14,065 research outputs found
Two short mass-loss events that unveil the binary heart of Minkowski's Butterfly Nebula
Studying the appearance and properties of bipolar winds is critical to
understand the stellar evolution from the AGB to the planetary nebula (PN)
phase. Many uncertainties exist regarding the presence and role of binary
stellar systems, mainly due to the deficit of conclusive observational
evidences. We investigate the extended equatorial distribution around the early
bipolar planetary nebula M 2-9 ("Minkowski's Butterfly Nebula") to gather new
information on the mechanism of the axial ejections. Interferometric millimeter
observations of molecular emission provide the most comprehensive view of the
equatorial mass distribution and kinematics in early PNe. Here we present
subarcsecond angular-resolution observations of the 12CO J=2-1 line and
continuum emission with the Plateau de Bure interferometer. The data reveal two
ring-shaped and eccentric structures at the equatorial basis of the two coaxial
optical lobes. The two rings were formed during short mass-loss episodes (~ 40
yr), separated by ~ 500 yr. Their positional and dynamical imprints provide
evidence of the presence of a binary stellar system at the center, which yields
critical information on its orbital characteristics, including a mass estimate
for the secondary of ~< 0.2 \ms. The presence of a stellar system with a
modest-mass companion at the center of such an elongated bipolar PN strongly
supports the binary-based models, because these are more easily able to explain
the frequent axisymmetric ejections in PNe.Comment: 8 page
Evidence for Fermi surface reconstruction in the static stripe phase of LaEuSrCuO,
We present a photoemission study of LaEuSrCuO
with doping level =1/8, where the charge carriers are expected to order
forming static stripes. Though the local probes in direct space seem to be
consistent with this idea, there has been little evidence found for such
ordering in quasiparticle dispersions. We show that the Fermi surface topology
of the 1/8 compound develops notable deviations from that observed for LaSrCuO in a way consistent with the FS reconstruction expected for
the scattering on the antiphase stripe order
Dissipative dynamics of topological defects in frustrated Heisenberg spin systems
We study the dynamics of topological defects of a frustrated spin system
displaying spiral order. As a starting point we consider the SO(3) nonlinear
sigma model to describe long-wavelength fluctuations around the noncollinear
spiral state. Besides the usual spin-wave magnetic excitations, the model
allows for topologically non-trivial static solutions of the equations of
motion, associated with the change of chirality (clockwise or counterclockwise)
of the spiral. We consider two types of these topological defects, single
vortices and vortex-antivortex pairs, and quantize the corresponding solutions
by generalizing the semiclassical approach to a non-Abelian field theory. The
use of the collective coordinates allows us to represent the defect as a
particle coupled to a bath of harmonic oscillators, which can be integrated out
employing the Feynman-Vernon path-integral formalism. The resulting effective
action for the defect indicates that its motion is damped due to the scattering
by the magnons. We derive a general expression for the damping coefficient of
the defect, and evaluate its temperature dependence in both cases, for a single
vortex and for a vortex-antivortex pair. Finally, we consider an application of
the model for cuprates, where a spiral state has been argued to be realized in
the spin-glass regime. By assuming that the defect motion contributes to the
dissipative dynamics of the charges, we can compare our results with the
measured inverse mobility in a wide range of temperature. The relatively good
agreement between our calculations and the experiments confirms the possible
relevance of an incommensurate spiral order for lightly doped cuprates.Comment: 22 pages, 7 figures, final published versio
Bolometric luminosity variations in the Luminous Blue Variable AFGL2298
We characterise the variability in the physical properties of the luminous
blue variable AFGL2298 between 1989-2008. In conjunction with published data
from 1989-2001, we have undertaken a long term (2001-2008) near-IR
spectroscopic and photometric observational campaign for this star and utilise
a non-LTE model atmosphere code to interpret these data. We find AFGL2298 to
have been highly variable during the two decades covered by the observational
datasets. Photometric variations of >1.6 mag have been observed in the JHK
wavebands; however, these are not accompanied by correlated changes in near-IR
colour. Non-LTE model atmosphere analysis of 4 epochs of K band spectroscopy
obtained between 2001-7 suggests that the photometric changes were driven by
expansion and contraction of the stellar photosphere accompanied by
comparatively small changes in the stellar temperature. Unclumped mass loss
rates throughout this period were modest and directly comparable to those of
other highly luminous LBVs. However, the bolometric luminosity of AFGL2298
appears to have varied by at least a factor of ~2 between 1989-2008, with it
being one of the most luminous stars in the Galaxy during maximum. Comparison
to other LBVs that have undergone non bolometric luminosity conserving
`eruptions' shows such events to be heterogeneous, with AFGL2298 the least
extreme example. These results - and the diverse nature of both the quiescent
LBVs and associated ejecta - may offer support to the suggestion that more than
one physical mechanism is responsible for such behaviour. [ABRIDGED]Comment: 12 pages, 6 figures, accepted for publication in Astronomy and
Astrophysic
Dynamics of lattice pinned charge stripes
We study the transversal dynamics of a charged stripe (quantum string) and
show that zero temperature quantum fluctuations are able to depin it from the
lattice. If the hopping amplitude t is much smaller than the string tension J,
the string is pinned by the underlying lattice. At t>>J, the string is depinned
and allowed to move freely, if we neglect the effect of impurities. By mapping
the system onto a 1D array of Josephson junctions, we show that the quantum
depinning occurs at t/J = 2 / pi^2. Besides, we exploit the relation of the
stripe Hamiltonian to the sine-Gordon theory and calculate the infrared
excitation spectrum of the quantum string for arbitrary t/J values.Comment: 4 pages, 2 figure
X-boson cumulant approach to the periodic Anderson model
The Periodic Anderson Model (PAM) can be studied in the infinite U limit by
employing the Hubbard X operators to project out the unwanted states. We have
already studied this problem employing the cumulant expansion with the
hybridization as perturbation, but the probability conservation of the local
states (completeness) is not usually satisfied when partial expansions like the
Chain Approximation (CHA) are employed. Here we treat the problem by a
technique inspired in the mean field approximation of Coleman's slave-bosons
method, and we obtain a description that avoids the unwanted phase transition
that appears in the mean-field slave-boson method both when the chemical
potential is greater than the localized level Ef at low temperatures (T) and
for all parameters at intermediate T.Comment: Submited to Physical Review B 14 pages, 17 eps figures inserted in
the tex
suppression in co-doped striped cuprates
We propose a model that explains the reduction of due to the pinning of
stripes by planar impurity co-doping in cuprates. A geometrical argument about
the planar fraction of carriers affected by stripe pinning leads to a a linear
suppression as a function of impurity concentration . The critical
value for the vanishing of superconductivity is shown to scale like
in the under-doped regime and becomes universal in the optimally- and
over-doped regimes. Our theory agrees very well with the experimental data in
single- and bi-layer cuprates co-doped with Zn, Li, Co, etc...Comment: 4 pages, 4 figure
The structure and chemistry of the massive shell around AFGL 2343: 29SiO and HCN as tracers of high-excitation regions
The yellow hypergiant stars (YHGs) are very massive objects that are expected
to pass through periods of intense mass loss during their evolution. Despite of
this, massive circumstellar envelopes have been found only in two of them,
IRC+10420 and AFGL 2343. The envelopes around these objects and the processes
that form them are poorly known. We aim to study the structure, dynamics and
chemistry of the envelope around AFGL 2343. We have obtained interferometric
maps of the rotational lines 29SiO J= 2-1, HCN J= 1-0 and SO J(K)= 2(2)-1(1)
towards AFGL 2343. We have used an LVG excitation model to analyze the new
observations and some previously published line profiles of AFGL 2343. The
analysis of the observational data and the fitting results show the presence of
a thin, hot and dense component within the previously identified CO shell. This
component can be associated with recently shocked gas, but it could also be due
to a phase of extremely copious mass loss. We suggest that this shell is the
responsible for the whole 29SiO emission and significantly contributes to the
HCN emission. The presence of such a dense shell rich in SiO can be related
with that previously found for IRC+10420, which was also suggested to result
from a shock. This may be a common feature in the evolution of these stars, as
a consequence of the episodic mass loss periods that they pass during their
evolution. We present new results for the mass loss pattern, the total mass of
the circumstellar envelope and the molecular abundances of some species in AFGL
2343.Comment: 9 pages, 9 figure
Condition of Brazilian sardine, Sardinella brasiliensis (Steindacher, 1879) larvae in the Sao Sebastiao inner and middle continental shelf (Sao Paulo, Brazil)
Stripes, Vibrations and Superconductivity
We propose a model of a spatially modulated collective charge state of
superconducting cuprates. The regions of higher carrier density (stripes) are
described in terms of Luttinger liquids and the regions of lower density as a
two-dimensional interacting bosonic gas of d_{x^2-y^2} hole pairs. The
interactions among the elementary excitations are repulsive and the transition
to the superconducting state is driven by decay processes. Vibrations of the
CCS and the lattice, although not participating directly in the binding
mechanism, are fundamental for superconductivity. The superfluid density and
the lattice have a strong tendency to modulation implying a still unobserved
dimerized stripe phase in cuprates. The phase diagram of the model has a
crossover from 1D to 2D behavior and a pseudogap region where the amplitude of
the order parameters are finite but phase coherence is not established. We
discuss the nature of the spin fluctuations and the unusual isotope effect
within the model.Comment: 51 pages, 20 figures. Post-March Meeting version: New references are
added, some of the typos are corrected, and a few new discussions are
include
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