8,450 research outputs found
From solar-like to anti-solar differential rotation in cool stars
Stellar differential rotation can be separated into two main regimes:
solar-like when the equator rotates faster than the poles and anti-solar when
the polar regions rotate faster than the equator. We investigate the transition
between these two regimes with 3-D numerical simulations of rotating spherical
shells. We conduct a systematic parameter study which also includes models from
different research groups. We find that the direction of the differential
rotation is governed by the contribution of the Coriolis force in the force
balance, independently of the model setup (presence of a magnetic field,
thickness of the convective layer, density stratification). Rapidly-rotating
cases with a small Rossby number yield solar-like differential rotation, while
weakly-rotating models sustain anti-solar differential rotation. Close to the
transition, the two kinds of differential rotation are two possible bistable
states. This study provides theoretical support for the existence of anti-solar
differential rotation in cool stars with large Rossby numbers.Comment: 5 pages, 6 figures, accepted for publication in MNRA
A homomorphism between link and XXZ modules over the periodic Temperley-Lieb algebra
We study finite loop models on a lattice wrapped around a cylinder. A section
of the cylinder has N sites. We use a family of link modules over the periodic
Temperley-Lieb algebra EPTL_N(\beta, \alpha) introduced by Martin and Saleur,
and Graham and Lehrer. These are labeled by the numbers of sites N and of
defects d, and extend the standard modules of the original Temperley-Lieb
algebra. Beside the defining parameters \beta=u^2+u^{-2} with u=e^{i\lambda/2}
(weight of contractible loops) and \alpha (weight of non-contractible loops),
this family also depends on a twist parameter v that keeps track of how the
defects wind around the cylinder. The transfer matrix T_N(\lambda, \nu) depends
on the anisotropy \nu and the spectral parameter \lambda that fixes the model.
(The thermodynamic limit of T_N is believed to describe a conformal field
theory of central charge c=1-6\lambda^2/(\pi(\lambda-\pi)).)
The family of periodic XXZ Hamiltonians is extended to depend on this new
parameter v and the relationship between this family and the loop models is
established. The Gram determinant for the natural bilinear form on these link
modules is shown to factorize in terms of an intertwiner i_N^d between these
link representations and the eigenspaces of S^z of the XXZ models. This map is
shown to be an isomorphism for generic values of u and v and the critical
curves in the plane of these parameters for which i_N^d fails to be an
isomorphism are given.Comment: Replacement of "The Gram matrix as a connection between periodic loop
models and XXZ Hamiltonians", 31 page
What controls the magnetic geometry of M dwarfs?
Context: observations of rapidly rotating M dwarfs show a broad variety of
large-scale magnetic fields encompassing dipole-dominated and multipolar
geometries. In dynamo models, the relative importance of inertia in the force
balance -- quantified by the local Rossby number -- is known to have a strong
impact on the magnetic field geometry. Aims: we aim to assess the relevance of
the local Rossby number in controlling the large-scale magnetic field geometry
of M dwarfs. Methods: we explore the similarities between anelastic dynamo
models in spherical shells and observations of active M-dwarfs, focusing on
field geometries derived from spectropolarimetric studies. To do so, we
construct observation-based quantities aimed to reflect the diagnostic
parameters employed in numerical models. Results: the transition between
dipole-dominated and multipolar large-scale fields in early to mid M dwarfs is
tentatively attributed to a Rossby number threshold. We interpret late M dwarfs
magnetism to result from a dynamo bistability occurring at low Rossby number.
By analogy with numerical models, we expect different amplitudes of
differential rotation on the two dynamo branches.Comment: 4 pages, 4 figures, accepted for publication in A&
Comparison of the induction of pulmonary neoplasms in Sprague-Dawley rats by fission neutrons and radon daughters
Validity of the Adiabatic Approximation
We analyze the validity of the adiabatic approximation, and in particular the
reliability of what has been called the "standard criterion" for validity of
this approximation. Recently, this criterion has been found to be insufficient.
We will argue that the criterion is sufficient only when it agrees with the
intuitive notion of slowness of evolution of the Hamiltonian. However, it can
be insufficient in cases where the Hamiltonian varies rapidly but only by a
small amount. We also emphasize the distinction between the adiabatic {\em
theorem} and the adiabatic {\em approximation}, two quite different although
closely related ideas.Comment: 4 pages, 1 figur
Lung carcinomas in Sprague-Dawley rats after exposure to low doses of radon daughters, fission neutrons, or Îł-rays
Quadrupolar Order in Isotropic Heisenberg Models with Biquadratic Interaction
Through Quantum Monte Carlo simulation, we study the biquadratic-interaction
model with the SU(2) symmetry in two and three dimensions. The zero-temperature
phase diagrams for the two cases are identical and exhibit an intermediate
phase characterized by finite quadrupole moment, in agreement with mean-field
type arguments and the semi-classical theory. In three dimensions, we
demonstrate that the model in the quadrupolar regime has a phase transition at
a finite temperature. In contrast to predictions by mean-field theories, the
phase transition to the quadrupolar phase turns out to be of the second order.
We also examine the critical behavior in the two marginal cases with the SU(3)
symmetry.Comment: 4 pages 5 figure
Thermodynamic metrics and optimal paths
A fundamental problem in modern thermodynamics is how a molecular-scale
machine performs useful work, while operating away from thermal equilibrium
without excessive dissipation. To this end, we derive a friction tensor that
induces a Riemannian manifold on the space of thermodynamic states. Within the
linear-response regime, this metric structure controls the dissipation of
finite-time transformations, and bestows optimal protocols with many useful
properties. We discuss the connection to the existing thermodynamic length
formalism, and demonstrate the utility of this metric by solving for optimal
control parameter protocols in a simple nonequilibrium model.Comment: 5 page
BORAZANs:â Tunable Fluorophores Based on 2-(Pyrazolyl)aniline Chelates of Diphenylboron
The reaction between 2-pyrazolyl-4-X-anilines, H(pzAnX), (X = para-OMe (L1), Me (L2), H (L3), Cl (L4), CO2Et (L5), CF3 (L6), CN (L7)) and triphenylboron in boiling toluene affords the respective, highly emissive N,Nâ-boron chelate complexes, BPh2(pzAnX) (X = para-OMe (1), Me (2), H (3), Cl (4), CO2Et (5), CF3 (6), CN (7)) in high yield. The structural, electrochemical, and photophysical properties of the new boron complexes can be fine-tuned by varying the electron-withdrawing or -donating power of the para-aniline substituent (delineated by the substituent\u27s Hammett parameter). Those complexes with electron-withdrawing para-aniline substituents such as CO2Et (5), CF3 (6), and CN (7) have more planar chelate rings, more âquinoidal\u27 disortion in the aniline rings, greater chemical stability, higher oxidation potentials, and more intense (ÏF = 0.81 for 7 in toluene), higher-energy (blue) fluorescent emission compared to those with electron-donating substituents. Thus, for 1 the oxidation potential is 0.53 V versus Ag/AgCl (compared to 1.12 V for 7), and the emission is tuned to the yellow-green but at an expense in terms of lower quantum yields (ÏF = 0.07 for 1 in toluene) and increased chemical reactivity. Density functional calculations (B3LYP/6-31G*) on PM3 energy-minimized structures of the ligands and boron complexes reproduced experimentally observed data and trends and provided further insight into the nature of the electronic transitions
Random replicators with high-order interactions
We use tools of the equilibrium statistical mechanics of disordered systems
to study analytically the statistical properties of an ecosystem composed of N
species interacting via random, Gaussian interactions of order p >= 2, and
deterministic self-interactions u <= 0. We show that for nonzero u the effect
of increasing the order of the interactions is to make the system more
cooperative, in the sense that the fraction of extinct species is greatly
reduced. Furthermore, we find that for p > 2 there is a threshold value which
gives a lower bound to the concentration of the surviving species, preventing
then the existence of rare species and, consequently, increasing the robustness
of the ecosystem to external perturbations.Comment: 7 pages, 4 Postscript figure
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