18,185 research outputs found
The topological strong spatial mixing property and new conditions for pressure approximation
In the context of stationary nearest-neighbour Gibbs measures
satisfying strong spatial mixing, we present a new combinatorial
condition (the topological strong spatial mixing property (TSSM)) on the
support of sufficient for having an efficient approximation algorithm for
topological pressure. We establish many useful properties of TSSM for studying
strong spatial mixing on systems with hard constraints. We also show that TSSM
is, in fact, necessary for strong spatial mixing to hold at high rate. Part of
this work is an extension of results obtained by D. Gamarnik and D. Katz
(2009), and B. Marcus and R. Pavlov (2013), who gave a special representation
of topological pressure in terms of conditional probabilities.Comment: 40 pages, 8 figures. arXiv admin note: text overlap with
arXiv:1309.1873 by other author
The QUEST RR Lyrae Survey: III. The Low Galactic Latitude Catalogue
We present results for the QUEST RR Lyrae Survey at low galactic latitude,
conducted entirely with observations obtained with the QUEST mosaic camera and
the 1.0/1.5m J\"urgen Stock Schmidt telescope at the National Observatory of
Venezuela. The survey spans an area of 476 sq. deg on the sky, with multi-epoch
observations in the V, R and I photometric bands for 6.5x10^6 stars in the
galactic latitude range 30<= b(deg) <=+25, in a direction close to the Galactic
Anticenter 190<= l(deg)<= 230. The variability survey has a typical number of
30 observations per object in V and I and ~25 in R, with up to ~120-150 epochs
in V and I and up to ~100 in R in the best sampled regions. The completeness
magnitudes of the survey are V=R=18.5 mag, and I=18.0 mag. We identified 211 RR
Lyrae stars, 160 bona fide stars of type ab and 51 candidates of type c, ours
being the first deep RR Lyrae survey conducted at low galactic latitude.The
completeness of the RR Lyrae survey was estimated in >95 per cent and ~85 per
cent for RRab and RRc stars respectively. Photometric metallicities were
computed based on the light curves and individual extinctions calculated from
minimum light colours for each RRab star. Distances were obtained with typical
errors ~7 per cent. The RR Lyrae survey simultaneously spans a large range of
heliocentric distances 0.5<= R_hel(kpc)<=40 and heights above the plane
-15<=z(kpc)<=+20, with well known completeness across the survey area, making
it an ideal set for studying the struc ture of the Galactic thick disk.Comment: 24 pages, 17 figures, 8 tables. Accepted for publication in MNRA
Testing the existence of optical linear polarization in young brown dwarfs
Linear polarization can be used as a probe of the existence of atmospheric
condensates in ultracool dwarfs. Models predict that the observed linear
polarization increases withthe degree of oblateness, which is inversely
proportional to the surface gravity. We aimed to test the existence of optical
linear polarization in a sample of bright young brown dwarfs, with spectral
types between M6 and L2, observable from the Calar Alto Observatory, and
cataloged previously as low gravity objects using spectroscopy. Linear
polarimetric images were collected in I and R-band using CAFOS at the 2.2 m
telescope in Calar Alto Observatory (Spain). The flux ratio method was employed
to determine the linear polarization degrees. With a confidence of 3,
our data indicate that all targets have a linear polarimetry degree in average
below 0.69% in the I-band, and below 1.0% in the R-band, at the time they were
observed. We detected significant (i.e. P/ 3) linear polarization
for the young M6 dwarf 2MASS J04221413+1530525 in the R-band, with a degree of
= 0.81 0.17 %.Comment: Accepted for publication in MNRA
A Long-Lived Accretion Disk Around a Lithium-Depleted Binary T Tauri Star
We present a high dispersion optical spectrum of St 34 and identify the
system as a spectroscopic binary with components of similar luminosity and
temperature (both M3+/-0.5). Based on kinematics, signatures of accretion, and
location on an H-R diagram, we conclude that St 34 is a classical T Tauri star
belonging to the Taurus-Auriga T Association. Surprisingly, however, neither
component of the binary shows LiI 6708 A, absorption, the most universally
accepted criterion for establishing stellar youth. In this uniquely known
instance, the accretion disk appears to have survived longer than the lithium
depletion timescale. We speculate that the long-lived accretion disk is a
consequence of the sub-AU separation companion tidally inhibiting, though not
preventing, circumstellar accretion. Comparisons with pre-main sequence
evolutionary models imply, for each component of St 34, a mass of 0.37+/-0.08
Msun and an isochronal age of 8+/-3 Myr, which is much younger than the
predicted lithium depletion timescale of ~ 25 Myr. Although a distance 38%
closer than that of Taurus-Auriga or a hotter temperature scale could reconcile
this discrepancy at 21-25 Myr, similar discrepancies in other systems and the
implications of an extremely old accreting Taurus-Auriga member suggest instead
a possible problem with evolutionary models. Regardless, the older age implied
by St 34's depleted lithium abundance is the first compelling evidence for a
substantial age spread in this region. Additionally, since St 34's coeval
co-members with early M spectral types would likewise fail the lithium test for
youth, current membership lists may be incomplete.Comment: 4 pages, including 2 figures. Accepted for publication in ApJ Let
Progress report on the relativistic three-particle quantization condition
We describe recent work on the relativistic three-particle quantization
condition, generalizing and applying the original formalism of Hansen and
Sharpe, and of Brice\~no, Hansen and Sharpe. In particular, we sketch three
recent developments: the generalization of the formalism to include K-matrix
poles; the numerical implementation of the quantization condition in the
isotropic approximation; and ongoing work extending the description of the
three-particle divergence-free K matrix beyond the isotropic approximation.Comment: 7 pages, 1 figure, Proceedings of Lattice 201
Three-Particle Systems With Resonant Subprocesses in a Finite Volume
In previous work, we have developed a relativistic, model-independent three-particle quantization condition, but only under the assumption that no poles are present in the two-particle K matrices that appear as scattering subprocesses [M. T. Hansen and S. R. Sharpe, Phys. Rev. D 90, 116003 (2014); M. T. Hansen and S. R. Sharpe, Phys. Rev. D 92, 114509 (2015); R. A. Briceño et al., Phys. Rev. D 95, 074510 (2017).]. Here we lift this restriction, by deriving the quantization condition for identical scalar particles with a G-parity symmetry, in the case that the two-particle K matrix has a pole in the kinematic regime of interest. As in earlier work, our result involves intermediate infinite-volume quantities with no direct physical interpretation, and we show how these are related to the physical three-to-three scattering amplitude by integral equations. This work opens the door to study processes such as a2→ρπ→πππ, in which the ρ is rigorously treated as a resonance state
Discovery of the Pre-Main Sequence Population of the Stellar Association LH 95 in the Large Magellanic Cloud with Hubble Space Telescope ACS Observations
We report the discovery of an extraordinary number of pre-main sequence (PMS)
stars in the vicinity of the stellar association LH 95 in the Large Magellanic
Cloud (LMC). Using the {\em Advanced Camera for Surveys} on-board the {\em
Hubble} Space Telescope in wide-field mode we obtained deep high-resolution
imaging of the main body of the association and of a nearby representative LMC
background field. These observations allowed us to construct the
color-magnitude diagram (CMD) of the association in unprecedented detail, and
to decontaminate the CMD for the average LMC stellar population. The most
significant result is the direct detection of a substantial population of PMS
stars and their clustering properties with respect to the distribution of the
higher mass members of the association. Although LH 95 represents a rather
modest star forming region, our photometry, with a detection limit \lsim 28
mag, reveals in its vicinity more than 2,500 PMS stars with masses down to
M{\solar}. Our observations offer, thus, a new perspective of a
typical LMC association: The stellar content of LH 95 is found to extend from
bright OB stars to faint red PMS stars, suggesting a fully populated Initial
Mass Function (IMF) from the massive blue giants down to the sub-solar mass
regime.Comment: Accepted for Publication in ApJ Letters - 4 Pages ApJ paper format -
3 figures in low-resolution/grayscal
Progress in three-particle scattering from LQCD
We present the status of our formalism for extracting three-particle
scattering observables from lattice QCD (LQCD). The method relies on relating
the discrete finite-volume spectrum of a quantum field theory with its
scattering amplitudes. As the finite-volume spectrum can be directly determined
in LQCD, this provides a method for determining scattering observables, and
associated resonance properties, from the underlying theory. In a pair of
papers published over the last two years, two of us have extended this approach
to apply to relativistic three-particle scattering states. In this talk we
summarize recent progress in checking and further extending this result. We
describe an extension of the formalism to include systems in which two-to-three
transitions can occur. We then present a check of the previously published
formalism, in which we reproduce the known finite-volume energy shift of a
three-particle bound state.Comment: 9 pages, 3 figures, proceedings for XIIth Quark Confinement and the
Hadron Spectrum (CONF12
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