2,531 research outputs found
The Properties of Reconnection Current Sheets in GRMHD Simulations of Radiatively Inefficient Accretion Flows
Non-ideal MHD effects may play a significant role in determining the
dynamics, thermal properties, and observational signatures of radiatively
inefficient accretion flows onto black holes. In particular, particle
acceleration during magnetic reconnection events may influence black hole
spectra and flaring properties. We use representative GRMHD simulations of
black hole accretion flows to identify and explore the structures and
properties of current sheets as potential sites of magnetic reconnection. In
the case of standard and normal (SANE) disks, we find that, in the reconnection
sites, the plasma beta ranges from to , the magnetization ranges
from to , and the guide fields are weak compared to the
reconnecting fields. In magnetically arrested (MAD) disks, we find typical
values for plasma beta from to , magnetizations from
to , and typically stronger guide fields, with strengths comparable to or
greater than the reconnecting fields. These are critical parameters that govern
the electron energy distribution resulting from magnetic reconnection and can
be used in the context of plasma simulations to provide microphysics inputs to
global simulations. We also find that ample magnetic energy is available in the
reconnection regions to power the fluence of bright X-ray flares observed from
the black hole in the center of the Milky Way.Comment: 8 pages, 8 figures, submitted to Ap
Electron and Proton Acceleration in Trans-Relativistic Magnetic Reconnection: Dependence on Plasma Beta and Magnetization
Non-thermal electron acceleration via magnetic reconnection is thought to
play an important role in powering the variable X-ray emission from radiatively
inefficient accretion flows around black holes. The trans-relativistic regime
of magnetic reconnection, where the magnetization , defined as the
ratio of magnetic energy density to enthalpy density, is , is
frequently encountered in such flows. By means of a large suite of
two-dimensional particle-in-cell simulations, we investigate electron and
proton acceleration in the trans-relativistic regime. We focus on the
dependence of the electron energy spectrum on and the proton
(i.e., the ratio of proton thermal pressure to magnetic pressure). We find that
the electron spectrum in the reconnection region is non-thermal and can be
generally modeled as a power law. At , the
slope, , is independent of and it hardens with increasing
as . Electrons are primarily accelerated by the
non-ideal electric field at X-points, either in the initial current layer or in
current sheets generated in between merging magnetic islands. At higher values
of , the electron power law steepens for all values of . At
values of near , when both electrons
and protons are relativistically hot prior to reconnection, the spectra of both
species display an additional component at high energies, containing a few
percent of particles. These particles are accelerated via a Fermi-like process
by bouncing in between the reconnection outflow and a stationary magnetic
island. We provide an empirical prescription for the dependence of the
power-law slope and the acceleration efficiency on and , which
can be used in global simulations of collisionless accretion disks.Comment: 19 pages, 23 figures, submitted to Ap
Modular bismacycles for the selective C–H arylation of phenols and naphthols
Given the important role played by 2-hydroxybiaryls in organic, medicinal and materials chemistry, concise methods for the synthesis of this common motif are extremely valuable. In seeking to extend the synthetic chemists’ lexicon in this regard, we have developed an expedient and general strategy for the ortho-arylation of phenols and naphthols using readily-available boronic acids. Our methodology relies on in situ generation of a uniquely reactive Bi(V) arylating agent from a bench stable Bi(III) precursor via telescoped B-to-Bi transmetallation and oxidation. By exploiting reactivity 2 that is orthogonal to conventional metal-catalyzed manifolds, diverse aryl and heteroaryl partners can be rapidly coupled to phenols and naphthols under mild conditions. Following arylation, highyielding recovery of the Bi(III) precursor allows for its efficient re-use in subsequent reactions. Mechanistic interrogation of each key step of the methodology informs its practical application and provides fundamental insight into the under-exploited reactivity of organobismuth compounds
Reproduction numbers for epidemic models with households and other social structures II: comparisons and implications for vaccination
In this paper we consider epidemic models of directly transmissible SIR (susceptible - infective - recovered) and SEIR (with an additional latent class) infections in fully-susceptible populations with a social structure, consisting either of households or of households and workplaces. We review most reproduction numbers defined in the literature for these models, including the basic reproduction number R0 introduced in the companion paper of this, for which we provide a simpler, more elegant derivation. Extending previous work, we provide a complete overview of the inequalities among these reproduction numbers and resolve some open questions. Special focus is put on the exponential-growth-associated reproduction number Rr, which is loosely defined as the estimate of R0 based on the observed exponential growth of an emerging epidemic obtained when the social structure is ignored. We show that for the vast majority of the models considered in the literature Rr >= R0 when R0 >=1 and Rr <= R0 when R0 <= 1. We show that, in contrast to models without social structure, vaccination of a fraction 1-1/R0 of the population, chosen uniformly at random, with a perfect vaccine is usually insufficient to prevent large epidemics. In addition, we provide significantly sharper bounds than the existing ones for bracketing the critical vaccination coverage between two analytically tractable quantities, which we illustrate by means of extensive numerical examples
Lung imaging: How to get better look inside the lung
In the last years, imaging has played a key role in the diagnosis and monitoring and critical illness, including acute respiratory distress syndrome (ARDS). Chest X-ray (CXR) and computed tomography (CT) are the conventional techniques most performed in the critically ill patients, the latter being the gold standard to assess lung aeration in ARDS patients. In addition, two bedside techniques are now gaining popularity alongside the conventional ones: lung ultrasound (LUS) and electrical impedance tomography (EIT). These techniques do not involve the use of ionizing radiations, are non-invasive and relatively easy to use, and are under extensive investigation as a complement, and for some application a substitution of conventional techniques. At last, positron emission tomography (PET) and magnetic resonance imaging (MRI) can provide functional information on the lung and respiratory function, and are increasingly used in research to improve the understanding of the pathophysiological mechanisms underlying ARDS. The purpose of this review is to give an up-to-date overview of the conventional and emerging imaging techniques available the diagnosis and management of patients with ARDS
Power to mechanical power to minimize ventilator-induced lung injury?
Mechanical ventilation is a life-supportive therapy, but can also promote damage to pulmonary structures, such as epithelial and endothelial cells and the extracellular matrix, in a process referred to as ventilator-induced lung injury (VILI). Recently, the degree of VILI has been related to the amount of energy transferred from the mechanical ventilator to the respiratory system within a given timeframe, the so-called mechanical power. During controlled mechanical ventilation, mechanical power is composed of parameters set by the clinician at the bedside-such as tidal volume (VT), airway pressure (Paw), inspiratory airflow (V'), respiratory rate (RR), and positive end-expiratory pressure (PEEP) level-plus several patient-dependent variables, such as peak, plateau, and driving pressures. Different mathematical equations are available to calculate mechanical power, from pressure-volume (PV) curves to more complex formulas which consider both dynamic (kinetic) and static (potential) components; simpler methods mainly consider the dynamic component. Experimental studies have reported that, even at low levels of mechanical power, increasing VT causes lung damage. Mechanical power should be normalized to the amount of ventilated pulmonary surface; the ratio of mechanical power to the alveolar area exposed to energy delivery is called "intensity." Recognizing that mechanical power may reflect a conjunction of parameters which may predispose to VILI is an important step toward optimizing mechanical ventilation in critically ill patients. However, further studies are needed to clarify how mechanical power should be taken into account when choosing ventilator settings
Evolution of truncated moments of singlet parton distributions
We define truncated Mellin moments of parton distributions by restricting the
integration range over the Bjorken variable to the experimentally accessible
subset x_0 < x < 1 of the allowed kinematic range 0 < x < 1. We derive the
evolution equations satisfied by truncated moments in the general (singlet)
case in terms of an infinite triangular matrix of anomalous dimensions which
couple each truncated moment to all higher moments with orders differing by
integers. We show that the evolution of any moment can be determined to
arbitrarily good accuracy by truncating the system of coupled moments to a
sufficiently large but finite size, and show how the equations can be solved in
a way suitable for numerical applications. We discuss in detail the accuracy of
the method in view of applications to precision phenomenology.Comment: 23 pages, 6 figures, LaTeX; factors of 2nf in Appendix C correcte
Ventilatory settings in the initial 72Â h and their association with outcome in out-of-hospital cardiac arrest patients: a preplanned secondary analysis of the targeted hypothermia versus targeted normothermia after out-of-hospital cardiac arrest (TTM2) trial
Purpose: The optimal ventilatory settings in patients after cardiac arrest and their association with outcome remain unclear. The aim of this study was to describe the ventilatory settings applied in the first 72 h of mechanical ventilation in patients after out-of-hospital cardiac arrest and their association with 6-month outcomes.
Methods: Preplanned sub-analysis of the Target Temperature Management-2 trial. Clinical outcomes were mortality and functional status (assessed by the Modified Rankin Scale) 6 months after randomization.
Results: A total of 1848 patients were included (mean age 64 [Standard Deviation, SD = 14] years). At 6 months, 950 (51%) patients were alive and 898 (49%) were dead. Median tidal volume (VT) was 7 (Interquartile range, IQR = 6.2-8.5) mL per Predicted Body Weight (PBW), positive end expiratory pressure (PEEP) was 7 (IQR = 5-9) cmH20, plateau pressure was 20 cmH20 (IQR = 17-23), driving pressure was 12 cmH20 (IQR = 10-15), mechanical power 16.2 J/min (IQR = 12.1-21.8), ventilatory ratio was 1.27 (IQR = 1.04-1.6), and respiratory rate was 17 breaths/minute (IQR = 14-20). Median partial pressure of oxygen was 87 mmHg (IQR = 75-105), and partial pressure of carbon dioxide was 40.5 mmHg (IQR = 36-45.7). Respiratory rate, driving pressure, and mechanical power were independently associated with 6-month mortality (omnibus p-values for their non-linear trajectories: p < 0.0001, p = 0.026, and p = 0.029, respectively). Respiratory rate and driving pressure were also independently associated with poor neurological outcome (odds ratio, OR = 1.035, 95% confidence interval, CI = 1.003-1.068, p = 0.030, and OR = 1.005, 95% CI = 1.001-1.036, p = 0.048). A composite formula calculated as [(4*driving pressure) + respiratory rate] was independently associated with mortality and poor neurological outcome.
Conclusions: Protective ventilation strategies are commonly applied in patients after cardiac arrest. Ventilator settings in the first 72 h after hospital admission, in particular driving pressure and respiratory rate, may influence 6-month outcomes
Lung hyperaeration assessment by computed tomography: Correction of reconstruction-induced bias
Background: Computed tomography (CT) reconstruction parameters, such as slice thickness and convolution kernel, significantly affect the quantification of hyperaerated parenchyma (VHYPER%). The aim of this study was to investigate the mathematical relation between VHYPER% calculated at different reconstruction settings, in mechanically ventilated and spontaneously breathing patients with different lung pathology. Methods: In this retrospective observational study, CT scans of patients of the intensive care unit and emergency department were collected from two CT scanners and analysed with different kernel-thickness combinations (reconstructions): 1.25 mm soft kernel, 5 mm soft kernel, 5 mm sharp kernel in the first scanner; 2.5 mm slice thickness with a smooth (B41s) and a sharp (B70s) kernel on the second scanner. A quantitative analysis was performed with Maluna® to assess lung aeration compartments as percent of total lung volume. CT variables calculated with different reconstructions were compared in pairs, and their mathematical relationship was analysed by using quadratic and power functions. Results: 43 subjects were included in the present analysis. Image reconstruction parameters influenced all the quantitative CT-derived variables. The most relevant changes occurred in the hyperaerated and normally aerated volume compartments. The application of a power correction formula led to a significant reduction in the bias between VHYPER% estimations (p 0.15 in all cases). Conclusions: Hyperaerated percent volume at different reconstruction settings can be described by a fixed mathematical relationship, independent of lung pathology, ventilation mode, and type of CT scanner
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