronomical Journal,
March 200
Matrix Product State and mean field solutions for one-dimensional systems can be found efficiently
We consider the problem of approximating ground states of one-dimensional
quantum systems within the two most common variational ansatzes, namely the
mean field ansatz and Matrix Product States. We show that both for mean field
and for Matrix Product States of fixed bond dimension, the optimal solutions
can be found in a way which is provably efficient (i.e., scales polynomially).
This implies that the corresponding variational methods can be in principle
recast in a way which scales provably polynomially. Moreover, our findings
imply that ground states of one-dimensional commuting Hamiltonians can be found
efficiently.Comment: 5 pages; v2: accepted version, Journal-ref adde
Flat deformation of a spacetime admitting two Killing fields
It is shown that given an analytic Lorentzian metric on a 4-manifold, ,
which admits two Killing vector fields, then it exists a local deformation law
, where is a 2-dimensional projector, such that is
flat and admits the same Killing vectors. We also characterize the particular
case when the projector coincides with the quotient metric. We apply some
of our results to general stationary axisymmetric spacetime
Efficient out-coupling and beaming of Tamm optical states via surface plasmon polariton excitation
We present evidence of optical Tamm states to surface plasmon polariton (SPP) coupling. We experimentally demonstrate that for a Bragg stack with a thin metal layer on the surface, hybrid Tamm-SPP modes may be excited when a grating on the air-metal interface is introduced. Out-coupling via the grating to free space propagation is shown to enhance the transmission as well as the directionality and polarization selection for the transmitted beam. We suggest that this system will be useful on those devices, where a metallic electrical contact as well as beaming and polarization control is needed
Stanilov-Tsankov-Videv Theory
We survey some recent results concerning Stanilov-Tsankov-Videv theory,
conformal Osserman geometry, and Walker geometry which relate algebraic
properties of the curvature operator to the underlying geometry of the
manifold.Comment: This is a contribution to the Proceedings of the 2007 Midwest
Geometry Conference in honor of Thomas P. Branson, published in SIGMA
(Symmetry, Integrability and Geometry: Methods and Applications) at
http://www.emis.de/journals/SIGMA
Biaxial Strain in the Hexagonal Plane of MnAs Thin Films: The Key to Stabilize Ferromagnetism to Higher Temperature
The alpha-beta magneto-structural phase transition in MnAs/GaAs(111)
epilayers is investigated by elastic neutron scattering. The in-plane parameter
of MnAs remains almost constant with temperature from 100 K to 420 K, following
the thermal evolution of the GaAs substrate. This induces a temperature
dependent biaxial strain that is responsible for an alpha-beta phase
coexistence and, more important, for the stabilization of the ferromagnetic
alpha-phase at higher temperature than in bulk. We explain the premature
appearance of the beta-phase at 275 K and the persistence of the ferromagnetic
alpha-phase up to 350 K with thermodynamical arguments based on the MnAs phase
diagram. It results that the biaxial strain in the hexagonal plane is the key
parameter to extend the ferromagnetic phase well over room temperature.Comment: 4 pages, 3 figures, accepted for publication in Physical Review
Letter
Resolving parameter degeneracies in long-baseline experiments by atmospheric neutrino data
In this work we show that the physics reach of a long-baseline (LBL) neutrino
oscillation experiment based on a superbeam and a megaton water Cherenkov
detector can be significantly increased if the LBL data are combined with data
from atmospheric neutrinos (ATM) provided by the same detector. ATM data are
sensitive to the octant of and to the type of the neutrino mass
hierarchy, mainly through three-flavor effects in e-like events. This allows to
resolve the so-called - and sign()-parameter
degeneracies in LBL data. As a consequence it becomes possible to distinguish
the normal from the inverted neutrino mass ordering at CL from a
combined LBL+ATM analysis if . The potential
to identify the true values of and the CP-phase
is significantly increased through the lifting of the
degeneracies. These claims are supported by a detailed simulation of the T2K
(phase II) LBL experiment combined with a full three-flavor analysis of ATM
data in the HyperKamiokande detector.Comment: 25 pages, 10 figure
- …