53 research outputs found
Broken discrete and continuous symmetries in two dimensional spiral antiferromagnets
We study the occurrence of symmetry breakings, at zero and finite
temperatures, in the J_1-J_3 antiferromagnetic Heisenberg model on the square
lattice using Schwinger boson mean field theory. For spin-1/2 the ground state
breaks always the SU(2) symmetry with a continuous quasi-critical transition at
J_3/J_1=0.38, from N\'eel to spiral long range order, although local spin
fluctuations considerations suggest an intermediate disordered regime around
0.35 < J_3/J_1 < 0.5, in qualitative agreement with recent numerical results.
At low temperatures we find a Z_2 broken symmetry region with short range
spiral order characterized by an Ising-like nematic order parameter that
compares qualitatively well with classical Monte Carlo results. At intermediate
temperatures the phase diagram shows regions with collinear short range orders:
for J_3/J_11 a novel phase
consisting of four decoupled third neighbour sublattices with N\'eel (\pi,\pi)
correlations in each one. We conclude that the effect of quantum and thermal
fluctuations is to favour collinear correlations even in the strongly
frustrated regime.Comment: 17 pages, accepted for publication in Journal of Physics: condensed
Matte
A test of the bosonic spinon theory for the triangular antiferromagnet spectrum
We compute the dynamical structure factor of the spin-1/2 triangular
Heisenberg model using the mean field Schwinger boson theory. We find that a
reconstructed dispersion, resulting from a non trivial redistribution of the
spectral weight, agrees quite well with the spin excitation spectrum recently
found with series expansions. In particular, we recover the strong
renormalization with respect to linear spin wave theory along with the
appearance of roton-like minima. Furthermore, near the roton-like minima the
contribution of the two spinon continuum to the static structure factor is
about 40 % of the total weight. By computing the density-density dynamical
structure factor, we identify an unphysical weak signal of the spin excitation
spectrum with the relaxation of the local constraint of the Schwinger bosons at
the mean field level. Based on the accurate description obtained for the static
and dynamic ground state properties, we argue that the bosonic spinon theory
should be considered seriously as a valid alternative to interpret the physics
of the triangular Heisenberg model.Comment: 6 pages, 5 figures, extended version including: a table with ground
state energy and magnetization; and the density-density dynamical structure
factor. Accepted for publication in Europhysics Letter
Classical Antiferromagnetism in Kinetically Frustrated Electronic Models
We study the infinite U Hubbard model with one hole doped away half-filling,
in triangular and square lattices with frustrated hoppings that invalidate
Nagaoka's theorem, by means of the density matrix renormalization group. We
find that these kinetically frustrated models have antiferromagnetic ground
states with classical local magnetization in the thermodynamic limit. We
identify the mechanism of this kinetic antiferromagnetism with the release of
the kinetic energy frustration as the hole moves in the established
antiferromagnetic background. This release can occurs in two different ways: by
a non-trivial spin-Berry phase acquired by the hole or by the effective
vanishing of the hopping amplitude along the frustrating loops.Comment: 12 pages and 4 figures, with Supplementary Material. To be published
in Phys. Rev. Let
SuperWASP Observations of the 2007 Outburst of Comet 17P/Holmes
We present wide-field imaging of the 2007 outburst of Comet 17P/Holmes
obtained serendipitously by SuperWASP-North on 17 nights over a 42-night period
beginning on the night (2007 October 22-23) immediately prior to the outburst.
Photometry of 17P's unresolved coma in SuperWASP data taken on the first night
of the outburst is consistent with exponential brightening, suggesting that the
rapid increase in the scattering cross-section of the coma could be largely due
to the progressive fragmentation of ejected material produced on a very short
timescale at the time of the initial outburst, with fragmentation timescales
decreasing from t(frag)~2x10^3 s to t(frag)~1x10^3 s over our observing period.
Analysis of the expansion of 17P's coma reveals a velocity gradient suggesting
that the outer coma was dominated by material ejected in an instantaneous,
explosive manner. We find an expansion velocity at the edge of the dust coma of
v(exp) = 0.55+/-0.02 km/s and a likely outburst date of t_0=2007 October
23.3+/-0.3, consistent with our finding that the comet remained below
SuperWASP's detection limit of m(V)~15 mag until at least 2007 October 23.3.
Modelling of 17P's gas coma indicates that its outer edge, which was observed
to extend past the outer dust coma, is best explained with a single pulse of
gas production, consistent with our conclusions concerning the production of
the outer dust coma.Comment: 36 pages, 8 figures, accepted for publication in MNRA
Study of the plutino object (208996) 2003 AZ84 from stellar occultations: size, shape and topographic features
We present results derived from four stellar occultations by the plutino
object (208996) 2003~AZ, detected at January 8, 2011 (single-chord
event), February 3, 2012 (multi-chord), December 2, 2013 (single-chord) and
November 15, 2014 (multi-chord). Our observations rule out an oblate spheroid
solution for 2003~AZ's shape. Instead, assuming hydrostatic equilibrium,
we find that a Jacobi triaxial solution with semi axes ~km % axis ratios and
, can better account for all our occultation observations.
Combining these dimensions with the rotation period of the body (6.75~h) and
the amplitude of its rotation light curve, we derive a density ~g~cm a geometric albedo . A grazing chord
observed during the 2014 occultation reveals a topographic feature along
2003~AZ's limb, that can be interpreted as an abrupt chasm of width
~km and depth ~km or a smooth depression of width ~km
and depth ~km (or an intermediate feature between those two extremes)
Asteroids' physical models from combined dense and sparse photometry and scaling of the YORP effect by the observed obliquity distribution
The larger number of models of asteroid shapes and their rotational states
derived by the lightcurve inversion give us better insight into both the nature
of individual objects and the whole asteroid population. With a larger
statistical sample we can study the physical properties of asteroid
populations, such as main-belt asteroids or individual asteroid families, in
more detail. Shape models can also be used in combination with other types of
observational data (IR, adaptive optics images, stellar occultations), e.g., to
determine sizes and thermal properties. We use all available photometric data
of asteroids to derive their physical models by the lightcurve inversion method
and compare the observed pole latitude distributions of all asteroids with
known convex shape models with the simulated pole latitude distributions. We
used classical dense photometric lightcurves from several sources and
sparse-in-time photometry from the U.S. Naval Observatory in Flagstaff,
Catalina Sky Survey, and La Palma surveys (IAU codes 689, 703, 950) in the
lightcurve inversion method to determine asteroid convex models and their
rotational states. We also extended a simple dynamical model for the spin
evolution of asteroids used in our previous paper. We present 119 new asteroid
models derived from combined dense and sparse-in-time photometry. We discuss
the reliability of asteroid shape models derived only from Catalina Sky Survey
data (IAU code 703) and present 20 such models. By using different values for a
scaling parameter cYORP (corresponds to the magnitude of the YORP momentum) in
the dynamical model for the spin evolution and by comparing synthetics and
observed pole-latitude distributions, we were able to constrain the typical
values of the cYORP parameter as between 0.05 and 0.6.Comment: Accepted for publication in A&A, January 15, 201
The Size, Shape, Albedo, Density, and Atmospheric Limit of Transneptunian Object (50000) Quaoar from Multi-chord Stellar Occultations
We present results derived from the first multi-chord stellar occultations by the transneptunian object (50000) Quaoar, observed on 2011 May 4 and 2012 February 17, and from a single-chord occultation observed on 2012 October 15. If the timing of the five chords obtained in 2011 were correct, then Quaoar would possess topographic features (crater or mountain) that would be too large for a body of this mass. An alternative model consists in applying time shifts to some chords to account for possible timing errors. Satisfactory elliptical fits to the chords are then possible, yielding an equivalent radius R [SUB]equiv[/SUB] = 555 ± 2.5 km and geometric visual albedo p[SUB]V[/SUB] = 0.109 ± 0.007. Assuming that Quaoar is a Maclaurin spheroid with an indeterminate polar aspect angle, we derive a true oblateness of \epsilon = 0.087^{+0.0268}_{-0.0175}, an equatorial radius of 569^{+24}_{-17} km, and a density of 1.99 ± 0.46 g cm[SUP]–3[/SUP]. The orientation of our preferred solution in the plane of the sky implies that Quaoar's satellite Weywot cannot have an equatorial orbit. Finally, we detect no global atmosphere around Quaoar, considering a pressure upper limit of about 20 nbar for a pure methane atmosphere.Peer reviewe
Stellar occultations enable milliarcsecond astrometry for Trans-Neptunian objects and Centaurs
Trans-Neptunian objects (TNOs) and Centaurs are remnants of our planetary
system formation, and their physical properties have invaluable information for
evolutionary theories. Stellar occultation is a ground-based method for
studying these small bodies and has presented exciting results. These
observations can provide precise profiles of the involved body, allowing an
accurate determination of its size and shape. The goal is to show that even
single-chord detections of TNOs allow us to measure their milliarcsecond
astrometric positions in the reference frame of the Gaia second data release
(DR2). Accurated ephemerides can then be generated, allowing predictions of
stellar occultations with much higher reliability. We analyzed data from
stellar occultations to obtain astrometric positions of the involved bodies.
The events published before the Gaia era were updated so that the Gaia DR2
catalog is the reference. Previously determined sizes were used to calculate
the position of the object center and its corresponding error with respect to
the detected chord and the International Celestial Reference System (ICRS)
propagated Gaia DR2 star position. We derive 37 precise astrometric positions
for 19 TNOs and 4 Centaurs. Twenty-one of these events are presented here for
the first time. Although about 68\% of our results are based on single-chord
detection, most have intrinsic precision at the submilliarcsecond level. Lower
limits on the diameter and shape constraints for a few bodies are also
presented as valuable byproducts. Using the Gaia DR2 catalog, we show that even
a single detection of a stellar occultation allows improving the object
ephemeris significantly, which in turn enables predicting a future stellar
occultation with high accuracy. Observational campaigns can be efficiently
organized with this help, and may provide a full physical characterization of
the involved object.Comment: 16 pages, 28 figures. The manuscript was accepted and is to be
publishe
New insights on the binary Asteroid 121 Hermione
We report on the results of a six-month photometric study of the main-belt binary C-type asteroid 121 Hermione, performed during its 2007 opposition. We took advantage of the rare observational opportunity afforded by one of the annual equinoxes of Hermione occurring close to its opposition in June 2007. The equinox provides an edge-on aspect for an Earth-based observer, which is well suited to a thorough study of Hermione’s physical characteristics. The catalog of observations carried out with small telescopes is presented in this work, together with new adaptive optics (AO) imaging obtained between 2005 and 2008 with the Yepun 8-m VLT telescope and the 10-m Keck telescope. The most striking result is confirmation that Hermione is a bifurcated and elongated body, as suggested by Marchis et al., (2005). A new effective diameter of 187 ± 6 km was calculated from the combination of AO, photometric and thermal observations. The new diameter is some 10% smaller than the hitherto accepted radiometric diameter based on IRAS data. The reason for the discrepancy is that IRAS viewed the system almost pole-on. New thermal observations with the Spitzer Space Telescope agree with the diameter derived from AO and lightcurve observations. On the basis of the new AO astrometric observations of the small 32-km diameter satellite we have refined the orbit solution and derived a new value of the bulk density of Hermione of 1.4 +0.5/-0.2 g cm-3. We infer a macroscopic porosity of ~33 +5/-20%
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