907 research outputs found
Convergence of Discretized Light Cone Quantization in the small mass limit
I discuss the slow convergence of Discretized Light Cone Quantization (DLCQ)
in the small mass limit and suggest a solution.Comment: 8 pages, 5 Postscript figures, uses boxedeps.te
Sulphur-bearing molecules in AGB stars I: The occurrence of hydrogen sulfide
Through a survey of (sub-)millimetre emission lines of various
sulphur-bearing molecules, we aim to determine which molecules are the primary
carriers of sulphur in different types of AGB stars. In this paper, the first
in a series, we investigate the occurrence of HS in AGB circumstellar
envelopes and determine its abundance, where possible. We have surveyed 20 AGB
stars with a range of mass-loss rates and of different chemical types using the
APEX telescope to search for rotational transition lines of five key
sulphur-bearing molecules: CS, SiS, SO, SO and HS. Here we present our
results for HS, including detections, non-detections and detailed radiative
transfer modelling of the detected lines. We compare results based on different
descriptions of the molecular excitation of HS and different abundance
distributions, including those derived from chemical modelling results. We
detected HS towards five AGB stars, all of which have high mass-loss rates
of yr and are oxygen-rich. HS
was not detected towards the carbon or S-type stars that fall in a similar
mass-loss range. For the stars in our sample with detections, we find peak
o-HS abundances relative to H between and . Overall, we conclude that HS can play a significant role in
oxygen-rich AGB stars with higher mass-loss rates, but is unlikely to play a
key role in stars of other chemical types or the lower mass-loss rate
oxygen-rich stars. For two sources, V1300 Aql and GX Mon, HS is most likely
the dominant sulphur-bearing molecule in the circumstellar envelope.Comment: 14 pages, 7 figures, accepted in A&
Chemical modelling of dust–gas chemistry within AGB outflows – II. Effect of the dust-grain size distribution
Asymptotic giant branch (AGB) stars are, together with supernovae, the main contributors of stellar dust to the interstellar medium (ISM). Dust grains formed by AGB stars are thought to be large. However, as dust nucleation and growth within their outflows are still not understood, the dust-grain size distribution (GSD) is unknown. This is an important uncertainty regarding our knowledge of the chemical and physical history of interstellar dust, as AGB dust forms ∼70 per cent∼70 per cent of the starting point of its evolution. We expand on our chemical kinetics model, which uniquely includes a comprehensive dust–gas chemistry. The GSD is now allowed to deviate from the commonly assumed canonical Mathis, Rumpl & Nordsieck distribution. We find that the specific GSD can significantly influence the dust–gas chemistry within the outflow. Our results show that the level of depletion of gas-phase species depends on the average grain surface area of the GSD. Gas-phase abundance profiles and their possible depletions can be retrieved from observations of molecular emission lines when using a range of transitions. Because of degeneracies within the prescription of GSD, specific parameters cannot be retrieved, only (a lower limit to) the average grain surface area. None the less, this can discriminate between dust composed of predominantly large or small grains. We show that when combined with other observables such as the spectral energy distribution and polarized light, depletion levels from molecular gas-phase abundance profiles can constrain the elusive GSD of the dust delivered to the ISM by AGB outflows
An ALMA view of CS and SiS around oxygen-rich AGB stars
We aim to determine the distributions of molecular SiS and CS in the
circumstellar envelopes of oxygen-rich asymptotic giant branch stars and how
these distributions differ between stars that lose mass at different rates. In
this study we analyse ALMA observations of SiS and CS emission lines for three
oxygen-rich galactic AGB stars: IK Tau, with a moderately high mass-loss rate
of M yr, and W Hya and R Dor with low mass loss
rates of M yr. These molecules are usually
more abundant in carbon stars but the high sensitivity of ALMA allows us to
detect their faint emission in the low mass-loss rate AGB stars. The high
spatial resolution of ALMA also allows us to precisely determine the spatial
distribution of these molecules in the circumstellar envelopes. We run
radiative transfer models to calculate the molecular abundances and abundance
distributions for each star. We find a spread of peak SiS abundances with
for R Dor, for W Hya, and for
IK Tau relative to H. We find lower peak CS abundances of
for R Dor, for W Hya and
for IK Tau, with some stratifications in the abundance
distributions. For IK Tau we also calculate abundances for the detected
isotopologues: CS, SiS, SiS, SiS, SiS,
SiS, and SiS. Overall the isotopic ratios we derive
for IK Tau suggest a lower metallicity than solar.Comment: 16 page
The Perils of `Soft' SUSY Breaking
We consider a two dimensional SU(N) gauge theory coupled to an adjoint
Majorana fermion, which is known to be supersymmetric for a particular value of
fermion mass. We investigate the `soft' supersymmetry breaking of the discrete
light cone quantization (DLCQ) of this theory. There are several DLCQ
formulations of this theory currently in the literature and they naively appear
to behave differently under `soft' supersymmetry breaking at finite resolution.
We show that all these formulations nevertheless yield identical bound state
masses in the decompactification limit of the light-like circle. Moreover, we
are able to show that the supersymmetry-inspired version of DLCQ (so called
`SDLCQ') provides the best rate of convergence of DLCQ bound state masses
towards the actual continuum values, except possibly near or at the critical
fermion mass. In this last case, we discuss improved extrapolation schemes that
must supplement the DLCQ algorithm in order to obtain correct continuum bound
state masses. Interestingly, when we truncate the Fock space to two particles,
the SDLCQ prescription presented here provides a scheme for improving the rate
of convergence of the massive t'Hooft model. Thus the supersymmetry-inspired
SDLCQ prescription is applicable to theories without supersymmetry.Comment: 11 pages, Latex; 2 figures (EPS); Numerical results extended;
conclusions revise
An ALMA view of SO and SO around oxygen-rich AGB stars
We present and analyse SO and SO, recently observed with high angular
resolution and sensitivity in a spectral line survey with ALMA, for two
oxygen-rich AGB stars: the low mass-loss rate R Dor and high mass-loss rate IK
Tau. We analyse 8 lines of SO detected towards both stars, 78 lines of
SOdetected towards R Dor and 52 lines of SO detected towards IK Tau. We
detect several lines of SO, SO and SO towards both
stars, and tentatively SO towards R Dor, and hence derive isotopic
ratios for these species. The spatially resolved observations show us that the
two sulphur oxides are co-located towards R Dor and trace out the same wind
structures in the circumstellar envelope (CSE). Much of the emission is well
reproduced with a Gaussian abundance distribution spatially centred on the
star. Emission from the higher energy levels of SO and SO towards R Dor
provide evidence in support of a rotating inner region of gas identified in
earlier work. The new observations allow us to refine the abundance
distribution of SO in IK Tau derived from prior observations with single
antennas, and confirm the distribution is shell-like with the peak in the
fractional abundance not centred on the star. The confirmation of different
types of SO abundance distributions will help fine-tune chemical models and
allows for an additional method to discriminate between low and high mass-loss
rates for oxygen-rich AGB stars.Comment: Accepted for publication in MNRA
Dutch patients, retail chicken meat and poultry share the same ESBL genes, plasmids and strains
Intestinal carriage of extended-spectrum beta-lactamase (ESBL) -producing bacteria in food-producing animals and contamination of retail meat may contribute to increased incidences of infections with ESBL-producing bacteria in humans. Therefore, distribution of ESBL genes, plasmids and strain genotypes in Escherichia coli obtained from poultry and retail chicken meat in the Netherlands was determined and defined as ‘poultry-associated’ (PA). Subsequently, the proportion of E. coli isolates with PA ESBL genes, plasmids and strains was quantified in a representative sample of clinical isolates. The E. coli were derived from 98 retail chicken meat samples, a prevalence survey among poultry, and 516 human clinical samples from 31 laboratories collected during a 3-month period in 2009. Isolates were analysed using an ESBL-specific microarray, sequencing of ESBL genes, PCR-based replicon typing of plasmids, plasmid multi-locus sequence typing (pMLST) and strain genotyping (MLST). Six ESBL genes were defined as PA (blaCTX-M-1, blaCTX-M-2, blaSHV-2, blaSHV-12, blaTEM-20, blaTEM-52): 35% of the human isolates contained PA ESBL genes and 19% contained PA ESBL genes located on IncI1 plasmids that were genetically indistinguishable from those obtained from poultry (meat). Of these ESBL genes, 86% were blaCTX-M-1 and blaTEM-52 genes, which were also the predominant genes in poultry (78%) and retail chicken meat (75%). Of the retail meat samples, 94% contained ESBL-producing isolates of which 39% belonged to E. coli genotypes also present in human samples. These findings are suggestive for transmission of ESBL genes, plasmids and E. coli isolates from poultry to humans, most likely through the food chain
Determining the effects of clumping and porosity on the chemistry in a non-uniform AGB outflow
(abridged) In the inner regions of AGB outflows, several molecules have been
detected with abundances much higher than those predicted from thermodynamic
equilibrium (TE) chemical models. The presence of the majority of these species
can be explained by shock-induced non-TE chemical models, where shocks caused
by the pulsating star take the chemistry out of TE in the inner region.
Moreover, a non-uniform density structure has been detected in several AGB
outflows. A detailed parameter study on the quantitative effects of a
non-homogeneous outflow has so far not been performed. We implement a porosity
formalism for treating the increased leakage of light associated with radiation
transport through a clumpy, porous medium. The effects from the altered UV
radiation field penetration on the chemistry, accounting also for the increased
reaction rates of two-body processes in the overdense clumps, are examined. We
present a parameter study of the effect of clumping and porosity on the
chemistry throughout the outflow. Both the higher density within the clumps and
the increased UV radiation field penetration have an important impact on the
chemistry, as they both alter the chemical pathways. The increased amount of UV
radiation in the inner region leads to photodissociation of parent species,
releasing the otherwise deficient elements. We find an increased abundance in
the inner region of all species not expected to be present assuming TE
chemistry, such as HCN in O-rich outflows, HO in C-rich outflows, and
NH in both. Outflows whose clumps have a large overdensity and that are
very porous to the interstellar UV radiation field yield abundances comparable
to those observed in O- and C-rich outflows for most of the unexpected species
investigated. The inner wind abundances of HO in C-rich outflows and of
NH in O- and C-rich outflows are however underpredicted.Comment: 33 pages, 20 figures, 15 tables, accepted for publication in
Astronomy & Astrophysic
Light-cone QCD on the lattice
Ideas and recent results for light-front Hamiltonian quantisation of lattice
gauge theories.Comment: LATTICE99(Plenary), 5 pages, 3 figure
ImageNet Large Scale Visual Recognition Challenge
The ImageNet Large Scale Visual Recognition Challenge is a benchmark in
object category classification and detection on hundreds of object categories
and millions of images. The challenge has been run annually from 2010 to
present, attracting participation from more than fifty institutions.
This paper describes the creation of this benchmark dataset and the advances
in object recognition that have been possible as a result. We discuss the
challenges of collecting large-scale ground truth annotation, highlight key
breakthroughs in categorical object recognition, provide a detailed analysis of
the current state of the field of large-scale image classification and object
detection, and compare the state-of-the-art computer vision accuracy with human
accuracy. We conclude with lessons learned in the five years of the challenge,
and propose future directions and improvements.Comment: 43 pages, 16 figures. v3 includes additional comparisons with PASCAL
VOC (per-category comparisons in Table 3, distribution of localization
difficulty in Fig 16), a list of queries used for obtaining object detection
images (Appendix C), and some additional reference
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