96 research outputs found
Recommended from our members
The stress-pressure relationship in simulations of MRI-induced turbulence
We determine how MRI-turbulent stresses depend on gas pressure via a suite of
unstratified shearing box simulations. Earlier numerical work reported only a
very weak dependence at best, results that call into question the canonical
alpha-disk model and the thermal stability results that follow from it. Our
simulations, in contrast, exhibit a stronger relationship, and show that
previous work was box-size limited: turbulent `eddies' were artificially
restricted by the numerical domain rather than by the scale height.
Zero-net-flux runs without physical diffusion coefficients yield a stress
proportional to , where P is pressure. The stresses are also
proportional to the grid length and hence remain numerically unconverged. The
same runs with physical diffusivities, however, give a result closer to an
alpha-disk: the stress is proportional to . Net-flux simulations
without explicit diffusion exhibit stresses proportional to , but
stronger imposed fields weaken this correlation. In summary, compressibility is
important for the saturation of the MRI, but the exact stress-pressure
relationship is difficult to ascertain in local simulations because of
numerical convergence issues and the influence of any imposed flux. As a
consequence, the interpretation of thermal stability behaviour in local
simulations is a problematic enterprise.Some of the simulations were run on the DiRAC Complexity system, operated by the University of Leicester IT Services, which forms part of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment is funded by BIS National E-Infrastructure capital grant ST/K000373/1 and STFC DiRAC Operations grant ST/K0003259/1. DiRAC is part of the UK National E-Infrastructure run. JR and HNL are partially funded by STFC grants ST/L000636/1 and ST/K501906/1. JG acknowledges support from the Max-Planck-Princeton Center for Plasma Physics.This is the final version of the article. It first appeared from Oxford University Press via http://dx.doi.org/10.1093/mnras/stv228
Novel Cytotoxic 4-Phenylfuranocoumarins from Calophyllum dispar
Eight new 4-phenylfuranocoumarins (1−8) have been isolated from the stem bark and the fruits of Calophyllum dispar, together with three known coumarins. The structures of 1−8 were established by means of spectroscopic analysis, including extensive 2D NMR studies. Some of these furanocoumarins exhibited significant cytotoxic activity against KB cells
ortho-Prenylphenol photooxygenation as a straightforward access to ortho-(2-hydroxy-3-methylbut-3-enyl)phenols
Explosion Mechanisms of Core-Collapse Supernovae
Supernova theory, numerical and analytic, has made remarkable progress in the
past decade. This progress was made possible by more sophisticated simulation
tools, especially for neutrino transport, improved microphysics, and deeper
insights into the role of hydrodynamic instabilities. Violent, large-scale
nonradial mass motions are generic in supernova cores. The neutrino-heating
mechanism, aided by nonradial flows, drives explosions, albeit low-energy ones,
of ONeMg-core and some Fe-core progenitors. The characteristics of the neutrino
emission from new-born neutron stars were revised, new features of the
gravitational-wave signals were discovered, our notion of supernova
nucleosynthesis was shattered, and our understanding of pulsar kicks and
explosion asymmetries was significantly improved. But simulations also suggest
that neutrino-powered explosions might not explain the most energetic
supernovae and hypernovae, which seem to demand magnetorotational driving. Now
that modeling is being advanced from two to three dimensions, more realism, new
perspectives, and hopefully answers to long-standing questions are coming into
reach.Comment: 35 pages, 11 figures (29 eps files; high-quality versions can be
obtained upon request); accepted by Annual Review of Nuclear and Particle
Scienc
Turbulent Magnetic Field Amplification from Spiral SASI Modes: Implications for Core-Collapse Supernovae and Proto-Neutron Star Magnetization
We extend our investigation of magnetic field evolution in three-dimensional
flows driven by the stationary accretion shock instability (SASI) with a suite
of higher-resolution idealized models of the post-bounce core-collapse
supernova environment. Our magnetohydrodynamic simulations vary in initial
magnetic field strength, rotation rate, and grid resolution. Vigorous
SASI-driven turbulence inside the shock amplifies magnetic fields
exponentially; but while the amplified fields reduce the kinetic energy of
small-scale flows, they do not seem to affect the global shock dynamics. The
growth rate and final magnitude of the magnetic energy are very sensitive to
grid resolution, and both are underestimated by the simulations. Nevertheless
our simulations suggest that neutron star magnetic fields exceeding G
can result from dynamics driven by the SASI, \emph{even for non-rotating
progenitors}.Comment: 28 pages, 17 figures, accepted for publication in the Ap
Dimension as a Key to the Neutrino Mechanism of Core-Collapse Supernova Explosions
We explore the dependence on spatial dimension of the viability of the
neutrino heating mechanism of core-collapse supernova explosions. We find that
the tendency to explode is a monotonically increasing function of dimension,
with 3D requiring 4050\% lower driving neutrino luminosity than 1D and
1525\% lower driving neutrino luminosity than 2D. Moreover, we find
that the delay to explosion for a given neutrino luminosity is always shorter
in 3D than 2D, sometimes by many hundreds of milliseconds. The magnitude of
this dimensional effect is much larger than the purported magnitude of a
variety of other effects, such as nuclear burning, inelastic scattering, or
general relativity, which are sometimes invoked to bridge the gap between the
current ambiguous and uncertain theoretical situation and the fact of robust
supernova explosions. Since real supernovae occur in three dimensions, our
finding may be an important step towards unraveling one of the most problematic
puzzles in stellar astrophysics. In addition, even though in 3D we do see
pre-explosion instabilities and blast asymmetries, unlike the situation in 2D,
we do not see an obvious axially-symmetric dipolar shock oscillation. Rather,
the free energy available to power instabilites seems to be shared by more and
more degrees of freedom as the dimension increases. Hence, the strong dipolar
axisymmetry seen in 2D and previously identified as a fundamental
characteristic of the shock hydrodynamics may not survive in 3D as a prominent
feature.Comment: Accepted to ApJ July 7th, Replaced with accepted versio
Isoflavonoids in non-leguminous families: an update
This review provides a listing of isoflavonoids reported in non-leguminous families. Reviews published to date have principally focused on plants with the richest isoflavonoid contents, the family Leguminosae. After a brief recall of the structure of isoflavonoids, we present all isoflavonoid structures encountered in non-leguminous families which may, thereby, become new plant sources for these compounds. Articles reporting on their different functions in plants are presented, as well as a brief summary discussing their potential benefits for human health. A list of 135 references is given
Genus Hydrangea: diversity of pigments and phenolic compounds
The most important collection of Hydrangea in Europe is located in Angers (France). It consists of over 700 germplasm accessions distributed in 13 species. Originating from Asia and America, they were introduced in Europe in the 18th century for their ornamental interest but medicinal properties may also be found in this genus since extracts from H. macrophylla are already described as exhibiting anti-diabetic [1], lipid lowering and anti-oxidative [2], anti-allergic [3] and antimalarial activities [4]. Management of the collection requires botanical, genetic and biochemical studies allowing good, reliable characterization of species, subspecies and varieties. In this context, the biochemical characterization of the inflorescences was undertaken to evaluate the intra and interspecific diversities of pigments and other phenolic compounds. Inflorescences are generally white, except for three species: H. macrophylla, H. involucrata and H .aspera which exhibit rose or blue flowers. Among them only H. macrophylla was previously studied for sepal color variation [5]. In this study, 80 accessions were analyzed by means of HPLC/DAD, LC-MS/MS and NMR experiments: 46 H. macrophylla, 13 H. aspera, 6 H. involucrata, 5 H. paniculata, 3 H. quercifolia, 2 H. arborescens, 2 H. anomala, 2 H. heteromala, 1 H. scandens, 1 H. seemannii and 1 H. integrifolia. About 50 phenolic derivatives - essentially phenolic acids and flavonols (quercetin and kaempferol) - and 20 anthocyanins could be identified. The contents of pigments and other phenolic compounds appeared as very diverse both qualitatively and quantitatively and some compounds could be identified as chemospecific. On this basis, a statistical study using Principal Component Analysis allowed a clear distinction between both species and subspecies. Besides, different biological evaluations of crude extracts and secondary metabolites isolated from Hydrangea sp will also be discussed
Mycolactone as Analgesic: Subcutaneous Bioavailability Parameters
is the bacillus responsible for Buruli ulcer, an infectious disease and the third most important mycobacterial disease worldwide, after tuberculosis and leprosy. infection is a type of panniculitis beginning mostly with a nodule or an oedema, which can progress to large ulcerative lesions. The lesions are caused by mycolactone, the polyketide toxin of . Mycolactone plays a central role for host colonization as it has immunomodulatory and analgesic effects. On one hand, mycolactone induces analgesia by targeting type-2 angiotensin II receptors (ATR), causing cellular hyperpolarization and neuron desensitization. Indeed, a single subcutaneous injection of mycolactone into the mouse footpad induces a long-lasting hypoesthesia up to 48 h. It was suggested that the long-lasting hypoesthesia may result from the persistence of a significant amount of mycolactone locally following its injection, which could be probably due to its slow elimination from tissues. To verify this hypothesis, we investigated the correlation between hypoesthesia and mycolactone bioavailability directly at the tissue level. Various quantities of mycolactone were then injected in mouse tissue and hypoesthesia was recorded with nociception assays over a period of 48 h. The hypoesthesia was maximal 6 h after the injection of 4 μg mycolactone. The basal state was reached 48 h after injection, which demonstrated the absence of nerve damage. Surprisingly, mycolactone levels decreased strongly during the first hours with a reduction of 70 and 90% after 4 and 10 h, respectively. Also, mycolactone did not diffuse in neighboring skin tissue and only poorly into the bloodstream upon direct injection. Nevertheless, the remaining amount was sufficient to induce hypoesthesia during 24 h. Our results thus demonstrate that intact mycolactone is rapidly eliminated and that very small amounts of mycolactone are sufficient to induce hypoesthesia. Taken together, our study points out that mycolactone ought to be considered as a promising analgesic
Inhibition of P-glycoprotein-mediated multidrug efflux by aminomethylene and ketomethylene analogs of reversins
Several aminomethylene analogs and a ketomethylene analog of reversins were synthesized in order to evaluate their ability to inhibit P-glycoprotein-mediated drug efflux in K562/R7 human leukemic cells overexpressing P-glycoprotein. These analogs retained good activity compared to cyclosporin A and the original reversins
- …