6,703 research outputs found
On the implementation of faults in finite-element glacial isostatic adjustment models
postprin
6Li detection in metal-poor stars: can 3D model atmospheres solve the second lithium problem?
The presence of 6Li in the atmospheres of metal-poor halo stars is usually
inferred from the detection of a subtle extra depression in the red wing of the
7Li doublet line at 670.8 nm. However, the intrinsic line asymmetry caused by
convective flows in the photospheres of cool stars is almost indistinguishable
from the asymmetry produced by a weak 6Li blend on a (presumed) symmetric 7Li
profile. Previous determinations of the 6Li/ 7Li isotopic ratio based on 1D
model atmospheres, ignoring the convection-induced line asymmetry, must
therefore be considered as upper limits. By comparing synthetic 1D LTE and 3D
non-LTE line profiles of the Li 670.8 nm feature, we quantify the differential
effect of the convective line asymmetry on the derived 6Li abundance as a
function of effective temperature, gravity, and metallicity. As expected, we
find that the asymmetry effect systematically reduces the resulting 6Li/7Li
ratios. Depending on the stellar parameters, the 3D-1D offset in 6Li/7Li ranges
between -0.005 and -0.020. When this purely theoretical correction is taken
into account for the Asplund 2006 sample of stars, the number of significant
6Li detections decreases from 9 to 5 (2 sigma criterion), or from 5 to 2 (3
sigma criterion).
We also present preliminary results of a re-analysis of high-resolution, high
S/N spectra of individual metal-poor turn-off stars, to see whether the "second
Lithium problem" actually disappears when accounting properly for convection
and non-LTE line formation in 3D stellar atmospheres. Out of 8 stars, HD84937
seems to be the only significant (2 sigma) detection of 6Li. In view of our
results, the existence of a 6Li plateau appears questionable.Comment: To appear in the proceedings of 'Lithium in the Cosmos', Paris, Feb.
27-29, 2012, Memorie della Societa' Astronomica Italiana Supplement
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Classical GR as a topological theory with linear constraints
We investigate a formulation of continuum 4d gravity in terms of a
constrained topological (BF) theory, in the spirit of the Plebanski
formulation, but involving only linear constraints, of the type used recently
in the spin foam approach to quantum gravity. We identify both the continuum
version of the linear simplicity constraints used in the quantum discrete
context and a linear version of the quadratic volume constraints that are
necessary to complete the reduction from the topological theory to gravity. We
illustrate and discuss also the discrete counterpart of the same continuum
linear constraints. Moreover, we show under which additional conditions the
discrete volume constraints follow from the simplicity constraints, thus
playing the role of secondary constraints. Our analysis clarifies how the
discrete constructions of spin foam models are related to a continuum theory
with an action principle that is equivalent to general relativity.Comment: 4 pages, based on a talk given at the Spanish Relativity Meeting 2010
(ERE2010, Granada, Spain
Cavity QED with separate photon storage and qubit readout modes
We present the realization of a cavity quantum electrodynamics setup in which
photons of strongly different lifetimes are engineered in different harmonic
modes of the same cavity. We achieve this in a superconducting transmission
line resonator with superconducting qubits coupled to the different modes. One
cavity mode is strongly coupled to a detection line for qubit state readout,
while a second long lifetime mode is used for photon storage and coherent
quantum operations. We demonstrate sideband based measurement of photon
coherence, generation of n photon Fock states and the scaling of the sideband
Rabi frequency with the square root of n using a scheme that may be extended to
realize sideband based two-qubit logic gates.Comment: 4 pages, 5 figures, version with high resolution figures available at
http://qudev.ethz.ch/content/science/PubsPapers.htm
Program transformations using temporal logic side conditions
This paper describes an approach to program optimisation based on transformations, where temporal logic is used to specify side conditions, and strategies are created which expand the repertoire of transformations and provide a suitable level of abstraction. We demonstrate the power of this approach by developing a set of optimisations using our transformation language and showing how the transformations can be converted into a form which makes it easier to apply them, while maintaining trust in the resulting optimising steps. The approach is illustrated through a transformational case study where we apply several optimisations to a small program
Using Sideband Transitions for Two-Qubit Operations in Superconducting Circuits
We demonstrate time resolved driving of two-photon blue sideband transitions
between superconducting qubits and a transmission line resonator. Using the
sidebands, we implement a pulse sequence that first entangles one qubit with
the resonator, and subsequently distributes the entanglement between two
qubits. We show generation of 75% fidelity Bell states by this method. The full
density matrix of the two qubit system is extracted using joint measurement and
quantum state tomography, and shows close agreement with numerical simulation.
The scheme is potentially extendable to a scalable universal gate for quantum
computation.Comment: 4 pages, 5 figures, version with high resolution figures available at
http://qudev.ethz.ch/content/science/PubsPapers.htm
Chemical composition of a sample of bright solar-metallicity stars
We present a detailed analysis of seven young stars observed with the
spectrograph SOPHIE at the Observatoire de Haute-Provence for which the
chemical composition was incomplete or absent in the literature. For five
stars, we derived the stellar parameters and chemical compositions using our
automatic pipeline optimized for F, G, and K stars, while for the other two
stars with high rotational velocity, we derived the stellar parameters by using
other information (parallax), and performed a line-by-line analysis.
Chromospheric emission-line fluxes from CaII are obtained for all targets. The
stellar parameters we derive are generally in good agreement with what is
available in the literature. We provide a chemical analysis of two of the stars
for the first time. The star HIP 80124 shows a strong Li feature at 670.8 nm
implying a high lithium abundance. Its chemical pattern is not consistent with
it being a solar sibling, as has been suggested.Comment: To be published on A
The Solar Photospheric Nitrogen Abundance: Determination with 3D and 1D Model Atmospheres
We present a new determination of the solar nitrogen abundance making use of
3D hydrodynamical modelling of the solar photosphere, which is more physically
motivated than traditional static 1D models. We selected suitable atomic
spectral lines, relying on equivalent width measurements already existing in
the literature. For atmospheric modelling we used the co 5 bold 3D radiation
hydrodynamics code. We investigated the influence of both deviations from local
thermodynamic equilibrium (non-LTE effects) and photospheric inhomogeneities
(granulation effects) on the resulting abundance. We also compared several
atlases of solar flux and centre-disc intensity presently available. As a
result of our analysis, the photospheric solar nitrogen abundance is A(N) =
7.86 +/- 0.12.Comment: 6 pages, 4 figure
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