Hysteresis effects and diagnostics of the shock formation in low angular momentum axisymmetric accretion in the Kerr metric     

Tapas K. Das  B. Czerny 《New Astronomy》2012,17(3):254-271

The secular evolution of the purely general relativistic low angular momentum accretion flow around a spinning black hole is shown to exhibit hysteresis effects. This confirms that a stationary shock is an integral part of such an accretion disc in the Kerr metric. The equations describing the space gradient of the dynamical flow velocity of the accreting matter have been shown to be equivalent to a first order autonomous dynamical systems. Fixed point analysis ensures that such flow must be multi-transonic for certain astrophysically relevant initial boundary conditions. Contrary to the existing consensus in the literature, the critical points and the sonic points are proved not to be isomorphic in general, they can form in a completely different length scales. Physically acceptable global transonic solutions must produce odd number of critical points. Homoclinic orbits for the flow possessing multiple critical points select the critical point with the higher entropy accretion rate, confirming that the entropy accretion rate is the degeneracy removing agent in the system. However, heteroclinic orbits are also observed for some special situation, where both the saddle type critical points of the flow configuration possesses identical entropy accretion rate. Topologies with heteroclinic orbits are thus the only allowed non-removable degenerate solutions for accretion flow with multiple critical points, and are shown to be structurally unstable. Depending on suitable initial boundary conditions, a homoclinic trajectory can be combined with a standard non-homoclinic orbit through an energy preserving Rankine-Hugoniot type of stationary shock, and multi-critical accretion flow then becomes truly multi-transonic. An effective Lyapunov index has been proposed to analytically confirm why certain class of transonic flow cannot accommodate shock solutions even if it produces multiple critical points.  相似文献   

Normal modes of black hole accretion disks     

Manuel Ortega-rodríguez  Alexander S. Silbergleit  Robert V. Wagoner 《地球物理与天体物理流体动力学》2013,107(1):75-115

This article studies the hydrodynamical problem of normal modes of small adiabatic oscillations of relativistic barotropic thin accretion disks around black holes (and compact weakly magnetic neutron stars). Employing Wentzel-Kramers-Brillouin (WKB) techniques, we obtain the eigenfrequencies and eigenfunctions of the modes for different values of the mass and angular momentum of the central black hole. We discuss the properties of the various types of modes and show for the first time that modes covering the whole extension of the disk (full-disk p-modes) can exist within the studied thin disk model. However, these modes have relatively short wavelengths.  相似文献   

What was the Volatile Composition of the Planetesimals that Formed the Earth?     

Joseph A. Nuth III 《Earth, Moon, and Planets》2008,102(1-4):435-445

Is there an asteroid type or meteorite class that best exemplifies the materials that went into the Earth? Carbonaceous chondrites were once the objects of choice, and in the minds of many this choice is still valid. However, the origin of primitive chondritic meteorites is unclear. At the extremes they could either be fragments of very small parent bodies that never became hot enough to undergo geochemical modification other than mild lithification, or remnants of the uppermost layers of a body that had undergone a significant degree of internal differentiation, while the top layers remained cool due to radiative heat loss or loss of volatiles to space. This latter case is problematic if one considers these objects as precursors to the Earth since the timescale for the evolution of such a small body could be longer than the timescale for the accretion of the Earth. Large-scale circulation of materials in the primitive solar nebula could greatly increase the diversity of materials near 1 AU while also making the entire inner solar system both more homogeneous and much wetter than previously expected. The total mass of the nebula is an important, but poorly constrained factor controlling the growth of planetesimals. There is also a selection effect that dominates our sampling of the planetesimals that may have existed 4.5 billion years ago; namely, small fragile bodies are more likely to be lost from the system or ground down by collisions between small bodies, yet these are precisely those that may have dominated the population from which the Earth accreted. The composition of these aggregates could have played a very important role in the early chemical evolution of the Earth. In particular, the Earth may have been much wetter and richer in hydrocarbons and other reducing materials than previously suspected.  相似文献   

Observational properties of accretion discs: Precessing and warped?     

Phil Charles  Will Clarkson  Remon Cornelisse  Chun Shih I 《New Astronomy Reviews》2008,51(10-12):768

Over the last 40 years, X-ray astronomy missions have revealed long-term, superorbital periods in a variety of X-ray binaries. These modulations can provide significant constraints on the physical properties of accretion discs. Some of these modulations are Her X-1-like and are interpreted as irradiation-driven, tilted, precessing accretion discs. Others show more complex light curves, with the period changing on timescales >1000 d, and are interpreted in terms of the Ogilvie and Dubus [Ogilvie, G.I., Dubus, G., 2001, MNRAS 320, 485 (OD01)] disc stability criteria. We suggest a categorisation of superorbital periods into six different types, based on their observed characteristics.  相似文献   

The inner magnetized regions of circumstellar accretion discs     

Jonathan Ferreira   《New Astronomy Reviews》2008,52(2-5):42

In this lecture, I will briefly address several phenomena expected when magnetic fields are present in the innermost regions of circumstellar accretion discs: (i) the magneto-rotational instability and related “dead zones”; (ii) the formation of magnetically-driven jets and the observational constraints derived from Classical T Tauri stars; (iii) the magnetic star–disc interactions and their expected role in the stellar spin down.It should be noted that the magnetic fields invoked here are organized large scale magnetic fields, not turbulent small scale ones. I will therefore first argue why one can safely expect these fields to be present in circumstellar accretion discs. Objects devoid of such large scale fields would not be able to drive jets. A global picture is thus gradually emerging where the magnetic flux is an important control parameter of the star formation process as a whole. High angular resolution technics, by probing the innermost circumstellar disc regions should provide valuable constraints.  相似文献   

http://dx.doi.org/10.1016/j.gsf.2016.06.011     

Yusuke Imaeda  Toshikazu Ebisuzaki 《地学前缘(英文版)》2017,8(2):223-231

We present a new united theory of planet formation,which includes magneto-rotational instability(MRl) and porous aggregation of solid particles in a consistent way.We show that the "tandem planet formation" regime is likely to result in solar system-like planetary systems.In the tandem planet formation regime,planetesimals form at two distinct sites:the outer and inner edges of the MRl suppressed region.The former is likely to be the source of the outer gas giants,and the latter is the source for the inner volatile-free rocky planets.Our study spans disks with a various range of accretion rates,and we find that tandem planet formation can occur for M = 10~(7.3)- 10~(-6.9)M_⊙yr~(-1).The rocky planets form between 0.4-2 AU,while the icy planets form between 6-30 All;no planets form in 2—6 AU region for any accretion rate.This is consistent with the gap in the solid component distribution in the solar system,which has only a relatively small Mars and a very small amount of material in the main asteroid belt from 2-6 AU.The tandem regime is consistent with the idea that the Earth was initially formed as a completely volatile-free planet.Water and other volatile elements came later through the accretion of icy material by occasional inward scattering from the outer regions.Reactions between reductive minerals,such as schreibersite(Fe-jP),and water are essential to supply energy and nutrients for primitive life on Earth.  相似文献   

Dynamics of charged dust particles in protoplanetary discs     

Mohsen Shadmehri 《Astrophysics and Space Science》2008,314(1-3):217-223

We study the effect of an imposed magnetic field on the motion of charged dust particles in magnetically active regions of a protoplanetary disc. Assuming a power law structure for the vertical and the toroidal components of the magnetic field for the regions beyond magnetically dead region of the disc, the radial and the vertical velocities of the charged particles, in the asymptotic case of small particles, are calculated analytically. While grains with radii smaller than a critical radius significantly are affected by the magnetic force, motion of the particles with larger radii is independent of the magnetic field. The critical radius depends on the magnetic geometry and the charge of the grains. Assuming that a grain particle has one elementary charge and the physical properties of the disc correspond to a minimum-mass solar nebula, we show that only micron-sized grains are affected by the magnetic force. Also, charge polarity determines direction of the radial velocity. For such small particles, both the radial and the vertical velocities increase due to the magnetic force.  相似文献   

Levee morphology and sedimentology along the lower Tuross River, south-eastern Australia   总被引：2，自引：0，他引：2  

Rob J. Ferguson  & Gary J. Brierley 《Sedimentology》1999,46(4):627-648

Levees on the lower Tuross River in south-eastern Australia reflect a complex interplay between depositional and erosional processes. Stream power, conditioned primarily by valley width, is the key determinant of levee morphology and sedimentology in this confined valley setting. Three styles of levee are described. The Rewlee levee is functionally linked to a flood channel in narrow valley settings (< 250 m). These levees contain a diverse facies assemblage characterized by various scales of erosion surfaces. Vertical accretion on levees has produced conditions under which stream power values exceed the threshold for catastrophic floodplain stripping. The levee at the Mortfield site is associated with less confined settings (valley width 500–600 m), which present lower flood stage and stream power conditions. This levee hosts a wide range of facies, but erosion surfaces are seldom observed. In the more open valley setting at the Central site (valley width 700–1000 m), levees comprise uniform, fine-grained deposits, which grade to pronounced distal floodplains with backswamps. As levees reflect a combination of within-channel and overbank processes, both depositional and erosional, these geomorphic features influence the character and sedimentology of adjacent landforms and the associated alluvial architecture of the basin.  相似文献   

潮滩冲淤扰动下外来与乡土红树植物幼苗稳定性差异     

潘炜杰  祝振昌  蔡宴朋  杨志峰 《热带海洋学报》2021,40(6):120-127

引种外来树种是我国各地在红树林修复实践中常用的措施。由于更容易在困难立林地存活, 外来树种被认为更具竞争优势。为弄清外来与乡土红树植物物种在困难立林地存活能力的差异及其原因, 本研究选取典型外来引种先锋树种拉关木(Laguncularia racemosa)与乡土先锋植物白骨壤(Avicennia marina)为研究对象, 通过控制实验探究潮滩冲淤扰动条件下幼苗稳定性差异及其形成机制。结果表明, 幼苗抗侵蚀能力与幼苗根冠比显著正相关, 泥沙淤积抑制幼苗根系生长, 而底床侵蚀则促进根系伸长。与白骨壤相比, 拉关木幼苗的主根更长, 根冠长度比更大, 更能抵抗底泥的侵蚀扰动, 且通过根系伸长以增强幼苗稳定性的能力也更强。拉关木幼苗比白骨壤更能适应潮滩冲淤扰动, 其竞争优势从幼苗阶段就已经建立。本研究结果可为红树林修复实践中树种和宜林地的选择提供参考, 亦可为评估外来红树物种入侵风险评价提供依据。  相似文献   

Spectral properties of shocked accretion flows— a self-consistent study     

Sandip K. Chakrabarti  Samir Mandal 《Astrophysics and Space Science》2007,309(1-4):163-166

Magnetized accretion flows around black holes which include standing or oscillating shock waves can produce very realistic spectrum till a few MeV. These shocks accelerate hot electrons which produce power-law spectrum. The post-shock region intercepts soft-photons from an external source, namely, a Keplerian disk and also from distributed sources such as the synchrotron photons emitted from thermal and non-thermal electrons originated in the pre-shock and post-shock flow. These photons are inverse Comptonized by the thermal and the non-thermal electrons present in the CENBOL region. Computations show that the emitted radiation is extended till a few MeV. We include the bulk motion Comptonization as well and discuss its importance vis-a-vis the power-law spectrum produced by non-thermal electrons.   相似文献