Neutrinos from Supernovae

Neutrinos are fundamental particles in the collapse of massive stars. Because of their weakly interacting nature, neutrinos can travel undisturbed through the stellar core and be direct probes of the still uncertain and fascinating supernova mechanism. Intriguing recent developments on the role of neutrinos during the stellar collapse are reviewed, as well as our current understanding of the flavor conversions in the stellar envelope. The detection perspectives of the next burst and of the diffuse supernova background will be also outlined. High-energy neutrinos in the GeV-PeV range can follow the MeV neutrino emission. Various scenarios concerning the production of high-energy neutrinos are discussed.     

Explosion mechanism in supernovae collapse

The prevailing number of supernovae have come from the collapse of iron stellar cores. The 1D-hydrodynamic theory has failed, for more than 20 years of its development, to find an effective mechanism of the supernova explosion, despite the fact that it describes the characteristics neutrino pulse satisfactorily. In the present paper a scenario of rotational nature of collapsing supernova explosion is formulated and discussed.In the first stage of the collapse, a fast rotating protoneutron star is formed (quasi-1D-hydrodynamic model by Imshennik and Nadyozhin 1977, 1992). Then in the second stage it fragmentations into a short-lived neutron binary star, which results in an inevitable supernova-scale explosion of the low-mass component: energy release is just about 10⁵¹ ergs. The important effects of fragmentation, gravitational radiation, mass transfer and the explosion are investigated by both analytical and numerical methods (Aksenov and Imshennik, 1994; Imshennik and Popov, 1994; Aksenov, Blinnikov, and Imshennik, 1994, in press). A comparison of the proposed scenario with the explosion of SN 1987A is made.     

在核爆炸冲击波环境中飞行飞机的结构动力响应分析

着重分析在核爆炸冲击波环境中飞机结构动力响应有关问题,推导了动力学基本方程,介绍了飞机在核爆炸冲击波中非定常气动载荷计算方法,给出了结构动力响应分析的思路和计算过程,提出了飞行飞机在核爆炸冲击波作用下总体结构安全边界值的估算方法,并对存在的问题进行了讨论。     

Short Timescale Core Dynamics: Theory and Observations

Fluid motions in the Earth’s core produce changes in the geomagnetic field (secular variation) and are also an important ingredient in the planet’s rotational dynamics. In this article we review current understanding of core dynamics focusing on short timescales of years to centuries. We describe both theoretical models and what may be inferred from geomagnetic and geodetic observations. The kinematic concepts of frozen flux and magnetic diffusion are discussed along with relevant dynamical regimes of magnetostrophic balance, tangential geostrophy, and quasi-geostrophy. An introduction is given to free modes and waves that are expected to be present in Earth’s core including axisymmetric torsional oscillations and non-axisymmetric Magnetic-Coriolis waves. We focus on important recent developments and promising directions for future investigations.     

Crucial Physical Dependencies of the Core-Collapse Supernova Mechanism

We explore with self-consistent 2D Fornax simulations the dependence of the outcome of collapse on many-body corrections to neutrino-nucleon cross sections, the nucleon-nucleon bremsstrahlung rate, electron capture on heavy nuclei, pre-collapse seed perturbations, and inelastic neutrino-electron and neutrino-nucleon scattering. Importantly, proximity to criticality amplifies the role of even small changes in the neutrino-matter couplings, and such changes can together add to produce outsized effects. When close to the critical condition the cumulative result of a few small effects (including seeds) that individually have only modest consequence can convert an anemic into a robust explosion, or even a dud into a blast. Such sensitivity is not seen in one dimension and may explain the apparent heterogeneity in the outcomes of detailed simulations performed internationally. A natural conclusion is that the different groups collectively are closer to a realistic understanding of the mechanism of core-collapse supernovae than might have seemed apparent.     

Supernova explosions — The role of a Rayleigh-Taylor instability

A two dimensional hydrodynamic study indicates that convectively unstable gradients which develop during core collapse and bounce give rise to large scale core overturn. It is also shown that the concomitant release of neutrini can deposit large amounts of energy and momentum in the infalling envelope and give rise to a powerful supernova explosion.     

Triggering Protostellar Collapse, Injection, and Disk Formation

Certain meteoritical inclusions contain evidence for the existence of short-lived radioactivities such as ²⁶Al and ⁴¹Ca at the time of their formation 4.566 billion years ago. Because the half-lives of these nuclides are so short, this evidence requires that no more than about a million years elapsed between their nucleosynthesis and their inclusion in cm-sized solids in the solar nebula. This abbreviated time span can be explained if these nuclides were synthesized in a stellar source such as a supernova, and were then transported across the interstellar medium by the resulting shock wave, which then triggered the gravitational collapse of the presolar molecular cloud core. Detailed 2D and 3D numerical hydrodynamical models are reviewed and show that such a scenario is consistent with the time scale constraint, and with the need to both trigger collapse and to inject shock-wave matter into the collapsing protostellar cloud and onto the protoplanetary disk formed by the collapse. This revised version was published online in June 2006 with corrections to the Cover Date.     

Aspects of neutrino astronomy

A broad overview of the current status of experimental neutrino astronomy is presented. Particular emphasis is given to the major recent developments that have occurred during the last few years. It is concluded that these developments and the next generation of experiments currently being installed signifies the coming of age of neutrino astronomy.     

Solar neutrinos: Theory versus observation

The current state of the solar neutrino problem is described. The predictions of solar models (standard and nonstandard) are reviewed. The neutrino absorption cross sections for all detectors of current interest are reviewed with special emphasis on the uncertainties that exist in the calculations for each target. A long-term program for neutrino spectroscopy of the solar interior is outlined. It is shown, in addition, that stellar collapses can be detected within the galaxy by the proposed solar neutrino detectors.     

The effect of rotation on the hydrodynamics of stellar collapse

Summary Using values of d, _min, and _max that Van Riper (1978) has found most promising for a hydrodynamic envelope ejection, we have shown that even a small amount of rotation in the initial core can stop its collapse before nuclear densities are reached. We expected _i > 0.02 to produce significant deviations from a spherically symmetric collapse, but have found that _i as much as ten times smaller than this will not allow the core to reach densities as high as in the spherical collapse. In no case, however, does the core flatten very much, nor does the value of become very large. Low final 's preclude the formation of an axisymmetric torus. They also indicate that deformation of an iron core into a triaxial configuration or fragmentation of the core during its collapse is an extremely unlikely event. (Note: Classically, must exceed 0.27 before a dynamic instability to non-axisymmetric perturbations is encountered.)The small degree of flattening of the core also suggests that the reduced moment of inertia I of the core will always be relatively small in magnitude and hence that the third time derivative of I, which is proportional to the energy emitted in gravity wave radiation, will not be very significant. Numerically calculated estimates of I- during some of these model evolutions supports this suspicion. If the _min and used here are found to be realistic values after the detailed physics of the core collapse is well understood, it is clear that gravitational radiation from a core collapse will be difficult to measure.Finally, we should point out that it is the relatively large values of Y_min (near 4/3) combined with values of d near unity that (a) prevented the core from flattening significantly in these models and (b) prevented the core from reaching high configurations. If realistic values of either one (or both) of these parameters are found to be much smaller in more complete models of the core collapse, then the core will have to become flatter (and denser) before pressure gradients will support it along the rotation axis. All of the conclusions drawn here would be modified accordingly under those circumstances. It should also be noted that in general relativistic models, the critical for spherical collapse is somewhat larger than 4/3 (Van Riper, 1979). Therefore, we predict that when fully general relativistic core collapses are performed including rotation, a given choice of _min and _i will produce a slightly flatter and slightly denser core than the corresponding model that has been presented here.     

Solar Neutrinos

The current status of solar neutrino experiments is reviewed. All the experimental measurements show deficits of solar neutrinos. Non monotonic suppression indicates that the problem may naturally be explained by neutrino oscillations, but not by modifying solar models. A new experiment shows very promising results. We hope that a definite answer to the question of whether solar neutrinos are oscillating will be obtained in the very near future. This revised version was published online in June 2006 with corrections to the Cover Date.     

Inside and outside of supernovae

A newly formed neutron star in a supernova finds itself in a dense environment, in which the gravitational energy of accreting matter can be lost to neutrinos. For the conditions in SN 1987A, 0.1M may have fallen back onto the central neutron star on a timescale of hours after the explosion, after which the accretion rate is expected to drop sharply. Radiation is trapped in the flow until the mass accretion rate drops to 2×10^–4 M yr^–1 at which point radiation can begin to escape from the shocked envelope at an Eddington limit luminosity. Between this neutrino limit and the Eddington limit, 3×10^–8 M yr^–1, there are no steady, spherical solutions for neutron star accretion. SN 1987A should have reached the neutrino limit within a year of the explosion; the current lack of an Eddington luminosity can be attributed to black hole formation or to a clearing of the neutron star envelope. There is no evidence for newly formed neutron stars in supernovae. Radio supernovae, which were initially interpreted as pulsar activity, probably involve circumstellar interaction; SN 1993J shows especially good evidence for outer shock phenomena.     

非等强度多道冲击波作用下空泡溃灭机制分析

航空航天、生物医学等领域均存在冲击波与空泡作用问题,冲击波尤其是多道冲击波作用下空泡溃灭包含着复杂的多相瞬变行为与物理现象。基于自主搭建的可压缩多相流并行数值平台,对液体中单一/多道冲击波与空泡的作用过程进行了数值模拟。通过对冲击波作用下空泡内波系性质,以及多道冲击波与空泡作用后的系列反射波系在液体中的相互作用过程进行详细解析,分析了不同冲击波作用下空泡的形变演化过程,探究了空泡的溃灭机制与特性。研究发现,相较于单一冲击波的作用,多道冲击波作用下空泡内部及周围液体流场中的波系结构更为复杂,然而无论是单一冲击波或是多道冲击波工况,其最终诱发的空泡溃灭形态均十分相似。而且,当空泡发生溃灭前所受到多道冲击波的总强度与单一冲击波强度相等时,空泡的溃灭波强度也十分相近,为多道冲击波与空泡作用效果的定量化评估提供了理论基础和依据。     

爆炸冲击波对反舰导弹发动机舱的毁伤效应

为研究爆炸冲击波对来袭反舰导弹发动机舱的毁伤效应,应用ANSYS/LS-DYNA软件,对定量TNT和压装8701炸药爆炸产生的冲击波毁伤不同距离处的反舰导弹发动机舱进行了数值模拟。计算结果表明:爆炸冲击波对导弹发动机舱的毁伤以壳体凹陷为主要形式,对凹陷壳体周围区域几乎没有影响；爆炸冲击波对反舰导弹发动机舱的毁伤效应,随起爆点距离增加而迅速减小,且初期衰减速度明显大于后期。10 kg的装药量、炸点位于3 m处时,TNT和压装8701炸药对反舰导弹发动机舱基本无法造成毁伤。     

水下冲击波聚焦作用下空化效应的实验研究

冲击波聚焦在聚焦区域形成局部较高压力的同时还会在焦区产生空化效应。基于旋转椭球面反射罩及置于其焦点的水中脉冲放电声源建立了水下冲击波聚焦系统。通过压力传感器测量了反射罩轴向的压力历程曲线及峰值压力分布。同时,搭建了高速摄影所需的光学装置,拍摄了空化现象的高速摄影图片,对水下冲击波聚焦过程和空化汽泡的产生、发展及湮灭的整个过程进行了研究。对压力历程曲线和高速摄影所得结果进行对比分析得到空化现象产生的物理过程。实验结果表明：负压是空化现象发生的主要原因,空化汽泡的塌缩时间与汽泡半径存在线性关系,并且汽泡膨胀阶段持续的时间大于塌缩阶段持续的时间。     

战斗部内爆对舰艇舱室的毁伤效应仿真

采用有限元方法建立了舰艇内爆模型,仿真计算了不同壁厚舱室结构在反舰导弹战斗部内爆作用下的毁伤效应。结果表明,战斗部在舰艇舱室内内爆时,冲击波是对舰艇舱室造成毁伤的主要因素,在舰艇舱室角隅部位,由于冲击波的反复叠加,造成的毁伤更为严重;同时,舰艇舱室的焊接质量是影响舱室结构强度的重要因素。     

Urca processes in dense matter and neutron star cooling

Urca-processes were introduced into astrophysics by Gamow and Schoenberg in 1941. Neutrino cooling resulting from urca-processes plays an important role at the latest stages of evolution of massive stars. Recent work on neutrino emissivity of dense matter shows that neutrino cooling via urca-processes could determine the thermal evolution of young neutron stars and depends dramatically on the composition of the neutron star core. In particular, if a neutron star contains a central core in which the direct urca-process is operative, the cooling timescale shortens by many orders of magnitude.     

A Survey of Field-Aligned Mach Number and Plasma Beta in the Solar Wind

We have surveyed solar wind plasma beta and field-aligned Alfvénic Mach number using Ulysses and Wind data. We show the characteristic timescale and occurrence frequency of ‘magnetically dominated’ solar wind, whose interaction with a planetary magnetosphere may produce a bow shock with multiple shock fronts. We discuss radial, latitudinal, and solar cycle effects. This revised version was published online in August 2006 with corrections to the Cover Date.     

Numerical simulation of magnetohydrodynamic shock propagation in the corona

Recent developments in the field of numerical simulation models for the study of shock wave propagation in the corona are presented. These models are based on gasdynamic (GD) and ideal (that is, dissipationless, except at shocks) magnetohydrodynamic (MHD) theories. The characteristics and physical interpretations of the results derived from these models are discussed in some detail.The most significant physical results obtained to date are provided by the two-dimensional non-planar, time-dependent, MHD numerical simulation model. In this model, the non-linear interaction among the three essential MHD waves, i.e., fast-, slow-, and Alfvén waves are demonstrated. Finally, the physical relevance of these numerical simulation models in relation to observed solar activity is presented.An invited paper presented at STIP Workshop on Shock Waves in the Solar Corona and Interplanetary Space, 15–19 June, 1980, Smolenice, Czechoslovakia.     

An Introduction to Theory and Models of CMEs, Shocks, and Solar Energetic Particles

We present a brief introduction to the essential physics of coronal mass ejections as well as a review of theory and models of CME initiation, solar energetic particle (SEP) acceleration, and shock propagation. A brief review of the history of CME models demonstrates steady progress toward an understanding of CME initiation, but it is clear that the question of what initiates CMEs has still not been solved. For illustration, we focus on the flux cancellation model and the breakout model. We contrast the similarities and differences between these models, and we examine how their essential features compare with observations. We review the generation of shocks by CMEs. We also outline the theoretical ideas behind the origin of a gradual SEP event at the evolving CME-driven coronal/interplanetary shock and the origin of “impulsive” SEP events at flare sites of magnetic reconnection below CMEs. We argue that future developments in models require focused study of “campaign events” to best utilize the wealth of available CME and SEP observations.