1,444 research outputs found
The unbiased frequency of planetary signatures around single and binary white dwarfs using and
This paper presents combined IRAC and COS
results for a double-blind survey of 195 single and 22 wide binary white dwarfs
for infrared excesses and atmospheric metals. The selection criteria include
cooling ages in the range 9 to 300 Myr, and hydrogen-rich atmospheres so that
the presence of atmospheric metals can be confidently linked to ongoing
accretion from a circumstellar disc. The entire sample has infrared photometry,
whereas 168 targets have corresponding ultraviolet spectra. Three stars with
infrared excesses due to debris discs are recovered, yielding a nominal
frequency of per cent, while in stark contrast, the
fraction of stars with atmospheric metals is per cent. Thus, only one
out of 30 polluted white dwarfs exhibits an infrared excess at 3-4 m in
IRAC photometry, which reinforces the fact that atmospheric metal pollution is
the most sensitive tracer of white dwarf planetary systems. The corresponding
fraction of infrared excesses around white dwarfs with wide binary companions
is consistent with zero, using both the infrared survey data and an independent
assessment of potential binarity for well-established dusty and polluted stars.
In contrast, the frequency of atmospheric pollution among the targets in wide
binaries is indistinct from apparently single stars, and moreover the
multiplicity of polluted white dwarfs in a complete and volume-limited sample
is the same as for field stars. Therefore, it appears that the delivery of
planetesimal material onto white dwarfs is ultimately not driven by stellar
companions, but by the dynamics of planetary bodies.Comment: 15 pages, 6 figures, 3 tables; accepted for publication in MNRA
Dimensionality of spin modulations in 1/8-doped lanthanum cuprates from the perspective of NQR and muSR experiments
We investigate the dimensionality of inhomogeneous spin modulation patterns
in the cuprate family of high-temperature superconductors with particular focus
on 1/8-doped lanthanum cuprates. We compare one-dimensional stripe modulation
pattern with two-dimensional checkerboard of spin vortices in the context of
nuclear quadrupole resonance(NQR) and muon spin rotation(muSR) experiments. In
addition, we also consider the third pattern, a two-dimensional superposition
of spin spirals. Overall, we have found that none of the above patterns leads
to a consistent interpretation of the two types of experiments considered.
This, in particular, implies that the spin vortex checkerboard cannot be ruled
out on the basis of available NQR/muSR experimental results.Comment: 6 pages, 2 figure
Spectral Duality Between Heisenberg Chain and Gaudin Model
In our recent paper we described relationships between integrable systems
inspired by the AGT conjecture. On the gauge theory side an integrable spin
chain naturally emerges while on the conformal field theory side one obtains
some special reduced Gaudin model. Two types of integrable systems were shown
to be related by the spectral duality. In this paper we extend the spectral
duality to the case of higher spin chains. It is proved that the N-site GL(k)
Heisenberg chain is dual to the special reduced k+2-points gl(N) Gaudin model.
Moreover, we construct an explicit Poisson map between the models at the
classical level by performing the Dirac reduction procedure and applying the
AHH duality transformation.Comment: 36 page
Dilepton production in proton-proton collisions at BEVALAC energies
The dilepton production in elementary reactions at
BEVALAC energies GeV is investigated. The calculations
include direct decays of the vector mesons , , and , Dalitz decays of the -, -, -, -, and -mesons, and of the baryon resonances
. The subthreshold vector meson production cross sections in
collisions are treated in a way sufficient to avoid double counting with the
inclusive vector meson production. The vector meson dominance model for the
transition form factors of the resonance Dalitz decays is
used in an extended form to ensure correct asymptotics which are in agreement
with the quark counting rules. Such a modification gives an unified and
consistent description of both radiative decays and meson decays.
The effect of multiple pion production on the experimental efficiency for the
detection of the dilepton pairs is studied. We find the dilepton yield in
reasonable agreement with the experimental data for the set of intermediate
energies whereas at the highest energy GeV the number of
dilepton pairs is likely to be overestimated experimentally in the mass range
MeV.Comment: 25 pages (IOP style), 5 figures, revised manuscript accepted for
publication in JP
Calculation of 1/m^3 terms in the total semileptonic width of D mesons.
We calculate the 1/ corrections in the inclusive semileptonic widths
of mesons. We show that these are due to the novel penguin type operators
that appear at this level in the transition operator. Taking into account the
nonperturbative corrections leads to the predicted value of the semileptonic
width significantly lower than the experimental value. The worsen the
situation or at the very least, within uncertainty, give small contribution. We
indicate possible ways out. It seems most probable that violations of duality
are noticeable in the energy range characteristic to the inclusive decays in
the charm family. Theoretically these deviations are related to divergence of
the high-order terms in the power expansion in the inverse heavy quark mass.Comment: Final version accepted for publication in Physical Review D (19
pages, 5 figures appended as two PS files at the end of the LATEX file
Projective Hilbert space structures at exceptional points
A non-Hermitian complex symmetric 2x2 matrix toy model is used to study
projective Hilbert space structures in the vicinity of exceptional points
(EPs). The bi-orthogonal eigenvectors of a diagonalizable matrix are
Puiseux-expanded in terms of the root vectors at the EP. It is shown that the
apparent contradiction between the two incompatible normalization conditions
with finite and singular behavior in the EP-limit can be resolved by
projectively extending the original Hilbert space. The complementary
normalization conditions correspond then to two different affine charts of this
enlarged projective Hilbert space. Geometric phase and phase jump behavior are
analyzed and the usefulness of the phase rigidity as measure for the distance
to EP configurations is demonstrated. Finally, EP-related aspects of
PT-symmetrically extended Quantum Mechanics are discussed and a conjecture
concerning the quantum brachistochrone problem is formulated.Comment: 20 pages; discussion extended, refs added; bug correcte
On algebraic models of dynamical systems
We describe a universal algebraic model which, being read appropriately, yields (periodic and infinite) discrete dynamical systems, as well as their âcontinuous limitsâ, which cover all differential scalar Lax systems. For this model we give: Two different constructions of an infinity of integrals; modified equations; deformations; infinitesimal automorphisms. The basic tools are supplied by symbolic calculus and the abstract Hamiltonian formalism.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/43214/1/11005_2004_Article_BF00401731.pd
The Growth of Bubbles in Cosmological Phase Transitions
We study how bubbles grow after the initial nucleation event in generic
first-order cosmological phase transitions characterised by the values of
latent heat, interface tension and correlation length, and driven by a scalar
order parameter . Equations coupling and the fluid variables
and and depending on a dissipative constant are derived and solved
numerically in the 1+1 dimensional case starting from a slightly deformed
critical bubble configuration. Parameters corresponding to QCD and electroweak
phase transitions are chosen and the whole history of the bubble with formation
of combustion and shock fronts is computed as a function of . Both
deflagrations and detonations can appear depending on the values of the
parameters. Reheating due to collisions of bubbles is also computed.Comment: 24 LaTeX-pages with 20 figures not included. The complete PostScript
file, including figures, is available by anonymous ftp from
fltxc.helsinki.fi, as /pub/bubble.ps, or as a hardcopy by airmail (a
dublicate lies at nic.funet.fi:/pub/sci/physics/papers/bubble.ps). Helsinki
Preprint HU-TFT-93-4
Endogenous cholinergic inputs and local circuit mechanisms govern the phasic mesolimbic dopamine response to nicotine
Nicotine exerts its reinforcing action by stimulating nicotinic acetylcholine receptors (nAChRs) and boosting dopamine (DA) output from the ventral tegmental area (VTA). Recent data have led to a debate about the principal pathway of nicotine action: direct stimulation of the DAergic cells through nAChR activation, or disinhibition mediated through desensitization of nAChRs on GABAergic interneurons. We use a computational model of the VTA circuitry and nAChR function to shed light on this issue. Our model illustrates that the α4ÎČ2-containing nAChRs either on DA or GABA cells can mediate the acute effects of nicotine. We account for in vitro as well as in vivo data, and predict the conditions necessary for either direct stimulation or disinhibition to be at the origin of DA activity increases. We propose key experiments to disentangle the contribution of both mechanisms. We show that the rate of endogenous acetylcholine input crucially determines the evoked DA response for both mechanisms. Together our results delineate the mechanisms by which the VTA mediates the acute rewarding properties of nicotine and suggest an acetylcholine dependence hypothesis for nicotine reinforcement.Peer reviewe
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