1,862 research outputs found
Modes of clustered star formation
The realization that most stars form in clusters, raises the question of
whether star/planet formation are influenced by the cluster environment. The
stellar density in the most prevalent clusters is the key factor here. Whether
dominant modes of clustered star formation exist is a fundamental question.
Using near-neighbour searches in young clusters Bressert et al. (2010) claim
this not to be the case and conclude that star formation is continuous from
isolated to densely clustered. We investigate under which conditions
near-neighbour searches can distinguish between different modes of clustered
star formation. Near-neighbour searches are performed for model star clusters
investigating the influence of the combination of different cluster modes,
observational biases, and types of diagnostic and find that the cluster density
profile, the relative sample sizes, limitations in observations and the choice
of diagnostic method decides whether modelled modes of clustered star formation
are detected. For centrally concentrated density distributions spanning a wide
density range (King profiles) separate cluster modes are only detectable if the
mean density of the individual clusters differs by at least a factor of ~65.
Introducing a central cut-off can lead to underestimating the mean density by
more than a factor of ten. The environmental effect on star and planet
formation is underestimated for half of the population in dense systems. A
analysis of a sample of cluster environments involves effects of superposition
that suppress characteristic features and promotes erroneous conclusions. While
multiple peaks in the distribution of the local surface density imply the
existence of different modes, the reverse conclusion is not possible. Equally,
a smooth distribution is not a proof of continuous star formation, because such
a shape can easily hide modes of clustered star formation (abridged)Comment: 9 pages, 6 figures, accepted by A&
Heavy Quark Interactions and Quarkonium Binding
We consider heavy quark interactions in quenched and unquenched lattice QCD.
In a region just above the deconfinement point, non-Abelian gluon polarization
leads to a strong increase in the binding. Comparing quark-antiquark and
quark-quark interaction, the dependence of the binding on the separation
distance is found to be the same for the colorless singlet and
the colored anti-triplet state. In a potential model description of
in-medium behavior, this enhancement of the binding leads to a
survival up to temperatures of 1.5 or higher; it could also result in
flow.Comment: 8 pages, 8 Figures; invited talk at "Strangeness in Quark Matter
2008", Beijing/China, to appear in the Proceeding
Conjugate gradient solvers on Intel Xeon Phi and NVIDIA GPUs
Lattice Quantum Chromodynamics simulations typically spend most of the
runtime in inversions of the Fermion Matrix. This part is therefore frequently
optimized for various HPC architectures. Here we compare the performance of the
Intel Xeon Phi to current Kepler-based NVIDIA Tesla GPUs running a conjugate
gradient solver. By exposing more parallelism to the accelerator through
inverting multiple vectors at the same time, we obtain a performance greater
than 300 GFlop/s on both architectures. This more than doubles the performance
of the inversions. We also give a short overview of the Knights Corner
architecture, discuss some details of the implementation and the effort
required to obtain the achieved performance.Comment: 7 pages, proceedings, presented at 'GPU Computing in High Energy
Physics', September 10-12, 2014, Pisa, Ital
Interplay between chiral and axial symmetries in a SU(2) Nambu--Jona-Lasinio Model with the Polyakov loop
We consider a two flavor Polyakov--Nambu--Jona-Lasinio (PNJL) model where the
Lagrangian includes an interaction term that explicitly breaks the U
anomaly. At finite temperature, the restoration of chiral and axial symmetries,
signaled by the behavior of several observables, is investigated. We compare
the effects of two regularizations at finite temperature, one of them, that
allows high momentum quarks states, leading to the full recovery of chiral
symmetry. From the analysis of the behavior of the topological susceptibility
and of the mesonic masses of the axial partners, it is found in the SU(2) model
that, unlike the SU(3) results, the recovery of the axial symmetry is not a
consequence of the full recovery of the chiral symmetry. Thus, one needs to use
an additional idea, by means of a temperature dependence of the anomaly
coefficient, that simulates instanton suppression effects.Comment: 21 pages, 5 figures; PRD versio
Numerical study of the equation of state for two flavor QCD at finite density
We discuss the equation of state for 2 flavor QCD at non-zero temperature and
density. Derivatives of with respect to quark chemical potential
up to fourth order are calculated, enabling estimates of the pressure,
quark number density and associated susceptibilities as functions of
via a Taylor series expansion. It is found that the fluctuations in the quark
number density increase in the vicinity of the phase transition temperature and
the susceptibilities start to develop a pronounced peak as is
increased. This suggests the presence of a critical endpoint in the plane.Comment: 5 pages, 4 figures, Talk at Confinement 200
String breaking in Lattice QCD
The separation of a heavy quark and antiquark pair leads to the formation of
a tube of flux, or string, which should break in the presence of light
quark-antiquark pairs. This expected zero temperature phenomenon has proven
elusive in simulations of lattice QCD. We present simulation results that show
that the string does break in the confining phase at nonzero temperature.Comment: LATTICE98(hightemp), 3 pages, 4 figures, LaTe
Effective potential for Polyakov loops from a center symmetric effective theory in three dimensions
We present lattice simulations of a center symmetric dimensionally reduced
effective field theory for SU(2) Yang Mills which employ thermal Wilson lines
and three-dimensional magnetic fields as fundamental degrees of freedom. The
action is composed of a gauge invariant kinetic term, spatial gauge fields and
a potential for the Wilson line which includes a "fuzzy" bag term to generate
non-perturbative fluctuations. The effective potential for the Polyakov loop is
extracted from the simulations including all modes of the loop as well as for
cooled configuration where the hard modes have been averaged out. The former is
found to exhibit a non-analytic contribution while the latter can be described
by a mean-field like ansatz with quadratic and quartic terms, plus a
Vandermonde potential which depends upon the location within the phase diagram.Comment: 10 pages, 22 figures, v2: published version (minor clarifications,
update of reference list
Redescription of Doryphoribius vietnamensis (Iharos, 1969) (Tardigrada) comb. nov. on the basis of the holotype and additional material from China
Hypsibius vietnamensis IHAROS, 1969 was described from Huong tich, 60 km south-west of Hanoi in Vietnam. RAMAZZOTTI andMAUCCI (1983) transferred it to the genus Isohypsibius on the basis of drawings of claws. BEASLEY and CLEVELAND (1996) reported Isohypsibius vietnamensis (IHAROS, 1969) from Yunnan Province in China. We have re-examined specimens described by IHAROS (holotype) and those from China and determined that they should be transferred to the genus Doryphoribius due to their bucco-pharyngeal apparatus structure. Many years ago Hypsibius (Isohypsibius) flavus IHAROS 1966 was also transferred to the genus Doryphoribius SCHUSTER and TOFTNER (1982). It may suggest that some other species of genus Isohypsibius should also by transferred to genus Doryphoribius. Genus Doryphoribius differs from Isohypsibius by the presence of the ventral reinforcement bar on the buccal tube
QCD at non-zero chemical potential and temperature from the lattice
A study of QCD at non-zero chemical potential, mu, and temperature, T, is
performed using the lattice technique. The transition temperature (between the
confined and deconfined phases) is determined as a function of mu and is found
to be in agreement with other work. In addition the variation of the pressure
and energy density with mu is obtained for small positive mu. These results are
of particular relevance for heavy-ion collision experiments.Comment: Invited paper presented at the Joint Workshop on Physics at the
Japanese Hadron Facility, March 2002, Adelaide. 10 pages, uses
ws-procs9x6.cls style file (provided
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