scholarly journals Improved hydrodynamic pulsation models for the pulsating extreme helium star V652 Herculis

2021 ◽  
Author(s):  
C Simon Jeffery ◽  
Pilar Montañés-Rodríguez ◽  
Hideyuki Saio
Keyword(s):  
Large Amplitude ◽  
Strong Shock ◽  
Model Space ◽  
Shock Acceleration ◽  
Pulsation Period ◽  
Hydrodynamic Models ◽  
Velocity Amplitude ◽  
Minimum Radius ◽  
Large Phase ◽  
Helium Star

Abstract New non-linear hydrodynamic models have been constructed to simulate the radial pulsations observed in the extreme helium star V652 Her. These use a finer zoning to allow higher radial resolution than in previous simulations. Models incorporate updated OPAL and OP opacity tables and adopt a composition based on the best atmospheric analyses to date. Key pulsation properties including period, velocity amplitude and shock acceleration are examined as a function of the mean stellar parameters (mass, luminosity, and effective temperature). The new models confirm that, for large amplitude pulsations, a strong shock develops at minimum radius, and is associated with a large phase delay between maximum brightness and minimum radius. Using the observed pulsation period to constrain parameter space in one dimension, other pulsation properties are used to constrain the model space further, and to critically discuss observational measurements. Similar models may be useful for the interpretation of other blue large amplitude pulsators, which may also exhibit pulsation-driven shocks.

scholarly journals Fermi bubbles from stochastic acceleration of electrons in a Galactic outflow

2019 ◽  
Vol 622 ◽  
pp. A203 ◽  
Author(s):  
P. Mertsch ◽  
V. Petrosian
Keyword(s):  
Large Scale ◽  
Transport Coefficients ◽  
Strong Shock ◽  
High Energy ◽  
Brightness Distribution ◽  
Shock Acceleration ◽  
Galactic Centre ◽  
Large Reservoir ◽  
Stochastic Acceleration ◽  
Multi Wavelength

The discovery of the Fermi bubbles – a huge bilobular structure seen in GeV gamma-rays above and below the Galactic centre – implies the presence of a large reservoir of high energy particles at ~10 kpc from the disk. The absence of evidence for a strong shock coinciding with the edge of the bubbles, and constraints from multi-wavelength observations point towards stochastic acceleration by turbulence as a likely mechanism of acceleration. We have investigated the time-dependent acceleration of electrons in a large-scale outflow from the Galactic centre. For the first time, we present a detailed numerical solution of the particle kinetic equation that includes the acceleration, transport and relevant energy loss processes. We also take into account the addition of shock acceleration of electrons at the bubble’s blast wave. Fitting to the observed spectrum and surface brightness distribution of the bubbles allows determining the transport coefficients, thereby shedding light on the origin of the Fermi bubbles.

scholarly journals The Radial Velocity Curve of V652 Her (BD+13°3224)

1985 ◽  
Vol 87 ◽  
pp. 109-115
Author(s):  
P.W. Hill ◽  
C.S. Jeffery
Keyword(s):  
Radial Velocity ◽  
Time Resolution ◽  
High Frequency ◽  
Velocity Curve ◽  
Velocity Data ◽  
Effective Surface ◽  
Radial Velocity Curve ◽  
Minimum Radius ◽  
Helium Star ◽  
Radial Velocity Data

AbstractNew radial velocity data for the pulsating extreme helium star V652 Her (BD+13°3224) have been obtained with a time resolution of 100 s. High frequency structure in the radial velocity curve is detected, and a comparison with previous data suggests that the detailed shape of the velocity curve is variable. The data imply that the effective surface gravity must increase by a factor of 4 at minimum radius.

scholarly journals Radiation-hydrodynamic Models of the Accretion Spots in Magnetic Cataclysmic Variables

2004 ◽  
Vol 190 ◽  
pp. 187-200 ◽  
Author(s):  
K. Beuermann
Keyword(s):  
Mass Flux ◽  
White Dwarf ◽  
Cataclysmic Variables ◽  
Strong Shock ◽  
Hydrodynamic Models ◽  
Mass Fluxes ◽  
Open Questions ◽  
Low Mass ◽  
Magnetic Cataclysmic Variables ◽  
Theoretical Foundations

AbstractThe structure of the near-polar accretion spots on accreting magnetic white dwarfs has been studied theoretically and observationally in numerous papers over the last decade. Detailed treatments are available for the regime of low mass flux, usually termed the bombardment case, and for higher mass fluxes which create a strong shock standing above the photosphere of the white dwarf. No general treatment is so far available for the case of shocks buried deep in the photosphere. I review the theoretical foundations, present some applications of theory, and discuss in short the open questions which still need to be addressed.

Strong shock propagation through decreasing density

1972 ◽  
Vol 54 (2) ◽  
pp. 297-304 ◽  
Author(s):  
D. A. Freiwald
Keyword(s):  
Strong Shock ◽  
Limited Range ◽  
Ideal Gas ◽  
Experimental Results ◽  
Shock Acceleration ◽  
Shock Propagation ◽  
Final State ◽  
Free Expansion ◽  
Real Gas Effects ◽  
Expansion Model

The acceleration of a shock wave in an ideal gas of decreasing density has previously been studied. The problem is reconsidered here with empirical inclusion of real-gas effects for strong shocks in hydrogen. Experimental results suggest that previous shock acceleration models are valid only for a limited range of the Knudsen number in finite geometries and that for large final-state Knudsen numbers a free-expansion model best describes the experimental results.

scholarly journals Radiation from the impact of broad-line region clouds onto AGN accretion disks

2020 ◽  
Vol 636 ◽  
pp. A92
Author(s):  
A. L. Müller ◽  
G. E. Romero
Keyword(s):  
Accretion Disk ◽  
Spectral Energy Distribution ◽  
Strong Shock ◽  
Shock Acceleration ◽  
Spectral Energy ◽  
Broad Line ◽  
Broad Line Region ◽  
Line Region ◽  
The Impact ◽  
Relativistic Particles

Context. Active galactic nuclei are supermassive black holes surrounded by an accretion disk, two populations of clouds, bipolar jets, and a dusty torus. The clouds move in Keplerian orbits at high velocities. In particular, the broad-line region (BLR) clouds have velocities ranging from 1000 to 10 000 km s−1. Given the extreme proximity of these clouds to the supermassive black hole, frequent collisions with the accretion disk should occur. Aims. The impact of BLR clouds onto the accretion disk can produce strong shock waves where particles might be accelerated. The goal of this work is to investigate the production of relativistic particles, and the associated non-thermal radiation in these events. In particular, we apply the model we develop to the Seyfert galaxy NGC 1068. Methods. We analyze the efficiency of diffusive shock acceleration in the shock of colliding clouds of the BLR with the accretion disk. We calculate the spectral energy distribution of photons generated by the relativistic particles and estimate the number of simultaneous impacts needed to explain the gamma radiation observed by Fermi in Seyfert galaxies. Results. We find that is possible to understand the measured gamma emission in terms of the interaction of clouds with the disk if the hard X-ray emission of the source is at least obscured between 20% and 40%. The total number of clouds contained in the BLR region might be between 3 × 108 and 6 × 108, which are values in good agreement with the observational evidence. The maximum energy achieved by the protons (∼PeV) in this context allows the production of neutrinos in the observing range of IceCube.

Diffusive Shock Acceleration by Multiple Shocks

1993 ◽  
Vol 10 (3) ◽  
pp. 222-224 ◽  
Author(s):  
D.B. Melrose ◽  
M.H. Pope
Keyword(s):  
Finite Number ◽  
Power Law ◽  
Momentum Distribution ◽  
Strong Shock ◽  
High Energy ◽  
Shock Acceleration ◽  
Diffusive Shock Acceleration ◽  
Single Shock ◽  
Multiple Shocks

AbstractDiffusive shock acceleration produces a power law momentum distribution f(p)α p−b, with b ≥ 4 for a single shock, and b = 4 for a single strong shock. It has been shown that the distribution for acceleration at a sequence of identical shocks is flatter, approaching f(p)α p−3 below a high energy knee, for an arbitrarily large number of shocks. We show how this flatter distribution arises and discuss the range of momenta over which it extends after a finite number of shocks.

scholarly journals Fully nonlinear electrostatic waves in electron–positron plasmas

2009 ◽  
Vol 76 (3-4) ◽  
pp. 267-275 ◽  
Author(s):  
GAIMIN LU ◽  
YUE LIU ◽  
YOUMEI WANG ◽  
L. STENFLO ◽  
S. I. POPEL ◽  
...  
Keyword(s):  
Large Amplitude ◽  
Small Amplitude ◽  
Governing Equations ◽  
Fully Nonlinear ◽  
Electrostatic Waves ◽  
Phase Velocities ◽  
Electron Positron ◽  
Large Phase

AbstractFully nonlinear electrostatic waves in a plasma containing electrons, positrons, and ions are investigated by solving the governing equations exactly. It is found that both smooth and spiky quasistationary waves exist, and large-amplitude waves necessarily have large-phase velocities, but small-amplitude waves can be both fast and slow.

scholarly journals Hydrogen Abundances and Shock Waves

2021 ◽  
pp. 187-219
Author(s):  
Donald V. Reames
Keyword(s):  
Shock Waves ◽  
Power Law ◽  
Strong Shock ◽  
Weak Shock ◽  
Shock Acceleration ◽  
Seed Particle ◽  
Abundance Patterns ◽  
Seed Population ◽  
Suprathermal Ions ◽  
Strong Shock Waves

AbstractHow well do protons fit into the abundance patterns of the other elements? Protons have Q = 1 and A/Q = 1 at all temperatures of interest. When does their relative abundance fit on the power law in A/Q defined by the elements with A/Q > 2? For small “pure” impulsive events, protons fit well, but for larger CME-associated impulsive events, where shock waves boost the intensities, protons are enhanced a factor of order ten by addition of seed protons from the ambient plasma. During most large gradual SEP events with strong shock waves, protons again fit the power law, but with weaker or quasi-perpendicular shock waves, dominated by residual impulsive seed particle abundances at high Z, again protons are enhanced. Proton enhancements occur when moderately weak shock waves happen to sample a two-component seed population with dominant protons from the ambient coronal plasma and impulsive suprathermal ions at high Z; thus proton-enhanced events are a surprising new signature of shock acceleration in jets. A/Q measures the rigidity dependence of both acceleration and transport but does not help us distinguish the two. Energy-spectral indices and abundances are correlated for most gradual events but not when impulsive ions are present; thus we end with powerful new correlations that probe both acceleration and transport.

Active Control of Passive and Active Particle Distribution at the Outlet of Double Y-Microchannel Using Pulsatile Flow

2020 ◽  
Vol 142 (8) ◽  
Author(s):  
Minh Chau Nguyen ◽  
Hassan Peerhossaini ◽  
Elnaz Pashmi ◽  
Mohammad Mehdi Salek ◽  
Mojtaba Jarrahi
Keyword(s):  
Phase Shift ◽  
Pulsatile Flow ◽  
Particle Distribution ◽  
Amplitude Ratio ◽  
Pulsation Period ◽  
Steady Flows ◽  
Control Parameters ◽  
Flow Conditions ◽  
Velocity Amplitude ◽  
Series Of Experiments

Abstract While a variety of active and passive techniques have been proposed for steady flows, pulsatile flow has received much less attention. Pulsation makes more control parameters available for passive methods and enables them to separate particles. The purpose of this work is to determine the effects of the phase shift between two entering flows (only one includes the particles) on particle separation inside a double Y-microchannel. Numerical simulations were carried out for both steady and pulsating flow conditions. The results showed that when the velocity amplitude ratio (β) is less than 2, the separation index increases with the phase shift (φ) and the highest efficiency occurs at φ = 180 deg. A similar trend can be observed for higher values of β only if the pulsation period is short enough. A series of experiments qualitatively validated the numerical results.

scholarly journals Importance of Precise Radial Velocities for Cepheid Binaries

1999 ◽  
Vol 170 ◽  
pp. 211-217 ◽  
Author(s):  
L. Szabados
Keyword(s):  
Radial Velocity ◽  
Binary Systems ◽  
Velocity Variation ◽  
Pulsation Period ◽  
Radial Pulsation ◽  
Velocity Amplitude ◽  
Radial Velocity Variation ◽  
Classical Cepheids ◽  
Orbital Effect ◽  
Low Amplitude

AbstractIn view of the high incidence of duplicity among Cepheids, a large number of binary systems remain to be discovered because of the low amplitude of the orbital effect superimposed on the radial velocity variation due to radial pulsation. This fact is shown by simple statistics. The importance of combining astrometric data with radial velocity measurements is also emphasized. Finally the systematics of the radial velocity amplitude as a function of the pulsation period is considered for classical Cepheids.

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