44,062 research outputs found
A generalized Monte Carlo loop algorithm for frustrated Ising models
We introduce a Generalized Loop Move (GLM) update for Monte Carlo simulations
of frustrated Ising models on two-dimensional lattices with bond-sharing
plaquettes. The GLM updates are designed to enhance Monte Carlo sampling
efficiency when the system's low-energy states consist of an extensive number
of degenerate or near-degenerate spin configurations, separated by large energy
barriers to single spin flips. Through implementation on several frustrated
Ising models, we demonstrate the effectiveness of the GLM updates in cases
where both degenerate and near-degenerate sets of configurations are favored at
low temperatures. The GLM update's potential to be straightforwardly extended
to different lattices and spin interactions allow it to be readily adopted on
many other frustrated Ising models of physical relevance.Comment: 14 pages, 12 figure
CO observations and investigation of triggered star formation towards N10 infrared bubble and surroundings
We studied the environment of the dust bubble N10 in molecular emission.
Infrared bubbles, first detected by the GLIMPSE survey at 8.0 m, are ideal
regions to investigate the effect of the expansion of the HII region on its
surroundings eventual triggered star formation at its borders. In this work, we
present a multi-wavelength study of N10. This bubble is especially interesting
as infrared studies of the young stellar content suggest a scenario of ongoing
star formation, possibly triggered, on the edge of the HII region. We carried
out observations of CO(1-0) and CO(1-0) emission at PMO 13.7-m
towards N10. We also analyzed the IR and sub-mm emission on this region and
compare those different tracers to obtain a detailed view of the interaction
between the expanding HII region and the molecular gas. We also estimated the
parameters of the denser cold dust condensation and of the ionized gas inside
the shell. Bright CO emission was detected and two molecular clumps were
identified, from which we have derived physical parameters. We also estimate
the parameters for the densest cold dust condensation and for the ionized gas
inside the shell. The comparison between the dynamical age of this region and
the fragmentation time scale favors the "Radiation-Driven Implosion" mechanism
of star formation. N10 reveals to be specially interesting case with gas
structures in a narrow frontier between HII region and surrounding molecular
material, and with a range of ages of YSOs situated in region indicating
triggered star formation.Comment: Version 2 - Submmited to ApJ (under review
Mapping warm molecular hydrogen with Spitzer's Infrared Array Camera (IRAC)
Photometric maps, obtained with Spitzer's Infrared Array Camera (IRAC), can
provide a valuable probe of warm molecular hydrogen within the interstellar
medium. IRAC maps of the supernova remnant IC443, extracted from the Spitzer
archive, are strikingly similar to spectral line maps of the H2 pure rotational
transitions that we obtained with the Infrared Spectrograph (IRS) instrument on
Spitzer. IRS spectroscopy indicates that IRAC Bands 3 and 4 are indeed
dominated by the H2 v=0-0 S(5) and S(7) transitions, respectively. Modeling of
the H2 excitation suggests that Bands 1 and 2 are dominated by H2 v=1-0 O(5)
and v=0-0 S(9). Large maps of the H2 emission in IC433, obtained with IRAC,
show band ratios that are inconsistent with the presence of gas at a single
temperature. The relative strengths of IRAC Bands 2, 3, and 4 are consistent
with pure H2 emission from shocked material with a power-law distribution of
gas temperatures. CO vibrational emissions do not contribute significantly to
the observed Band 2 intensity. Assuming that the column density of H2 at
temperatures T to T+dT is proportional to T raised to the power -b for
temperatures up to 4000 K, we obtained a typical estimate of 4.5 for b. The
power-law index, b, shows variations over the range 3 to 6 within the set of
different sight-lines probed by the maps, with the majority of sight-lines
showing b in the range 4 to 5. The observed power-law index is consistent with
the predictions of simple models for paraboloidal bow shocks.Comment: 27 pages, including 11 figures. Accepted for publication in Ap
B-meson signatures of a Supersymmetric U(2) flavor model
We discuss B-meson signatures of a Supersymmetric U(2) flavor model, with
relatively light (electroweak scale masses) third generation right-handed
scalars. We impose current B and K meson experimental constraints on such a
theory, and obtain expectations for B->X_s gamma, B->X_s glue, B->X_s l+ l-,
B->phi K_s, B_s-B_sbar mixing, and the dilepton asymmetry in B_s. We show that
such a theory is compatible with all current data, and furthermore, could
reconcile the apparent deviations from Standard Model predictions that have
been found in some experiments.Comment: 37 pages, 21 figures, RevTeX4; v.2 - minor modifications to improve
readability. Published versio
Distributed Coordinate Descent for L1-regularized Logistic Regression
Solving logistic regression with L1-regularization in distributed settings is
an important problem. This problem arises when training dataset is very large
and cannot fit the memory of a single machine. We present d-GLMNET, a new
algorithm solving logistic regression with L1-regularization in the distributed
settings. We empirically show that it is superior over distributed online
learning via truncated gradient
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