Radiative shocks in spherical accretion

Author(s):  
Jun Fukue

ABSTRACT In order to explore various aspects of radiative shocks, we examine standing radiative shock waves in spherical accretion flows onto a central gravitating body under the equilibrium diffusion approximation. In contrast to the usual one-dimensional shock, in radiative shocks a radiative precursor appears in the pre-shock region before the shock front, due to the radiative diffusion effect. Furthermore, in spherical flows around a central object the gravitational potential varies in this radiative precursor, and a curvature effect also exists. We first formulate such radiative shocks in spherical flows, derive the overall jump conditions, and solve the structure of the radiative precursor for both the gas and radiation pressure dominated cases. Since the jump conditions contain the coordinates of both ends of the radiative precursor, we must obtain both the solution and the endpoints of the precursor simultaneously. We find that the gravitational effect is not significant, although it cannot be ignored. The curvature effect exerts a strong influence on the structure and width of the precursor. The precursor starting point x1 normalized by the shock radius is roughly expressed by $x_1={\cal M}_1^{1/7}$ for a radiation pressure dominated shock, while $x_1=1.21^{({\cal M}_1-1)}$ for a gas pressure one, where ${\cal M}_1$ is the pre-shock Mach number.

2000 ◽  
Vol 195 ◽  
pp. 381-384
Author(s):  
D. M. Caditz ◽  
S. Tsuruta

Accretion flows onto compact astronomical sources are likely to be supersonic, and shock waves may therefore be common in such flows. Plasma passing through a shock front will be compressed and heated according to the jump conditions across the shock discontinuity. Shocks in accretion flows may therefore have important consequences for the flow structure and emission characteristics. The equations governing adiabatic (nonradiative) shocks in relativistic plasmas are presented including the effects of radiation pressure and energy density, and pair equilibria in the postshock flow. We find that postshock states for accretion flows within cool, optically thick, accretion-driven sources such as AGN become radiation- or pair-dominated, and the postshock plasma will likely become optically thin before returning to steady-state conditions.


Author(s):  
J. Schreiber ◽  
F. Bell ◽  
Z. Najmudin

Abstract Experiments have shown that the ion energy obtained by laser–ion acceleration can be optimized by choosing either the appropriate pulse duration or the appropriate target thickness. We demonstrate that this behavior can be described either by the target normal sheath acceleration model of Schreiber et al. or by the radiation pressure acceleration model of Bulanov and coworkers. The starting point of our considerations is that the essential property of a laser system for ion acceleration is its pulse energy and not its intensity. Maybe surprisingly we show that higher ion energies can be reached with reduced intensities.


2011 ◽  
Vol 20 (13) ◽  
pp. 2507-2523 ◽  
Author(s):  
CHANDRA B. SINGH ◽  
SANDIP K. CHAKRABARTI

Solutions of black hole accretion flows with axisymmetric shocks are obtained self-consistently when the dissipation at the post-shock flow is taken into account. The Rankine–Hugoniot relationships had to be modified suitably to incorporate the energy loss as well as possible matter loss due to outflows in the post-shock region. The outflow rate from the post-shock region is also computed self-consistently. This was done by considering the quantities in the subsonic post-shock flow as the initial condition for the conical outflow. We have several major results: we find the analytical expression of the ratio of the outflow rate and the inflow rate Rṁ. We find that Rṁ strongly depends on the model assumptions which govern the flow geometry. It appears that, (a) the outflow rate is at most a few percent of the inflow rate, (b) the outflow is absent when the shock is relatively weak, (c) the outflow rate decreases with the increase in the energy loss at the post-shock region. These conclusions are very important as they have direct bearings on the observational effects. Since spectrally soft states are generally believed to be caused by the dominance of the soft photons and almost total loss of thermal energy of the Compton cloud by inverse Comptonization, a spectrally softer state should have less outflows. The opposite is generally true: A spectrally harder state will have a stronger outflow, but the result depends on the compression ratio and the adopted model. The other major result is that the model independence of the transonic properties of the flow does not hold in presence of the loss of the energy (radiation) and mass (outflow).


2004 ◽  
Vol 56 (3) ◽  
pp. 547-552 ◽  
Author(s):  
Toru Okuda ◽  
Vincenzo Teresi ◽  
Elena Toscano ◽  
Diego Molteni

Author(s):  
L.R. Wallenberg ◽  
J.-O. Bovin ◽  
G. Schmid

Metallic clusters are interesting from various points of view, e.g. as a mean of spreading expensive catalysts on a support, or following heterogeneous and homogeneous catalytic events. It is also possible to study nucleation and growth mechanisms for crystals with the cluster as known starting point.Gold-clusters containing 55 atoms were manufactured by reducing (C6H5)3PAuCl with B2H6 in benzene. The chemical composition was found to be Au9.2[P(C6H5)3]2Cl. Molecular-weight determination by means of an ultracentrifuge gave the formula Au55[P(C6H5)3]Cl6 A model was proposed from Mössbauer spectra by Schmid et al. with cubic close-packing of the 55 gold atoms in a cubeoctahedron as shown in Fig 1. The cluster is almost completely isolated from the surroundings by the twelve triphenylphosphane groups situated in each corner, and the chlorine atoms on the centre of the 3x3 square surfaces. This gives four groups of gold atoms, depending on the different types of surrounding.


2019 ◽  
Vol 476 (24) ◽  
pp. 3687-3704 ◽  
Author(s):  
Aphrodite T. Choumessi ◽  
Manuel Johanns ◽  
Claire Beaufay ◽  
Marie-France Herent ◽  
Vincent Stroobant ◽  
...  

Root extracts of a Cameroon medicinal plant, Dorstenia psilurus, were purified by screening for AMP-activated protein kinase (AMPK) activation in incubated mouse embryo fibroblasts (MEFs). Two isoprenylated flavones that activated AMPK were isolated. Compound 1 was identified as artelasticin by high-resolution electrospray ionization mass spectrometry and 2D-NMR while its structural isomer, compound 2, was isolated for the first time and differed only by the position of one double bond on one isoprenyl substituent. Treatment of MEFs with purified compound 1 or compound 2 led to rapid and robust AMPK activation at low micromolar concentrations and increased the intracellular AMP:ATP ratio. In oxygen consumption experiments on isolated rat liver mitochondria, compound 1 and compound 2 inhibited complex II of the electron transport chain and in freeze–thawed mitochondria succinate dehydrogenase was inhibited. In incubated rat skeletal muscles, both compounds activated AMPK and stimulated glucose uptake. Moreover, these effects were lost in muscles pre-incubated with AMPK inhibitor SBI-0206965, suggesting AMPK dependency. Incubation of mouse hepatocytes with compound 1 or compound 2 led to AMPK activation, but glucose production was decreased in hepatocytes from both wild-type and AMPKβ1−/− mice, suggesting that this effect was not AMPK-dependent. However, when administered intraperitoneally to high-fat diet-induced insulin-resistant mice, compound 1 and compound 2 had blood glucose-lowering effects. In addition, compound 1 and compound 2 reduced the viability of several human cancer cells in culture. The flavonoids we have identified could be a starting point for the development of new drugs to treat type 2 diabetes.


2020 ◽  
Vol 5 (5) ◽  
pp. 1175-1187
Author(s):  
Rachel Glade ◽  
Erin Taylor ◽  
Deborah S. Culbertson ◽  
Christin Ray

Purpose This clinical focus article provides an overview of clinical models currently being used for the provision of comprehensive aural rehabilitation (AR) for adults with cochlear implants (CIs) in the Unites States. Method Clinical AR models utilized by hearing health care providers from nine clinics across the United States were discussed with regard to interprofessional AR practice patterns in the adult CI population. The clinical models were presented in the context of existing knowledge and gaps in the literature. Future directions were proposed for optimizing the provision of AR for the adult CI patient population. Findings/Conclusions There is a general agreement that AR is an integral part of hearing health care for adults with CIs. While the provision of AR is feasible in different clinical practice settings, service delivery models are variable across hearing health care professionals and settings. AR may include interprofessional collaboration among surgeons, audiologists, and speech-language pathologists with varying roles based on the characteristics of a particular setting. Despite various existing barriers, the clinical practice patterns identified here provide a starting point toward a more standard approach to comprehensive AR for adults with CIs.


2006 ◽  
Vol 37 (3) ◽  
pp. 131-139 ◽  
Author(s):  
Juliane Degner ◽  
Dirk Wentura ◽  
Klaus Rothermund

Abstract: We review research on response-latency based (“implicit”) measures of attitudes by examining what hopes and intentions researchers have associated with their usage. We identified the hopes of (1) gaining better measures of interindividual differences in attitudes as compared to self-report measures (quality hope); (2) better predicting behavior, or predicting other behaviors, as compared to self-reports (incremental validity hope); (3) linking social-cognitive theories more adequately to empirical research (theory-link hope). We argue that the third hope should be the starting point for using these measures. Any attempt to improve these measures should include the search for a small-scale theory that adequately explains the basic effects found with such a measure. To date, small-scale theories for different measures are not equally well developed.


2005 ◽  
Vol 36 (3) ◽  
pp. 103-115 ◽  
Author(s):  
Robert A. Wicklund

Abstract: Solidarity in the classic sense pertains to a cohesion among humans that entails physical contact, shared emotions, and common goals or projects. Characteristic cases are to be found among families, close friends, or co-workers. The present paper, in contrast, treats a phenomenon of the solidarity of distance, a solidarity based in fear of certain others and in incompetence to interact with them. The starting point for this analysis is the person who is motivated to interact with others who are unfamiliar or fear-provoking. Given that the fear and momentary social incompetence do not allow a full interaction to ensue, the individual will move toward solidarity with those others on a symbolic level. In this manner the motivation to approach the others is acted upon while physical and emotional distance is retained.


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