1,749 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&
Charmonium in Medium: From Correlators to Experiment
We set up a framework in which in-medium charmonium properties are
constrained by thermal lattice QCD and subsequently implemented into a thermal
rate equation enabling the comparison with experimental data in heavy-ion
collisions. Specifically, we evaluate phenomenological consequences for
charmonium production originating from two different scenarios in which either
the free or the internal energy are identified with the in-medium 2-body
potential between charm and anti-charm quarks. These two scenarios represent
"melting temperatures" of approximately 1.25\, ("weak binding")
and 2\, ("strong binding"), respectively. Within current uncertainties in
dissociation rates and charm-quark momentum spectra, both scenarios can
reproduce the centrality dependence of inclusive yields in nuclear
collisions at SPS and RHIC reasonably well. However, the "strong-binding"
scenario associated the the internal energy as the potential tends to better
reproduce current data on transverse momentum spectra at both SPS and RHIC.Comment: 18 pages, 30 figure
A non-perturbative estimate of the heavy quark momentum diffusion coefficient
We estimate the momentum diffusion coefficient of a heavy quark within a pure
SU(3) plasma at a temperature of about 1.5Tc. Large-scale Monte Carlo
simulations on a series of lattices extending up to 192^3*48 permit us to carry
out a continuum extrapolation of the so-called colour-electric imaginary-time
correlator. The extrapolated correlator is analyzed with the help of
theoretically motivated models for the corresponding spectral function.
Evidence for a non-zero transport coefficient is found and, incorporating
systematic uncertainties reflecting model assumptions, we obtain kappa = (1.8 -
3.4)T^3. This implies that the "drag coefficient", characterizing the time
scale at which heavy quarks adjust to hydrodynamic flow, is (1.8 - 3.4)
(Tc/T)^2 (M/1.5GeV) fm/c, where M is the heavy quark kinetic mass. The results
apply to bottom and, with somewhat larger systematic uncertainties, to charm
quarks.Comment: 18 pages. v2: clarifications adde
Critical point and scale setting in SU(3) plasma: An update
We explore a method developed in statistical physics which has been argued to
have exponentially small finite-volume effects, in order to determine the
critical temperature Tc of pure SU(3) gauge theory close to the continuum
limit. The method allows us to estimate the critical coupling betac of the
Wilson action for temporal extents up to Nt ~ 20 with < 0.1% uncertainties.
Making use of the scale setting parameters r0 and sqrt{t0} in the same range of
beta-values, these results lead to the independent continuum extrapolations Tc
r0 = 0.7457(45) and Tc sqrt{t0} = 0.2489(14), with the latter originating from
a more convincing fit. Inserting a conversion of r0 from literature
(unfortunately with much larger errors) yields Tc / LambdaMSbar = 1.24(10).Comment: 12 pages. v2: clarifications and references added, published versio
Two-way interconversion of millimeter-wave and optical fields in Rydberg gases
We show that cold Rydberg gases enable an efficient six-wave mixing process
where terahertz or microwave fields are coherently converted into optical
fields and vice versa. This process is made possible by the long lifetime of
Rydberg states, the strong coupling of millimeter waves to Rydberg transitions
and by a quantum interference effect related to electromagnetically induced
transparency (EIT). Our frequency conversion scheme applies to a broad spectrum
of millimeter waves due to the abundance of transitions within the Rydberg
manifold, and we discuss two possible implementations based on focussed
terahertz beams and millimeter wave fields confined by a waveguide,
respectively. We analyse a realistic example for the interconversion of
terahertz and optical fields in rubidium atoms and find that the conversion
efficiency can in principle exceed 90\%.Comment: 11 pages, 6 figures and supplementary informatio
Atom chips with two-dimensional electron gases: theory of near surface trapping and ultracold-atom microscopy of quantum electronic systems
We show that current in a two-dimensional electron gas (2DEG) can trap
ultracold atoms m away with orders of magnitude less spatial noise than
a metal trapping wire. This enables the creation of hybrid systems, which
integrate ultracold atoms with quantum electronic devices to give extreme
sensitivity and control: for example, activating a single quantized conductance
channel in the 2DEG can split a Bose-Einstein condensate (BEC) for atom
interferometry. In turn, the BEC offers unique structural and functional
imaging of quantum devices and transport in heterostructures and graphene.Comment: 5 pages, 4 figures, minor change
Towards the continuum limit in transport coefficient computations
The analytic continuation needed for the extraction of transport coefficients
necessitates in principle a continuous function of the Euclidean time variable.
We report on progress towards achieving the continuum limit for 2-point
correlator measurements in thermal SU(3) gauge theory, with specific attention
paid to scale setting. In particular, we improve upon the determination of the
critical lattice coupling and the critical temperature of pure SU(3) gauge
theory, estimating r0*Tc ~ 0.7470(7) after a continuum extrapolation. As an
application the determination of the heavy quark momentum diffusion coefficient
from a correlator of colour-electric fields attached to a Polyakov loop is
discussed.Comment: 7 pages. To appear in the Proceedings of the 31st International
Symposium on Lattice Field Theory, July 29 - August 3, 2013, Mainz, German
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