4,454 research outputs found
Structuring and support by Alfven waves around prestellar cores
Observations of molecular clouds show the existence of starless, dense cores,
threaded by magnetic fields. Observed line widths indicate these dense
condensates to be embedded in a supersonically turbulent environment. Under
these conditions, the generation of magnetic waves is inevitable. In this
paper, we study the structure and support of a 1D plane-parallel,
self-gravitating slab, as a monochromatic, circularly polarized Alfven wave is
injected in its central plane. Dimensional analysis shows that the solution
must depend on three dimensionless parameters. To study the nonlinear,
turbulent evolution of such a slab, we use 1D high resolution numerical
simulations. For a parameter range inspired by molecular cloud observations, we
find the following. 1) A single source of energy injection is sufficient to
force persistent supersonic turbulence over several hydrostatic scale heights.
2) The time averaged spatial extension of the slab is comparable to the
extension of the stationary, analytical WKB solution. Deviations, as well as
the density substructure of the slab, depend on the wave-length of the injected
wave. 3) Energy losses are dominated by loss of Poynting-flux and increase with
increasing plasma beta. 4) Good spatial resolution is mandatory, making similar
simulations in 3D currently prohibitively expensive.Comment: 13 pages, 8 figures, accepted for publication in A&A. The manuscript
with full color, high-resolution, figures can be downloaded from
http://www.astro.phys.ethz.ch/papers/folini/folini_p_nf.htm
Radiative instabilities in simulations of spherically symmetric supernova blast waves
High-resolution simulations of the cooling regions of spherically symmetric
supernova remnants demonstrate a strong radiative instability. This
instability, whose presence is dependent on the shock velocity, causes
large-amplitude fluctuations in the shock velocity. The fluctuations begin
almost immediately after the radiative phase begins (upon shell formation) if
the shock velocity lies in the unstable range; they last until the shock slows
to speeds less than approximately 130 km/s. We find that shock-velocity
fluctuations from the reverberations of waves within the remnant are small
compared to those due to the instability. Further, we find (in plane-parallel
simulations) that advected inhomogeneities from the external medium do not
interfere with the qualitative nature of the instability-driven fluctuations.
Large-amplitude inhomogeneities may alter the phases of shock-velocity
fluctuations, but do not substantially reduce their amplitudes.Comment: 18 pages text, LaTeX/AASTeX (aaspp4); 10 figures; accepted by Ap
Effects of magnetic fields on radiatively overstable shock waves
We discuss high-resolution simulations of one-dimensional, plane-parallel
shock waves with mean speeds between 150 and 240 km/s propagating into gas with
Alfven velocities up to 40 km/s and outline the conditions under which these
radiative shocks experience an oscillatory instability in the cooling length,
shock velocity, and position of the shock front. We investigate two forms of
postshock cooling: a truncated single power law and a more realistic piecewise
power law. The degree of nonlinearity of the instability depends strongly on
the cooling power law and the Alfven Mach number: for power-law indices \alpha
< 0 typical magnetic field strengths may be insufficient either to stabilize
the fundamental oscillatory mode or to prevent the oscillations from reaching
nonlinear amplitudes.Comment: 11 text pages, LaTeX/AASTeX (aaspp4); 5 figures; accepted by Ap
Lithium depletion in solar-like stars: effect of overshooting based on realistic multi-dimensional simulations
We study lithium depletion in low-mass and solar-like stars as a function of
time, using a new diffusion coefficient describing extra-mixing taking place at
the bottom of a convective envelope. This new form is motivated by
multi-dimensional fully compressible, time implicit hydrodynamic simulations
performed with the MUSIC code. Intermittent convective mixing at the convective
boundary in a star can be modeled using extreme value theory, a statistical
analysis frequently used for finance, meteorology, and environmental science.
In this letter, we implement this statistical diffusion coefficient in a
one-dimensional stellar evolution code, using parameters calibrated from
multi-dimensional hydrodynamic simulations of a young low-mass star. We propose
a new scenario that can explain observations of the surface abundance of
lithium in the Sun and in clusters covering a wide range of ages, from
50 Myr to 4 Gyr. Because it relies on our physical model of convective
penetration, this scenario has a limited number of assumptions. It can explain
the observed trend between rotation and depletion, based on a single additional
assumption, namely that rotation affects the mixing efficiency at the
convective boundary. We suggest the existence of a threshold in stellar
rotation rate above which rotation strongly prevents the vertical penetration
of plumes and below which rotation has small effects. In addition to providing
a possible explanation for the long standing problem of lithium depletion in
pre-main sequence and main sequence stars, the strength of our scenario is that
its basic assumptions can be tested by future hydrodynamic simulations.Comment: 7 pages, 3 figures, Accepted for publication in ApJ Letter
Prevention of Bloodstream Infections With Central Venous Catheters Treated With Anti-Infective Agents Depends on Catheter Type and Insertion Time: Evidence From a Meta-Analysis
Objective: To test the evidence that the risk of infection related to central venous catheters (CVCs) is decreased by anti-infective coating or cuffing. Design: Systematic review of randomized, controlled trials comparing anti-infective with inactive (control) CVCs. Interventions: Average insertion times were taken as a measurement of the length of insertion. Dichotomous data were combined using a fixed effect model and expressed as odds ratio (OR) with 95% confidence interval (CI95). Results: Two trials on antibiotic coating (343 CVCs) had an average insertion time of 6 days; the risk of BSI decreased from 5.1% with control to 0% with anti-infective catheters. There were no trials with longer average insertion times. In three trials on silver collagen cuffs (422 CVCs), the average insertion time ranged from 5 to 8.2 days (median, 7 days); the risk of BSI was 5.6% with control and 3.2% with anti-infective catheters. In another trial on silver collagen cuffs (101 CVCs), the average insertion time was 38 days; the risk of BSI was 3.7% with control and 4.3% with anti-infective catheters. In five trials on chlorhexidine-silver sulfadiazine coating (1,269 CVCs), the average insertion time ranged from 5.2 to 7.5 days (median, 6 days); the risk of BSI decreased from 4.1% with control to 1.9% with anti-infective catheters. In five additional trials on chlorhexidine-silver sulfadiazine coating (1,544 CVCs), the average insertion time ranged from 7.8 to 20 days (median, 12 days); the risk of BSI was 4.5% with control and 4.2% with anti-infective catheters. Conclusions: Antibiotic and chlorhexidine-silver sulfadiazine coatings are anti-infective for short (approximately 1 week) insertion times. For longer insertion times, there are no data on antibiotic coating, and there is evidence of lack of effect for chlorhexidine-silver sulfadiazine coating. For silver-impregnated collagen cuffs, there is evidence of lack of effect for both short- and long-term insertio
Efficient non-parametric Bayesian hawkes processes
© 2019 International Joint Conferences on Artificial Intelligence. All rights reserved. In this paper, we develop an efficient nonparametric Bayesian estimation of the kernel function of Hawkes processes. The non-parametric Bayesian approach is important because it provides flexible Hawkes kernels and quantifies their uncertainty. Our method is based on the cluster representation of Hawkes processes. Utilizing the stationarity of the Hawkes process, we efficiently sample random branching structures and thus, we split the Hawkes process into clusters of Poisson processes. We derive two algorithms - a block Gibbs sampler and a maximum a posteriori estimator based on expectation maximization - and we show that our methods have a linear time complexity, both theoretically and empirically. On synthetic data, we show our methods to be able to infer flexible Hawkes triggering kernels. On two large-scale Twitter diffusion datasets, we show that our methods outperform the current state-of-the-art in goodness-of-fit and that the time complexity is linear in the size of the dataset. We also observe that on diffusions related to online videos, the learned kernels reflect the perceived longevity for different content types such as music or pets videos
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