38,372 research outputs found
SNAP-8 third loop optimization
Eutectic sodium potassium and OS-124 considered as coolant fluids for SNAP-8 third loop - optimum loop operating parameter
Explosive Events and the Evolution of the Photospheric Magnetic Field
Transition region explosive events have long been suggested as direct
signatures of magnetic reconnection in the solar atmosphere. In seeking further
observational evidence to support this interpretation, we study the relation
between explosive events and the evolution of the solar magnetic field as seen
in line-of-sight photospheric magnetograms. We find that about 38% of events
show changes of the magnetic structure in the photosphere at the location of an
explosive event over a time period of 1 h. We also discuss potential
ambiguities in the analysis of high sensitivity magnetograms
Discovery of a wide companion near the deuterium burning mass limit in the Upper Scorpius association
We present the discovery of a companion near the deuterium burning mass limit
located at a very wide distance, at an angular separation of 4.6+/-0.1 arcsec
(projected distance of ~ 670 AU) from UScoCTIO108, a brown dwarf of the very
young Upper Scorpius association. Optical and near-infrared photometry and
spectroscopy confirm the cool nature of both objects, with spectral types of M7
and M9.5, respectively, and that they are bona fide members of the association,
showing low gravity and features of youth. Their masses, estimated from the
comparison of their bolometric luminosities and theoretical models for the age
range of the association, are 60+/-20 and 14^{+2}_{-8} MJup, respectively. The
existence of this object around a brown dwarf at this wide orbit suggests that
the companion is unlikely to have formed in a disk based on current planet
formation models. Because this system is rather weakly bound, they did not
probably form through dynamical ejection of stellar embryos.Comment: 10 pages, including 4 figures and 2 table
3d Spinfoam Quantum Gravity: Matter as a Phase of the Group Field Theory
An effective field theory for matter coupled to three-dimensional quantum
gravity was recently derived in the context of spinfoam models in
hep-th/0512113. In this paper, we show how this relates to group field theories
and generalized matrix models. In the first part, we realize that the effective
field theory can be recasted as a matrix model where couplings between matrices
of different sizes can occur. In a second part, we provide a family of
classical solutions to the three-dimensional group field theory. By studying
perturbations around these solutions, we generate the dynamics of the effective
field theory. We identify a particular case which leads to the action of
hep-th/0512113 for a massive field living in a flat non-commutative space-time.
The most general solutions lead to field theories with non-linear redefinitions
of the momentum which we propose to interpret as living on curved space-times.
We conclude by discussing the possible extension to four-dimensional spinfoam
models.Comment: 17 pages, revtex4, 1 figur
Classical Setting and Effective Dynamics for Spinfoam Cosmology
We explore how to extract effective dynamics from loop quantum gravity and
spinfoams truncated to a finite fixed graph, with the hope of modeling
symmetry-reduced gravitational systems. We particularize our study to the
2-vertex graph with N links. We describe the canonical data using the recent
formulation of the phase space in terms of spinors, and implement a
symmetry-reduction to the homogeneous and isotropic sector. From the canonical
point of view, we construct a consistent Hamiltonian for the model and discuss
its relation with Friedmann-Robertson-Walker cosmologies. Then, we analyze the
dynamics from the spinfoam approach. We compute exactly the transition
amplitude between initial and final coherent spin networks states with support
on the 2-vertex graph, for the choice of the simplest two-complex (with a
single space-time vertex). The transition amplitude verifies an exact
differential equation that agrees with the Hamiltonian constructed previously.
Thus, in our simple setting we clarify the link between the canonical and the
covariant formalisms.Comment: 38 pages, v2: Link with discretized loop quantum gravity made
explicit and emphasize
Two-Gaussian excitations model for the glass transition
We develop a modified "two-state" model with Gaussian widths for the site
energies of both ground and excited states, consistent with expectations for a
disordered system. The thermodynamic properties of the system are analyzed in
configuration space and found to bridge the gap between simple two state models
("logarithmic" model in configuration space) and the random energy model
("Gaussian" model in configuration space). The Kauzmann singularity given by
the random energy model remains for very fragile liquids but is suppressed or
eliminated for stronger liquids. The sharp form of constant volume heat
capacity found by recent simulations for binary mixed Lennard Jones and soft
sphere systems is reproduced by the model, as is the excess entropy and heat
capacity of a variety of laboratory systems, strong and fragile. The ideal
glass in all cases has a narrow Gaussian, almost invariant among molecular and
atomic glassformers, while the excited state Gaussian depends on the system and
its width plays a role in the thermodynamic fragility. The model predicts the
existence of first-order phase transition for fragile liquids.Comment: 12 pages, 12 figure
Entropy in the Classical and Quantum Polymer Black Hole Models
We investigate the entropy counting for black hole horizons in loop quantum
gravity (LQG). We argue that the space of 3d closed polyhedra is the classical
counterpart of the space of SU(2) intertwiners at the quantum level. Then
computing the entropy for the boundary horizon amounts to calculating the
density of polyhedra or the number of intertwiners at fixed total area.
Following the previous work arXiv:1011.5628, we dub these the classical and
quantum polymer models for isolated horizons in LQG. We provide exact
micro-canonical calculations for both models and we show that the classical
counting of polyhedra accounts for most of the features of the intertwiner
counting (leading order entropy and log-correction), thus providing us with a
simpler model to further investigate correlations and dynamics. To illustrate
this, we also produce an exact formula for the dimension of the intertwiner
space as a density of "almost-closed polyhedra".Comment: 24 page
Bifurcations and Slow-Fast Analysis in a Cardiac Cell Model for Investigation of Early Afterdepolarizations
In this study, we teased out the dynamical mechanisms underlying the generation of arrhythmogenic early afterdepolarizations (EADs) in a three-variable model of a mammalian ventricular cell. Based on recently published studies, we consider a 1-fast, 2-slow variable decomposition of the system describing the cellular action potential. We use sweeping techniques, such as the spike-counting method, and bifurcation and continuation methods to identify parametric regions with EADs. We show the existence of isolas of periodic orbits organizing the different EAD patterns and we provide a preliminary classification of our fast-slow decomposition according to the involved dynamical phenomena. This investigation represents a basis for further studies into the organization of EAD patterns in the parameter space and the involved bifurcations
Bubble divergences from cellular cohomology
We consider a class of lattice topological field theories, among which are
the weak-coupling limit of 2d Yang-Mills theory, the Ponzano-Regge model of 3d
quantum gravity and discrete BF theory, whose dynamical variables are flat
discrete connections with compact structure group on a cell 2-complex. In these
models, it is known that the path integral measure is ill-defined in general,
because of a phenomenon called `bubble divergences'. A common expectation is
that the degree of these divergences is given by the number of `bubbles' of the
2-complex. In this note, we show that this expectation, although not realistic
in general, is met in some special cases: when the 2-complex is simply
connected, or when the structure group is Abelian -- in both cases, the
divergence degree is given by the second Betti number of the 2-complex.Comment: 5 page
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