56 research outputs found
Kinetic simulations of turbulent magnetic-field growth by streaming cosmic rays
Efficient acceleration of cosmic rays (via the mechanism of diffusive shock
acceleration) requires turbulent, amplified magnetic fields in the shock's
upstream region. We present results of multidimensional particle-in-cell
simulations aimed at observing the magnetic field amplification that is
expected to arise from the cosmic-ray current ahead of the shock, and the
impact on the properties of the upstream interstellar medium. We find that the
initial structure and peak strength of the amplified field is somewhat
sensitive to the choice of parameters, but that the field growth saturates in a
similar manner in all cases: the back-reaction on the cosmic rays leads to
modification of their rest-frame distribution and also a net transfer of
momentum to the interstellar medium, substantially weakening their relative
drift while also implying the development of a modified shock. The upstream
medium becomes turbulent, with significant spatial fluctuations in density and
velocity, the latter in particular leading to moderate upstream heating; such
fluctuations will also have a strong influence on the shock structure.Comment: 8 pages, 6 figures, accepted by Ap
Inverse Compton cooling of thermal plasma in colliding-wind binaries
The inverse-Compton effect (IC) is a widely recognized cooling mechanism for
both relativistic and thermal electrons in various astrophysical environments,
including the intergalactic medium and X-ray emitting plasmas. Its effect on
thermal electrons is however frequently overlooked in theoretical and numerical
models of colliding-wind binaries (CWB). In this article, we provide a
comprehensive investigation of the impact of IC cooling in CWBs, presenting
general results for when the photon fields of the stars dominate the cooling of
the thermal plasma and when shocks at the stagnation point are expected to be
radiative. Our analysis shows that IC cooling is the primary cooling process
for the shocked-wind layer over a significant portion of the relevant parameter
space, particularly in eccentric systems with large wind-momentum ratios, e.g.,
those containing a Wolf-Rayet and O-type star. Using the binary system WR 140
as a case study, we demonstrate that IC cooling leads to a strongly radiative
shocked wind near periastron, which may otherwise remain adiabatic if only
collisional cooling was considered. Our results are further supported by 2D and
3D simulations of wind-wind collisions. Specifically, 3D magnetohydrodynamic
simulations of WR 140 show a significant decrease in hard-X-ray emission around
periastron, in agreement with observations but in contrast to equivalent
simulations that omit IC cooling. A novel method is proposed for constraining
mass-loss rates of both stars in eccentric binaries where the wind-collision
zone switches from adiabatic to radiative approaching periastron. IC scattering
is an important cooling process in the thermal plasma of CWBs.Comment: Accepted to MNRAS, 17 pages, 13 figure
Could Cosmic Rays Affect Instabilities in the Transition Layer of Nonrelativistic Collisionless Shocks?
There is an observational correlation between astrophysical shocks and
non-thermal particle distributions extending to high energies. As a first step
toward investigating the possible feedback of these particles on the shock at
the microscopic level, we perform particle-in-cell (PIC) simulations of a
simplified environment consisting of uniform, interpenetrating plasmas, both
with and without an additional population of cosmic rays. We vary the relative
density of the counterstreaming plasmas, the strength of a homogeneous parallel
magnetic field, and the energy density in cosmic rays. We compare the early
development of the unstable spectrum for selected configurations without cosmic
rays to the growth rates predicted from linear theory, for assurance that the
system is well represented by the PIC technique. Within the parameter space
explored, we do not detect an unambiguous signature of any cosmic-ray-induced
effects on the microscopic instabilities that govern the formation of a shock.
We demonstrate that an overly coarse distribution of energetic particles can
artificially alter the statistical noise that produces the perturbative seeds
of instabilities, and that such effects can be mitigated by increasing the
density of computational particles.Comment: 22 pages, 5 figures, published in Ap
Coronin-1C Protein and Caveolin Protein Provide Constitutive and Inducible Mechanisms of Rac1 Protein Trafficking
Sustained directional fibroblast migration requires both polarized activation of the protrusive signal, Rac1, and redistribution of inactive Rac1 from the rear of the cell so that it can be redistributed or degraded. In this work, we determine how alternative endocytic mechanisms dictate the fate of Rac1 in response to the extracellular matrix environment. We discover that both coronin-1C and caveolin retrieve Rac1 from similar locations at the rear and sides of the cell. We find that coronin-1C-mediated extraction, which is responsible for Rac1 recycling, is a constitutive process that maintains Rac1 protein levels within the cell. In the absence of coronin-1C, the effect of caveolin-mediated endocytosis, which targets Rac1 for proteasomal degradation, becomes apparent. Unlike constitutive coronin-1C-mediated trafficking, caveolin-mediated Rac1 endocytosis is induced by engagement of the fibronectin receptor syndecan-4. Such an inducible endocytic/degradation mechanism would predict that, in the presence of fibronectin, caveolin defines regions of the cell that are resistant to Rac1 activation but, in the absence of fibronectin leaves more of the membrane susceptible to Rac1 activation and protrusion. Indeed, we demonstrate that fibronectin-stimulated activation of Rac1 is accelerated in the absence of caveolin and that, when caveolin is knocked down, polarization of active Rac1 is lost in FRET experiments and culminates in shunting migration in a fibrous fibronectin matrix. Although the concept of polarized Rac1 activity in response to chemoattractants has always been apparent, our understanding of the balance between recycling and degradation explains how polarity can be maintained when the chemotactic gradient has faded
Magnetic Field Amplification in Galaxy Clusters and its Simulation
We review the present theoretical and numerical understanding of magnetic
field amplification in cosmic large-scale structure, on length scales of galaxy
clusters and beyond. Structure formation drives compression and turbulence,
which amplify tiny magnetic seed fields to the microGauss values that are
observed in the intracluster medium. This process is intimately connected to
the properties of turbulence and the microphysics of the intra-cluster medium.
Additional roles are played by merger induced shocks that sweep through the
intra-cluster medium and motions induced by sloshing cool cores. The accurate
simulation of magnetic field amplification in clusters still poses a serious
challenge for simulations of cosmological structure formation. We review the
current literature on cosmological simulations that include magnetic fields and
outline theoretical as well as numerical challenges.Comment: 60 pages, 19 Figure
Experimental Observation of a Current-Driven Instability in a Neutral Electron-Positron Beam
We report on the first experimental observation of a current-driven instability developing in a quasineutral matter-antimatter beam. Strong magnetic fields (>= 1 T) are measured, via means of a proton radiography technique, after the propagation of a neutral electron-positron beam through a background electron-ion plasma. The experimentally determined equipartition parameter of epsilon(B) approximate to 10(-3) is typical of values inferred from models of astrophysical gamma-ray bursts, in which the relativistic flows are also expected to be pair dominated. The data, supported by particle-in-cell simulations and simple analytical estimates, indicate that these magnetic fields persist in the background plasma for thousands of inverse plasma frequencies. The existence of such long-lived magnetic fields can be related to analog astrophysical systems, such as those prevalent in lepton-dominated jets
Integration of oncology and palliative care : a Lancet Oncology Commission
Full integration of oncology and palliative care relies on the specific knowledge and skills of two modes of care: the tumour-directed approach, the main focus of which is on treating the disease; and the host-directed approach, which focuses on the patient with the disease. This Commission addresses how to combine these two paradigms to achieve the best outcome of patient care. Randomised clinical trials on integration of oncology and palliative care point to health gains: improved survival and symptom control, less anxiety and depression, reduced use of futile chemotherapy at the end of life, improved family satisfaction and quality of life, and improved use of health-care resources. Early delivery of patient-directed care by specialist palliative care teams alongside tumour-directed treatment promotes patient-centred care. Systematic assessment and use of patient-reported outcomes and active patient involvement in the decisions about cancer care result in better symptom control, improved physical and mental health, and better use of health-care resources. The absence of international agreements on the content and standards of the organisation, education, and research of palliative care in oncology are major barriers to successful integration. Other barriers include the common misconception that palliative care is end-of-life care only, stigmatisation of death and dying, and insufficient infrastructure and funding. The absence of established priorities might also hinder integration more widely. This Commission proposes the use of standardised care pathways and multidisciplinary teams to promote integration of oncology and palliative care, and calls for changes at the system level to coordinate the activities of professionals, and for the development and implementation of new and improved education programmes, with the overall goal of improving patient care. Integration raises new research questions, all of which contribute to improved clinical care. When and how should palliative care be delivered? What is the optimal model for integrated care? What is the biological and clinical effect of living with advanced cancer for years after diagnosis? Successful integration must challenge the dualistic perspective of either the tumour or the host, and instead focus on a merged approach that places the patient's perspective at the centre. To succeed, integration must be anchored by management and policy makers at all levels of health care, followed by adequate resource allocation, a willingness to prioritise goals and needs, and sustained enthusiasm to help generate support for better integration. This integrated model must be reflected in international and national cancer plans, and be followed by developments of new care models, education and research programmes, all of which should be adapted to the specific cultural contexts within which they are situated. Patient-centred care should be an integrated part of oncology care independent of patient prognosis and treatment intention. To achieve this goal it must be based on changes in professional cultures and priorities in health care
LES COMPLEXES D'YLURE DE PYRIDINIUM DU TUNGSTENE ET DU CHROME (SYNTHESE ET APPLICATIONS EN CHIMIE ORGANIQUE)
PARIS-BIUSJ-Thèses (751052125) / SudocCentre Technique Livre Ens. Sup. (774682301) / SudocPARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF
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