15,107 research outputs found
Potential formation sites of super star clusters in ultra-luminous infrared galaxies
Recent observational results on high spatial resolution images of
ultra-luminous infrared galaxies (ULIGs) have revealed very luminous, young,
compact, and heavily obscured super star clusters in their central regions,
suggested to be formed by gas-rich major mergers. By using stellar and gaseous
numerical simulations of galaxy mergers, we firstly demonstrate that the
central regions of ULIGs are the most promising formation sites of super star
clusters owing to the rather high gaseous pressure of the interstellar medium.
Based on simple analytical arguments, we secondly discuss the possibility that
super star clusters in an ULIG can be efficiently transferred into the nuclear
region owing to dynamical friction and consequently merge with one another to
form a single compact stellar nucleus with a seed massive black hole. We thus
suggest that multiple merging between super star clusters formed by nuclear
starbursts in the central regions of ULIGs can result in the formation of
massive black holes.Comment: 12 pages 4 figures, 2001, accepted by ApJ
Time lower bounds for nonadaptive turnstile streaming algorithms
We say a turnstile streaming algorithm is "non-adaptive" if, during updates,
the memory cells written and read depend only on the index being updated and
random coins tossed at the beginning of the stream (and not on the memory
contents of the algorithm). Memory cells read during queries may be decided
upon adaptively. All known turnstile streaming algorithms in the literature are
non-adaptive.
We prove the first non-trivial update time lower bounds for both randomized
and deterministic turnstile streaming algorithms, which hold when the
algorithms are non-adaptive. While there has been abundant success in proving
space lower bounds, there have been no non-trivial update time lower bounds in
the turnstile model. Our lower bounds hold against classically studied problems
such as heavy hitters, point query, entropy estimation, and moment estimation.
In some cases of deterministic algorithms, our lower bounds nearly match known
upper bounds
Circular String-Instabilities in Curved Spacetime
We investigate the connection between curved spacetime and the emergence of
string-instabilities, following the approach developed by Loust\'{o} and
S\'{a}nchez for de Sitter and black hole spacetimes. We analyse the linearised
equations determining the comoving physical (transverse) perturbations on
circular strings embedded in Schwarzschild, Reissner-Nordstr\"{o}m and de
Sitter backgrounds. In all 3 cases we find that the "radial" perturbations grow
infinitely for (ring-collapse), while the "angular"
perturbations are bounded in this limit. For we find that
the perturbations in both physical directions (perpendicular to the string
world-sheet in 4 dimensions) blow up in the case of de Sitter space. This
confirms results recently obtained by Loust\'{o} and S\'{a}nchez who considered
perturbations around the string center of mass.Comment: 24 pages Latex + 2 figures (not included). Observatoire de Paris,
Meudon No. 9305
The Holographic Universe
We present a holographic description of four-dimensional single-scalar
inflationary universes in terms of a three-dimensional quantum field theory.
The holographic description correctly reproduces standard inflationary
predictions in their regime of applicability. In the opposite case, wherein
gravity is strongly coupled at early times, we propose a holographic
description in terms of perturbative QFT and present models capable of
satisfying the current observational constraints while exhibiting a
phenomenology distinct from standard inflation. This provides a qualitatively
new method for generating a nearly scale-invariant spectrum of primordial
cosmological perturbations.Comment: 20 pages, 5 figs; extended version of arXiv:0907.5542 including
background material and detailed derivations. To appear in Proceedings of 1st
Mediterranean Conference on Classical and Quantum Gravit
Ab initio linear scaling response theory: Electric polarizability by perturbed projection
A linear scaling method for calculation of the static {\em ab inito} response
within self-consistent field theory is developed and applied to calculation of
the static electric polarizability. The method is based on density matrix
perturbation theory [Niklasson and Challacombe, cond-mat/0311591], obtaining
response functions directly via a perturbative approach to spectral projection.
The accuracy and efficiency of the linear scaling method is demonstrated for a
series of three-dimensional water clusters at the RHF/6-31G** level of theory.
Locality of the response under a global electric field perturbation is
numerically demonstrated by approximate exponential decay of derivative density
matrix elements.Comment: 4.25 pages in PRL format, 2 figure
Quasicontinuum -decay of Zr: benchmarking indirect () cross section measurements for the -process
Nuclear level densities (NLDs) and -ray strength functions
(SFs) have been extracted from particle- coincidences of the
Zr()Zr and Zr()Zr
reactions using the Oslo method. The new Zr SF data, combined
with photonuclear cross sections, cover the whole energy range from ~MeV up to the giant dipole resonance at ~MeV. The wide-range SF data display structures at ~MeV, compatible with a superposition of the spin-flip
resonance and a pygmy resonance. Furthermore, the SF shows a
minimum at ~MeV and an increase at lower -ray
energies. The experimentally constrained NLDs and SFs are shown to
reproduce known () and Maxwellian-averaged cross sections for
Zr using the {\sf TALYS} reaction code, thus serving as a benchmark
for this indirect method of estimating () cross sections for Zr
isotopes.Comment: 10 pages and 9 figure
Handling of chronic cases of pyaemia/osteomyelitis in finishing pigs in Denmark – is de-boning necessary to maintain food safety?
Meat inspection is up for debate and one issue deals with how to handle chronic cases of pyaemia/ osteomyelitis in finishing pigs. In Denmark, such carcasses are required to be de-boned to avoid presence of osteomyelitis not found in the rework area. Around 40,000 pigs (0.24%) are subjected to de-boning in Denmark per year, and the associated costs amount to approx. €3 million. The questions are: 1) is the meat from such pigs fit for human consumption? 2) Is de-boning necessary, or do the meat inspectors find what they should in the rework area? And 3) which alternative practices could replace de-boning? To address this, data covering 1 year were extracted from the Danish Slaughterhouse Database including information from the 7 largest Danish abattoirs. Registration schemes covering findings during de-boning and the result of de-boning (approved/ condemned) were provided by the individual abattoirs. Additionally, a questionnaire survey was undertaken regarding the de-boning personals’ experience related to de-boning. Furthermore, samples from 102 pigs sent for de-boning at one slaughterhouse were collected. These samples included abscesses found in pigs at the rework area plus one muscle sample per pig. All samples underwent microbiological investigation. As a control group, microbiological results obtained from a similar study from carcasses unconditionally approved at meat inspection were included. Staphylococcus aureus, which has the potential to cause human illness, was found in 15 abscesses and 1 muscle of the 102 pigs sent for de-boning. S. aureus was also found in 1 of the 60 control samples. The results were included in a risk assessment that revealed the same very low health risk related to consumption of meat from de-boned pigs as from fully accepted pigs. Abscesses were found at de-boning in a low proportion of the pigs, at different sites of the carcass, varying between abattoirs. The vast majority of pigs sent for de-boning were accepted after de-boning (99.7%). If routine de-boning is no longer required, then focus on a thorough inspection at the rework-area will most likely result in a higher probability of finding abscesses at that stage of inspection. Moreover, overlooked abscesses will be found during cutting. Therefore de-boning is not considered necessary and could be replaced by condemnation of the affected part(s) only
Passive-scheme analysis for solving untrusted source problem in quantum key distribution
As a practical method, the passive scheme is useful to monitor the photon
statistics of an untrusted source in a "Plug & Play" quantum key distribution
(QKD) system. In a passive scheme, three kinds of monitor mode can be adopted:
average photon number (APN) monitor, photon number analyzer (PNA) and photon
number distribution (PND) monitor. In this paper, the security analysis is
rigorously given for APN monitor, while for PNA, the analysis including
statistical fluctuation and random noise, is addressed with a confidence level.
The results show that the PNA can achieve better performance than the APN
monitor and can asymptotically approach the theoretical limit of the PND
monitor. Also, the passive scheme with the PNA works efficiently when the
signal-to-noise ratio () is not too low and so is highly applicable to
solve the untrusted source problem in the QKD system.Comment: 8 pages, 6 figures, published versio
Strings Propagating in the 2+1 Dimensional Black Hole Anti de Sitter Spacetime
We study the string propagation in the 2+1 black hole anti de Sitter
background (2+1 BH-ADS). We find the first and second order fluctuations around
the string center of mass and obtain the expression for the string mass. The
string motion is stable, all fluctuations oscillate with real frequencies and
are bounded, even at We compare with the string motion in the ordinary
black hole anti de Sitter spacetime, and in the black string background, where
string instabilities develop and the fluctuations blow up at We find the
exact general solution for the circular string motion in all these backgrounds,
it is given closely and completely in terms of elliptic functions. For the
non-rotating black hole backgrounds the circular strings have a maximal bounded
size they contract and collapse into No indefinitely growing
strings, neither multi-string solutions are present in these backgrounds. In
rotating spacetimes, both the 2+1 BH-ADS and the ordinary Kerr-ADS, the
presence of angular momentum prevents the string from collapsing into
The circular string motion is also completely solved in the black hole de
Sitter spacetime and in the black string background (dual of the 2+1 BH-ADS
spacetime), in which expanding unbounded strings and multi-string solutions
appear.Comment: Latex, 54 pages + 2 tables and 4 figures (not included). PARIS-DEMIRM
94/01
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