Richtmyer-Meshkov Experiments on the Omega Laser     

R. M. Hueckstaedt  S. H. Batha  M. M. Balkey  N. D. Delamater  J. R. Fincke  R. L. Holmes  N. E. Lanier  G. R. Magelssen  J. M. Scott  J. M. Taccetti  C. J. Horsfield  K. W. Parker  S. D. Rothman 《Astrophysics and Space Science》2005,298(1-2):255-259

Observations of the interstellar medium reveal a dynamic realm permeated by shocks. These shocks are generated on a large range of scales by galactic rotation, supernovae, stellar winds, and other processes. Whenever a shock encounters a density interface, Richtmyer-Meshkov instabilities may develop. Perturbations along the interface grow, leading to structure formation and material mixing. An understanding of the evolution of Richtmyer-Meshkov instabilities is essential for understanding galactic structure, molecular cloud morphology, and the early stages of star formation. An ongoing experimental campaign studies Richtmyer-Meshkov mixing in a convergent, compressible, miscible plasma at the Omega laser facility. Cylindrical targets, consisting of a low density foam core and an aluminum shell covered by an epoxy ablator, are directly driven by fifty laser beams. The aluminum shell is machined to produce different perturbation spectra. Surface types include unperturbed (smooth), single-mode sinusoids, multi-mode (rough), and multi-mode with particular modes accentuated (specified-rough). Experimental results are compared to theory and numerical simulations.  相似文献   

Simulation of stellar instabilities with vastly different time-scales using domain decomposition     

M. Grott  S. Chernigovski  W. Glatzel 《Monthly notices of the Royal Astronomical Society》2003,344(4):1119-1130

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Importance of perturbed gravitational potentials in differentially rotating Newtonian stars     

S. Karino 《Monthly notices of the Royal Astronomical Society》2003,343(1):175-180

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An analytical expression for apsidal superhump precession and comparisons with numerical simulations and dwarf nova observations     

M.M. Montgomery 《Monthly notices of the Royal Astronomical Society》2001,325(2):761-766

We compare analytical expressions of precession rates from apsidal (positive) superhumps in close binary systems with numerical disc simulation results and observed values. In the analytical expressions, we include both the dynamical effects on the precession of the disc and effects caused by pressure forces that have been theorized to provide a retrograde effect (i.e. slowing) on the prograde disc precession. We establish new limits on density wave pitch angle to a normalized disc sound speed 60≥Ω_orb  d  tan  i / c >2.214 . Using average values for the density wave pitch angle i and speed of sound c , we find good correlation between numerical simulations and the analytical expression for the apsidal superhump period excess, which includes both the prograde and retrograde effects, for mass ratios of 0.025≤ q ≤0.33 . We also show good correlations with the four known eclipsing systems, OY Car, Z Cha, HT Cas, and WZ Sge. Our analytical expression for apsidal superhump period excess as a function of orbital period is consistent with the trend found in observed systems.  相似文献   

Forced oscillations in a hydrodynamical accretion disk and QPOs     

J'erôme P'etri 《Astrophysics and Space Science》2006,302(1-4):117-139

This is the second of a series of papers aimed to look for an explanation on the generation of high frequency quasi-periodic oscillations (QPOs) in accretion disks around neutron star, black hole, and white dwarf binaries. The model is inspired by the general idea of a resonance mechanism in the accretion disk oscillations as was already pointed out by Abramowicz and Klu’zniak (2001). In a first paper (P'etri, 2005a, paper I), we showed that a rotating misaligned magnetic field of a neutron star gives rise to some resonances close to the inner edge of the accretion disk. In this second paper, we suggest that this process does also exist for an asymmetry in the gravitational potential of the compact object. We prove that the same physics applies, at least in the linear stage of the response to the disturbance in the system. This kind of asymmetry is well suited for neutron stars or white dwarfs possessing an inhomogeneous interior allowing for a deviation from a perfectly spherically symmetric gravitational field. After a discussion on the magnitude of this deformation applied to neutron stars, we show by a linear analysis that the disk initially in a cylindrically symmetric stationary state is subject to {three kinds of resonances: a corotation resonance, a Lindblad resonance due to a driven force and a parametric resonance}. In a second part, we focus on the linear response of a thin accretion disk in the 2D limit. {Waves are launched at the aforementioned resonance positions and propagate in some permitted regions inside the disk, according to the dispersion relation obtained by a WKB analysis}. In a last part, these results are confirmed and extended via non linear hydrodynamical numerical simulations performed with a pseudo-spectral code solving Euler's equations in a 2D cylindrical coordinate frame. {We found that for a weak potential perturbation, the Lindblad resonance is the only effective mechanism producing a significant density fluctuation}. In a last step, we replaced the Newtonian potential by the so called logarithmically modified pseudo-Newtonian potential in order to take into account some general-relativistic effects like the innermost stable circular orbit (ISCO). The latter potential is better suited to describe the close vicinity of a neutron star or a black hole. However, from a qualitative point of view, the resonance conditions remain the same. The highest kHz QPOs are then interpreted as the orbital frequency of the disk at locations where the response to the resonances are maximal. It is also found that strong gravity is not required to excite the resonances.  相似文献   

Fragmentation and the formation of primordial protostars: the possible role of collision-induced emission     

E. Ripamonti  T. Abel 《Monthly notices of the Royal Astronomical Society》2004,348(3):1019-1034

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Acceleration of anomalous cosmic rays at the heliospheric termination shock     

Berezhko  E. G.  Ksenofontov  L. T. 《Astronomy Letters》2005,31(12):836-843

Astronomy Letters - The acceleration of anomalous cosmic rays (ACRs) at the heliospheric termination shock and their influence on the shock structure and location are analyzed in terms of a...  相似文献   

Strong interplanetary field enhancements at Ulysses—evidence of dust trails' interaction with the solar wind?     

Geraint H Jones  André Balogh  Robert J MacDowall 《Icarus》2003,166(2):297-310

Interplanetary field enhancements were first discovered in the vicinity of Venus. These events are characterised by an increase in the magnitude of the heliospheric magnetic field with a near-symmetrical, sometimes thorn-shaped profile, and last from minutes to hours. Surveys of the events near Venus and Earth indicated clustering of the events in inertial space, which suggested that their sources were Solar System objects other than the Sun. A survey is presented of strong events of this type detected by the Ulysses spacecraft from 1990 to late 2001. Most of the events are accompanied by a discontinuity in the field direction near the events' centres. Other discontinuities are often symmetrical about the enhancement. The majority of events last less than two hours. When examined as a whole, the events tend to be accompanied by subtle changes in some plasma parameters. The majority of the enhancements are accompanied by magnetic holes on their fringes. The enhancements' occurrence rate increases with decreasing heliocentric distance. Possible formation mechanisms are discussed. No link was found with solar, or solar wind sources. Several aspects of the survey results are consistent with an origin related to cometary dust trails. Possible processes associated with a dust-solar wind interaction are discussed.  相似文献   

Geo-effectiveness of solar wind extremes     

Hari Om Vats 《Journal of Astrophysics and Astronomy》2006,27(2-3):227-235

Examples of extreme events of solar wind and their effect on geomagnetic conditions are discussed here. It is found that there are two regimes of high speed solar wind streams with a threshold of ∼ 850 km s^-1. Geomagnetic activity enhancement rate (GAER) is defined as an average increase in Ap value per unit average increase in the peak solar wind velocity (Vp) during the stream. GAER was found to be different in the two regimes of high speed streams with +ve and-ve IMF. GAER is 0.73 and 0.53 for solar wind streams with +ve and -ve IMF respectively for the extremely high speed streams (< 850 km s^-1). This indicates that streams above the threshold speed with +ve IMF are 1.4 times more effective in enhancing geomagnetic activity than those with -ve IMF. However, the high speed streams below the threshold with -ve IMF are 1.1 times more effective in enhancing geomagnetic activity than those with +ve IMF. The violent solar activity period (October–November 2003) of cycle 23 presents a very special case during which many severe and strong effects were seen in the environment of the Earth and other planets; however, the z-component of IMF (Bz) is mostly positive during this period. The most severe geomagnetic storm of this cycle occurred when Bz was positive.  相似文献   

Baryon loading and the Weibel instability in gamma-ray bursts     

M. Fiore  L. O. Silva  C. Ren  M. A. Tzoufras  W. B. Mori 《Monthly notices of the Royal Astronomical Society》2006,372(4):1851-1855

The dynamics of two counter-streaming electron–positron–ion unmagnetized plasma shells with zero net charge is analysed in the context of magnetic field generation in gamma-ray burst internal shocks due to the Weibel instability. The effects of large thermal motion of plasma particles, arbitrary mixture of plasma species and space charge effects are taken into account. We show that, although thermal effects slow down the instability, baryon loading leads to a non-negligible growth rate even for large temperatures and different shell velocities, thus guaranteeing the robustness and the occurrence of the Weibel instability for a wide range of scenarios.  相似文献