2,146 research outputs found
Hybrid propulsion systems for motor vehicles with predominantly intermittent modes of operation
A small delivery vehicle was equipped with a flywheel-hybrid drive and compared in test stand and driving tests with a conventional drive vehicle. It turned out that with the hybrid drive, energy can be saved and exhaust emissions can be reduced
The Influence of Dust Formation Modelling on Na I and K I Line Profiles in Substellar Atmospheres
We aim to understand the correlation between cloud formation and alkali line
formation in substellar atmospheres.We perform line profile calculations for Na
I and K I based on the coupling of our kinetic model for the formation and
composition of dust grains with 1D radiative transfer calculations in
atmosphere models for brown dwarfs and giant gas planets. The Na I and K I line
profiles sensibly depend on the way clouds are treated in substellar atmosphere
simulations. The kinetic dust formation model results in the highest
pseudo-continuum compared to the limiting cases.Comment: 5 pages, Accepted for publication in MNRA
A Special Case Of A Conjecture By Widom With Implications To Fermionic Entanglement Entropy
We prove a special case of a conjecture in asymptotic analysis by Harold
Widom. More precisely, we establish the leading and next-to-leading term of a
semi-classical expansion of the trace of the square of certain integral
operators on the Hilbert space . As already observed by Gioev and
Klich, this implies that the bi-partite entanglement entropy of the free Fermi
gas in its ground state grows at least as fast as the surface area of the
spatially bounded part times a logarithmic enhancement.Comment: 20 pages, 3 figures, improvement of the presentation, some references
added or updated, proof of Theorem 12 (formerly Lemma 11) adde
Groups of Fibonacci type revisited
This article concerns a class of groups of Fibonacci type introduced by Johnson and Mawdesley that includes Conway?s Fibonacci groups, the Sieradski groups, and the Gilbert-Howie groups. This class of groups provides an interesting focus for developing the theory of cyclically presented groups and, following questions by Bardakov and Vesnin and by Cavicchioli, Hegenbarth, and Repov?s, they have enjoyed renewed interest in recent years. We survey results concerning their algebraic properties, such as isomorphisms within the class, the classification of the finite groups, small cancellation properties, abelianizations, asphericity, connections with Labelled Oriented Graph groups, and the semigroups of Fibonacci type. Further, we present a new method of proving the classification of the finite groups that deals with all but three groups
Entanglement and particle correlations of Fermi gases in harmonic traps
We investigate quantum correlations in the ground state of noninteracting
Fermi gases of N particles trapped by an external space-dependent harmonic
potential, in any dimension. For this purpose, we compute one-particle
correlations, particle fluctuations and bipartite entanglement entropies of
extended space regions, and study their large-N scaling behaviors. The
half-space von Neumann entanglement entropy is computed for any dimension,
obtaining S_HS = c_l N^(d-1)/d ln N, analogously to homogenous systems, with
c_l=1/6, 1/(6\sqrt{2}), 1/(6\sqrt{6}) in one, two and three dimensions
respectively. We show that the asymptotic large-N relation S_A\approx \pi^2
V_A/3, between the von Neumann entanglement entropy S_A and particle variance
V_A of an extended space region A, holds for any subsystem A and in any
dimension, analogously to homogeneous noninteracting Fermi gases.Comment: 15 pages, 22 fig
Dust in brown dwarfs and extra-solar planets IV. Assessing TiO2 and SiO nucleation for cloud formation modeling
Clouds form in atmospheres of brown dwarfs and planets. The cloud particle
formation processes are similar to the dust formation process studied in
circumstellar shells of AGB stars and in Supernovae. Cloud formation modelling
in substellar objects requires gravitational settling and element replenishment
in addition to element depletion. All processes depend on the local conditions,
and a simultaneous treatment is required. We apply new material data in order
to assess our cloud formation model results regarding the treatment of the
formation of condensation seeds. We re-address the question of the primary
nucleation species in view of new (TiO2)_N-cluster data and new SiO vapour
pressure data. We apply the density functional theory using the computational
chemistry package Gaussian 09 to derive updated thermodynamical data for
(TiO2)_N-clusters as input for our TiO2 seed formation model. We test different
nucleation treatments and their effect on the overall cloud structure by
solving a system of dust moment equations and element conservation or a
pre-scribed Drift-Phoenix atmosphere structure. Updated Gibbs free energies for
the (TiO2)_N-clusters are presented, and a slightly temperature dependent
surface tension for T=500 ... 2000K with an average value of sigma_infty =
480.6 erg 1/cm2. The TiO2-seed formation rate changes only slightly with the
updated cluster data. A considerably larger effect on the rate of seed
formation, and hence on grain size and dust number density, results from a
switch to SiO-nucleation. Despite the higher abundance of SiO over TiO2 in the
gas phase, TiO2 remains considerably more efficient in forming condensation
seeds by homogeneous nucleation followed by heterogeneous grain growth. The
paper discussed the effect on the cloud structure in more detail.Comment: accepted for publication in A&A (abstract abridged
Complexity of links in 3-manifolds
We introduce a natural-valued complexity c(X) for pairs X=(M,L), where M is a
closed orientable 3-manifold and L is a link contained in M. The definition
employs simple spines, but for well-behaved X's we show that c(X) equals the
minimal number of tetrahedra in a triangulation of M containing L in its
1-skeleton. Slightly adapting Matveev's recent theory of roots for graphs, we
carefully analyze the behaviour of c under connected sum away from and along
the link. We show in particular that c is almost always additive, describing in
detail the circumstances under which it is not. To do so we introduce a certain
(0,2)-root for a pair X, we show that it is well-defined, and we prove that X
has the same complexity as its (0,2)-root. We then consider, for links in the
3-sphere, the relations of c with the crossing number and with the hyperbolic
volume of the exterior, establishing various upper and lower bounds. We also
specialize our analysis to certain infinite families of links, providing rather
accurate asymptotic estimates.Comment: 24 pages, 6 figure
Optical linear polarization in ultra cool dwarfs: A tool to probe dust in the ultra cool dwarf atmospheres
Aims.Recent studies have detected linear polarization in L dwarfs in the
optical I band. Theoretical models have been developed to explain this
polarization. These models predict higher polarization at shorter wavelengths.
We discuss the polarization in the R and I band of 4 ultra cool dwarfs.
Methods.We report linear polarization measurements of 4 ultra cool dwarfs in
the R and I bands using the Intermediate dispersion Spectrograph and Imaging
System (ISIS) mounted on the 4.2m William Herschel Telescope (WHT). Results.As
predicted by theoretical models, we find a higher degree of polarization in the
R band when compared to polarization in the I band for 3/4 of these ultra cool
dwarfs. This suggests that dust scattering asymmetry is caused by oblateness
>.We also show how these measurements fit the theoretical models. A case for
variability of linear polarization is found, which suggests the presence of
randomly distributed dust clouds. We also discuss one case for the presence of
a cold debris disk.Comment: 7 pages, 2 figure
Dynamic mineral clouds on HD 189733b. II. Monte Carlo radiative transfer for 3D cloudy exoplanet atmospheres : combining scattering and emission spectra
G.L. and Ch.H. highlight the financial support of the European community under the FP7 ERC starting grant 257431.Context. As the 3D spatial properties of exoplanet atmospheres are being observed in increasing detail by current and new generations of telescopes, the modelling of the 3D scattering effects of cloud forming atmospheres with inhomogeneous opacity structures becomes increasingly important to interpret observational data. Aims. We model the scattering and emission properties of a simulated cloud forming, inhomogeneous opacity, hot Jupiter atmosphere of HD 189733b. We compare our results to available Hubble Space Telescope (HST) and Spitzer data and quantify the effects of 3D multiple scattering on observable properties of the atmosphere. We discuss potential observational properties of HD 189733b for the upcoming Transiting Exoplanet Survey Satellite (TESS) and CHaracterising ExOPlanet Satellite (CHEOPS) missions. Methods. We developed a Monte Carlo radiative transfer code and applied it to post-process output of our 3D radiative-hydrodynamic, cloud formation simulation of HD 189733b. We employed three variance reduction techniques, i.e. next event estimation, survival biasing, and composite emission biasing, to improve signal to noise of the output. For cloud particle scattering events, we constructed a log-normal area distribution from the 3D cloud formation radiative-hydrodynamic results, which is stochastically sampled in order to model the Rayleigh and Mie scattering behaviour of a mixture of grain sizes. Results. Stellar photon packets incident on the eastern dayside hemisphere show predominantly Rayleigh, single-scattering behaviour, while multiple scattering occurs on the western hemisphere. Combined scattered and thermal emitted light predictions are consistent with published HST and Spitzer secondary transit observations. Our model predictions are also consistent with geometric albedo constraints from optical wavelength ground-based polarimetry and HST B band measurements. We predict an apparent geometric albedo for HD 189733b of 0.205 and 0.229, in the TESS and CHEOPS photometric bands respectively. Conclusions. Modelling the 3D geometric scattering effects of clouds on observables of exoplanet atmospheres provides an important contribution to the attempt to determine the cloud properties of these objects. Comparisons between TESS and CHEOPS photometry may provide qualitative information on the cloud properties of nearby hot Jupiter exoplanets.Publisher PDFPeer reviewe
A Review of Noncommutative Field Theories
We present a brief review of selected topics in noncommutative field theories
ranging from its revival in string theory, its influence on quantum field
theories, its possible experimental signatures and ending with some
applications in gravity and emergent gravity.Comment: Talk presented at the XIV Mexican School on Particles and Fields,
Morelia, Mexico, November 9-11, 2010; 8 pages. V2 reference adde
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