96 research outputs found
Renormalizing Partial Differential Equations
In this review paper, we explain how to apply Renormalization Group ideas to
the analysis of the long-time asymptotics of solutions of partial differential
equations. We illustrate the method on several examples of nonlinear parabolic
equations. We discuss many applications, including the stability of profiles
and fronts in the Ginzburg-Landau equation, anomalous scaling laws in
reaction-diffusion equations, and the shape of a solution near a blow-up point.Comment: 34 pages, Latex; [email protected]; [email protected]
Special fast diffusion with slow asymptotics. Entropy method and flow on a Riemannian manifold
We consider the asymptotic behaviour of positive solutions of the
fast diffusion equation
posed for x\in\RR^d, , with a precise value for the exponent
. The space dimension is so that , and even
for . This case had been left open in the general study \cite{BBDGV} since
it requires quite different functional analytic methods, due in particular to
the absence of a spectral gap for the operator generating the linearized
evolution.
The linearization of this flow is interpreted here as the heat flow of the
Laplace-Beltrami operator of a suitable Riemannian Manifold (\RR^d,{\bf g}),
with a metric which is conformal to the standard \RR^d metric.
Studying the pointwise heat kernel behaviour allows to prove {suitable
Gagliardo-Nirenberg} inequalities associated to the generator. Such
inequalities in turn allow to study the nonlinear evolution as well, and to
determine its asymptotics, which is identical to the one satisfied by the
linearization. In terms of the rescaled representation, which is a nonlinear
Fokker--Planck equation, the convergence rate turns out to be polynomial in
time. This result is in contrast with the known exponential decay of such
representation for all other values of .Comment: 37 page
Determination of dichlobenil and its major metabolite (BAM) in onions by PTVâGCâMS using PARAFAC2 and experimental design methodology
The optimization of a GCâMS analytical procedure which includes derivatization, Quick Easy Cheap Effective Rugged and Safe (QuEChERS) and programmed temperature vaporization (PTV) using design of experiments is performed to determine 2,6-dichlorobenzonitrile (dichlobenil) and 2,6-dichlorobenzamide (BAM) in onions, using 3,5-dichlorobenzonitrile and 2,4-dichlorobenzamide as internal standards. The use of a central composite design and two D-optimal designs, together with the desirability function, makes it possible to significantly reduce the economic, time and environmental cost of the study. The usefulness of PARAFAC2 for solving problems as the interference of unexpected derivatization artifacts unavoidably linked to some derivatization agents, or the presence of coeluents from the complex matrix, which share m/z ratios with the target compounds, is shown. The limits of decision (CCα) of the optimized procedure, 5.00 ÎŒg kgâ 1 for dichlobenil and 1.55 ÎŒg kgâ 1 for BAM (α = 0.05), are below the maximum residue limit (MRL) established by the EU for dichlobenil (20 ÎŒg kgâ 1) in this commodity.Ministerio de EconomĂa y Competitividad (CTQ2011-26022) and Junta
de Castilla y LeĂłn (BU108A11-2
Probing quantum gravity using photons from a flare of the active galactic nucleus Markarian 501 observed by the MAGIC telescope
We analyze the timing of photons observed by the MAGIC telescope during a
flare of the active galactic nucleus Mkn 501 for a possible correlation with
energy, as suggested by some models of quantum gravity (QG), which predict a
vacuum refractive index \simeq 1 + (E/M_{QGn})^n, n = 1,2. Parametrizing the
delay between gamma-rays of different energies as \Delta t =\pm\tau_l E or
\Delta t =\pm\tau_q E^2, we find \tau_l=(0.030\pm0.012) s/GeV at the 2.5-sigma
level, and \tau_q=(3.71\pm2.57)x10^{-6} s/GeV^2, respectively. We use these
results to establish lower limits M_{QG1} > 0.21x10^{18} GeV and M_{QG2} >
0.26x10^{11} GeV at the 95% C.L. Monte Carlo studies confirm the MAGIC
sensitivity to propagation effects at these levels. Thermal plasma effects in
the source are negligible, but we cannot exclude the importance of some other
source effect.Comment: 12 pages, 3 figures, Phys. Lett. B, reflects published versio
Anisotropy studies around the galactic centre at EeV energies with the Auger Observatory
Data from the Pierre Auger Observatory are analyzed to search for
anisotropies near the direction of the Galactic Centre at EeV energies. The
exposure of the surface array in this part of the sky is already significantly
larger than that of the fore-runner experiments. Our results do not support
previous findings of localized excesses in the AGASA and SUGAR data. We set an
upper bound on a point-like flux of cosmic rays arriving from the Galactic
Centre which excludes several scenarios predicting sources of EeV neutrons from
Sagittarius . Also the events detected simultaneously by the surface and
fluorescence detectors (the `hybrid' data set), which have better pointing
accuracy but are less numerous than those of the surface array alone, do not
show any significant localized excess from this direction.Comment: Matches published versio
The VLT-FLAMES Tarantula Survey
Context. The origin of massive runaway stars is an important unsolved problem in astrophysics. Two main scenarios have been proposed, namely: dynamical ejection or release from a binary at the first core collapse. However, their relative contribution remains heavily debated.
Aims. Taking advantage of two large spectroscopic campaigns towards massive stars in 30 Doradus, we aim to provide observational constraints on the properties of the O-type runaway population in the most massive active star-forming region in the Local Group.
Methods. We used radial velocity measurements of the O-type star populations in 30 Doradus obtained by the VLT-FLAMES Tarantula Survey and the Tarantula Massive Binary Monitoring to identify single and binary O-type runaways. Here, we discuss the rotational properties of the detected runaways and qualitatively compare the observations with expectations of ejection scenarios.
Results. We identified 23 single and one binary O-type runaway objects, most of them located outside the main star-forming regions in 30 Doradus. We find an overabundance of rapid rotators (ve sin iâ>â200 km sâ1) among the runaway population, thus providing an explanation for the observed overabundance of rapidly rotating stars in the 30 Doradus field. Considerations of the projected rotation rates and runaway line-of-sight velocities reveal a conspicuous absence of rapidly rotating (ve sin iâ>â210 km sâ1), fast-moving (vlosâ>â60 km sâ1) runaway stars in our sample, strongly suggesting the presence of two different populations of runaway stars: a population of rapidly spinning but slowly moving runaway stars and a population of fast-moving but slowly rotating ones. These are detected with a ratio close to 2:1 in our sample.
Conclusions. We argue that slowly moving but rapidly spinning runaway stars result from binary ejections, while rapidly moving but slowly spinning runaways could result from dynamical ejections. Given that detection biases will more strongly impact the slow-moving runaway population, our results suggest that the binary evolution scenario dominates the current massive runaway star population in 30 Doradus
Cluster Density and the IMF
Observed variations in the IMF are reviewed with an emphasis on environmental
density. The remote field IMF studied in the LMC by several authors is clearly
steeper than most cluster IMFs, which have slopes close to the Salpeter value.
Local field regions of star formation, like Taurus, may have relatively steep
IMFs too. Very dense and massive clusters, like super star clusters, could have
flatter IMFs, or inner-truncated IMFs. We propose that these variations are the
result of three distinct processes during star formation that affect the mass
function in different ways depending on mass range. At solar to intermediate
stellar masses, gas processes involving thermal pressure and supersonic
turbulence determine the basic scale for stellar mass, starting with the
observed pre-stellar condensations, and they define the mass function from
several tenths to several solar masses. Brown dwarfs require extraordinarily
high pressures for fragmentation from the gas, and presumably form inside the
pre-stellar condensations during mutual collisions, secondary fragmentations,
or in disks. High mass stars form in excess of the numbers expected from pure
turbulent fragmentation as pre-stellar condensations coalesce and accrete with
an enhanced gravitational cross section. Variations in the interaction rate,
interaction strength, and accretion rate among the primary fragments formed by
turbulence lead to variations in the relative proportions of brown dwarfs,
solar to intermediate mass stars, and high mass stars.Comment: 14 pages, 3 figures, to be published in ``IMF@50: A Fest-Colloquium
in honor of Edwin E. Salpeter,'' held at Abbazia di Spineto, Siena, Italy,
May 16-20, 2004. Kluwer Academic Publishers; edited by E. Corbelli, F. Palla,
and H. Zinnecke
Early life risk factors and their cumulative effects as predictors of overweight in Spanish children
Objectives: To explore early life risk factors of overweight/obesity at age 6 years and their cumulative effects on overweight/obesity at ages 2, 4 and 6 years.
Methods: Altogether 1031 Spanish children were evaluated at birth and during a 6-year follow-up. Early life risk factors included: parental overweight/obesity, parental origin/ethnicity, maternal smoking during pregnancy, gestational weight gain, gestational age, birth weight, caesarean section, breastfeeding practices and rapid infant weight gain collected via hospital records. Cumulative effects were assessed by adding up those early risk factors that significantly increased the risk of overweight/obesity. We conducted binary logistic regression models.
Results: Rapid infant weight gain (OR 2.29, 99% CI 1.54â3.42), maternal overweight/obesity (OR 1.93, 99% CI 1.27â2.92), paternal overweight/obesity (OR 2.17, 99% CI 1.44â3.28), Latin American/Roma origin (OR 3.20, 99% CI 1.60â6.39) and smoking during pregnancy (OR 1.61, 99% CI 1.01â2.59) remained significant after adjusting for confounders. A higher number of early life risk factors accumulated was associated with overweight/obesity at age 6 years but not at age 2 and 4 years.
Conclusions: Rapid infant weight gain, parental overweight/obesity, maternal smoking and origin/ethnicity predict childhood overweight/obesity and present cumulative effects. Monitoring children with rapid weight gain and supporting a healthy parental weight are important for childhood obesity prevention
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