A global model for violent relaxation

A global model for violent relaxation in dynamical systems is developed on basis of the virial equation and a second equation which describes the approach to equilibrium of the potential energy. It is shown under very general arguments, that the central region of the system must shed mass to an extended envelope in the collapse episode. Specific upper and lower limits for the mass in the core can be calculated.Comparison with numerical experiments agrees with the predictions of the model in a satisfactory fashion.     

Violent relaxation and mixing in non-uniform one-dimensional gravitational systems

A series of numerical experiments on non-uniform one-dimensional gravitational systems have been performed with the aim of testing the conflicting models of violent relaxation, developed by Lynden-Bell and by Shu. It is shown that the phase fluid model of Lynden-Bell ceases to apply to the particle systems investigated already at the end of the virialisation phase. From then on the particle nature of the system dominates its evolution: a very efficient mixing process, driven by the mean field fluctuations, washes out all memory of the initial phase densities. During this collisionless mixing process and in the absence of formation of structure, the system evolves towards a configuration characterized by a single temperature per unit mass in accordance with the Shu model of violent relaxation.     

Three-body dynamics: intermittent chaos with strange attractor

We have studied the structure of chaos in three-body dynamics using the concept of intermittency, implying that violent states of a system alternate in time with quasi-regular states producing together a non-stationary and evolving pattern of unpredictable behaviour. Computer simulations are produced to demonstrate explicitly sporadic short violent bursts in quasi-regular hierarchical states of the systems. This is seen both in orbits and in the long time series generated by the system. The time series prove to be similar in shape to what is observed in various physical experiments with laboratory chaotic systems when they reveal the so-called type-III intermittency. The new effective methods of time series analysis enable us to discover a strange attractor with a fractal dimension slightly above 2. This shows that three-body dynamics has the same intrinsic qualitative structure and quantitative measure of chaos as the widely known chaotic system, the Lorenz attractor.     

Predictions for triple stars with and without a pulsar in star clusters

Though about 80 pulsar binaries have been detected in globular clusters so far, no pulsar has been found in a triple system in which all three objects are of comparable mass. Here, we present predictions for the abundance of such triple systems, and for the most likely characteristics of these systems. Our predictions are based on an extensive set of more than 500 direct simulations of star clusters with primordial binaries, and a number of additional runs containing primordial triples. Our simulations employ a number N _tot of equal-mass stars from   N _tot= 512  to  19 661  and a primordial binary fraction from 0 to 50 per cent. In addition, we validate our results against simulations with   N = 19 661  that include a mass spectrum with a turn-off mass at  0.8 M_⊙  , appropriate to describe the old stellar populations of Galactic globular clusters. Based on our simulations, we expect that typical triple abundances in the core of a dense cluster are two orders of magnitude lower than the binary abundances, which in itself already suggests that we do not have to wait too long for the first comparable-mass triple with a pulsar to be detected.     

Dynamical theory of collisionless relaxation

In his theory of violent relaxation, Lynden-Bell gave a rigorous derivation of the equilibrium distribution, but only a qualitative discussion of the manner in which equilibrium is attained Here we present a fully explicit dynamical theory of collisionless relaxation towards Lynden-Bel equilibrium.The analysis proceeds from the coarse-graining in phase space of the collisionless Boltzmann equation the mesh size being determined by the precision of the observational data. The theoretical developmen leads to a kinetic equation generalizing that obtained by Kadomtsev and Pogutse in the rather differen context of homogeneous plasma turbulence. The collision integral differs from the classical Fokker Planck type essentially by the appearance of products of three distribution functions. It drives th systems towards the Lynden-Bell equilibrium state, on a time-scale which is inversely proportional to th coarse-graining mesh and, in the non-degenerate limit, to the fine-grained phase density. Owing to th various approximations introduced, the theory does not, however, describe the violent relaxation proces itself, but rather its late quiescent phases.     

Long-Term Evolution of FU Ori-Type Stars at Infrared Wavelengths

We investigated the brightness evolution of 5 FU Ori systems in the 1–200 μm wavelength range using observations from the Infrared Space Observatory, 2MASS and MSX data. The SEDs were compared with earlier ones derived from the IRAS photometry and ground-based observations around the epoch 1983. In three cases no difference was seen, in one object the near- and mid-infrared part of the spectrum became fainter by a factor of 2, and in the fifth case a marginal fading was observed. We study in detail the case of V1057 Cyg.     

Evolutionary synthesis modelling of young star clusters in merging galaxies

The observational properties of globular cluster systems (GCSs) are vital tools to investigate the violent star formation histories of their host galaxies. This violence is thought to have been triggered by galaxy interactions or mergers. The most basic properties of a GCS are its luminosity function (number of clusters per luminosity bin) and color distributions. A large number of observed GCS show bimodal color distributions, which can be translated into a bimodality in either metallicity and/or age. An additional uncertainty comes into play when one considers extinction. These effects can be disentangled either by obtaining spectroscopic data for the clusters or by imaging observations in at least four passbands. This allows us then to discriminate between various formation scenarios of GCSs, e.g. the merger scenario by Ashman and Zepf, and the multi-phase collapse model by Forbes et al. Young and metal-rich star cluster populations are seen to form in interacting and merging galaxies. We analyse multiwavelength broad-band observations of these young cluster systems provided by the ASTROVIRTEL project. This revised version was published online in August 2006 with corrections to the Cover Date.     

Statistical mechanics of Keplerian orbits

Theoretical predictions agree with computer simulations at least for those collisional systems in which the restitution coefficient is independent of impact velocity. An uncertainty principle for the orbits restricts the validity of the theory and its predictions. Discussion of the whole theory and of computer simulations shows that a velocity-dependent restitution coefficient provides the only astronomically interesting applications of the collisional processes. The Saturnian and Uranian ring systems correspond very well to theoretical expectations if the restitution coefficient is of this type.     

日冕磁场重建方法在研究太阳爆发活动中的应用

太阳是与地球关系最为密切的天体.发生在日面上的剧烈爆发性活动可能对人类的生存环境产生巨大影响甚至是灾难性后果.包含太阳耀斑、暗条爆发和日冕物质抛射在内的太阳爆发活动是同一物理过程的不同表现形式,其能量来源于爆发前储存在日冕中的磁场自由能.因此,了解日冕磁场的3维结构是理解太阳爆发的触发机制以及活动区的稳定性等现象的前提.由于观测技术限制,目前尚无法对日冕磁场进行常规观测,因此发展了多种利用可常规观测的光球磁场来重建日冕磁场的方法.主要评述近10 yr来各种日冕磁场重建方法在研究太阳爆发活动中的应用.     

DoH-functions always increase during violent relaxation?

Recent work on the violent relaxation of collisionless stellar systems has been based on the notion of a wide class of entropy functions. A theorem concerning entropy increase has been proved. We draw attention to some underlying assumptions that have been ignored in the applications of this theorem to stellar dynamical problems. Once these are taken into account, the use of this theorem is at best heuristic. We present a simple counter-example.     

A numerical comparison of theories of violent relaxation

Using N -body simulations with a large set of massless test particles, we compare the predictions of two theories of violent relaxation, the well-known Lynden-Bell theory and the more recent theory by Nakamura. We derive 'weakened' versions of both the theories in which we use the whole equilibrium coarse-grained distribution function     as a constraint instead of the total energy constraint. We use these weakened theories to construct expressions for the conditional probability   K_i (τ)  that a test particle initially at the phase-space coordinate τ would end-up in the i th macro-cell at equilibrium. We show that the logarithm of the ratio   R_ij (τ) ≡ K_i (τ)/ K_j (τ)  is directly proportional to the initial phase-space density   f ₀(τ)  for the Lynden-Bell theory and inversely proportional to   f ₀(τ)  for the Nakamura theory. We then measure   R_ij (τ)  using a set of N -body simulations of a system undergoing a gravitational collapse to check the validity of the two theories of violent relaxation. We find that both the theories are at odds with the numerical results, both qualitatively and quantitatively.     

3D Hydrodynamical Simulations of Colliding Wind Binaries: TheoryConfronts Observations

The link between gas dynamical models and observations is crucial. The general progress in numerical simulations must be accompanied by predictions for observable quantities, which not only allow to test the models or parts of them against observations but which also improve the understanding of observational data. In this paper we focus on predictions for observations, for three examples of 3Dhydrodynamical simulations of binary star systems, and the techniques required for their derivation. The examples include synthetic, optically thin Doppler broadened line profiles for colliding wind symbiotic binaries, the UV light curve of an accretion model for the symbiotic binary RW Hya, and the X-ray light curve of the WR+O binaryγ Velorum. The main purpose is to emphasize the importance of such studies and to illustrate the capabilities of the emploiedtools. The tools are all contained in the A-MAZE code package we have developed and are publicly available. Seminar for Applied Mathematics This revised version was published online in July 2006 with corrections to the Cover Date.     

Statistical analysis of the optical variability of QSO's

For 34 QSOs and 9 active galaxies with known optical variation and measured redshifts, the rate of optical variation appears to be positively correlated with the age: older quasars vary more violently than younger ones and active galaxies are the most violent of all.     

The role of tidal interactions in star formation

Nearly all of the initial angular momentum of the matter that goes into each forming star must somehow be removed or redistributed during the formation process. The possible transport mechanisms and the possible fates of the excess angular momentum are discussed, and it is argued that transport processes in discs are probably not sufficient by themselves to solve the angular momentum problem, while tidal interactions with other stars in forming binary or multiple systems are likely to be of very general importance in redistributing angular momentum during the star formation process. Most, if not all, stars probably form in binary or multiple systems, and tidal torques in these systems can transfer much of the angular momentum from the gas around each forming star to the orbital motions of the companion stars. Tidally generated waves in circumstellar discs may contribute to the overall redistribution of angular momentum. Stars may gain much of their mass by tidally triggered bursts of rapid accretion, and these bursts could account for some of the most energetic phenomena of the earliest stages of stellar evolution, such as jet-like outflows. If tidal interactions are indeed of general importance, planet-forming discs may often have a more chaotic and violent early evolution than in standard models, and shock heating events may be common. Interactions in a hierarchy of subgroups may play a role in building up massive stars in clusters and in determining the form of the upper initial mass function (IMF) . Many of the processes discussed here have analogues on galactic scales, and there may be similarities between the formation of massive stars by interaction-driven accretion processes in clusters and the buildup of massive black holes in galactic nuclei.     

Irradiation of the secondary star in X-ray Nova Scorpii 1994 (=GRO J165540)

The radiation temperature–redshift relation for Friedmann–Robertson–Walker geometries is rediscussed in connection with recent observational data based on the fine-structure splitting of atomic and singly ionized carbon lines in quasar absorption-line systems. Indirect measurement of T ( z ) is one of the most powerful cosmological tests available because it may exclude even the presence of a cosmological constant. Unlike recent claims, we argue that the temperature at high z may be smaller than the standard prediction, thereby opening a window to alternative (big bang) models. By including new ingredients like a phenomenological decaying vacuum energy density and gravitational 'adiabatic' photon creation as well as late inflationary models driven by a scalar field, a new temperature law is deduced and its predictions are compared with the standard result.     

Prospects in space-based gamma-ray astronomy

Observations of the gamma-ray sky reveal the most powerful sources and the most violent events in the Universe. While at lower wavebands the observed emission is generally dominated by thermal processes, the gamma-ray sky provides us with a view on the non-thermal Universe. Here particles are accelerated to extreme relativistic energies by mechanisms which are still poorly understood, and nuclear reactions are synthesizing the basic constituents of our world. Cosmic accelerators and cosmic explosions are the major science themes that are addressed in the gamma-ray regime.With the INTEGRAL observatory, ESA has provided a unique tool to the astronomical community revealing hundreds of sources, new classes of objects, extraordinary views of antimatter annihilation in our Galaxy, and fingerprints of recent nucleosynthesis processes. While INTEGRAL provides the global overview over the soft gamma-ray sky, there is a growing need to perform deeper, more focused investigations of gamma-ray sources. In soft X-rays a comparable step was taken going from the Einstein and the EXOSAT satellites to the Chandra and XMM/Newton observatories. Technological advances in the past years in the domain of gamma-ray focusing using Laue diffraction and multilayer-coated mirror techniques have paved the way towards a gamma-ray mission, providing major improvements compared to past missions regarding sensitivity and angular resolution. Such a future Gamma-Ray Imager will allow to study particle acceleration processes and explosion physics in unprecedented detail, providing essential clues on the innermost nature of the most violent and most energetic processes in the Universe.     

Statistics of three-body experiments: Escape energy,velocity and final binary eccentricity

The distribution of escape energy, escape velocity and final binary eccentricity in the breakup of a three-body system have been studied, using a recent survey of 25 000 numerical experiments. The dependence of these distributions on the initial parameters have been analysed and were found to agree well with analytical predictions in some cases. The escape velocity depends mainly on the total energy and angular momentum of the system, while the eccentricity is markedly different between planar and three-dimensional systems.     

Constraining the 13C neutron source in AGB stars through isotopic analysis of trace elements in presolar SiC

Abstract— Analyses of the isotopic compositions of multiple elements (Mo, Zr, and Ba) in individual mainstream presolar SiC grains were done by resonant ionization mass spectrometry (RIMS). While most heavy element compositions were consistent with model predictions for the slow neutron capture process (s‐process) in low‐mass (1.5–3 M_⊙) asymptotic giant branch stars of solar metallicity when viewed on single‐element three‐isotope plots, grains with compositions deviating from model predictions were identified on multi‐element plots. These grains have compositions that cannot result from any neutron capture process but can be explained by contamination in some elements with solar system material. Previous work in which only one heavy element per grain was examined has been unable to identify contaminated grains. The multi‐element analyses of this study detected contaminated grains which were subsequently eliminated from consideration. The uncontaminated grains form a data set with a greatly reduced spread on the three‐isotope plots of each element measured, corresponding to a smaller range of ¹³C pocket efficiencies in parent AGB stars. Furthermore, due to this reduced spread, the nature of the stellar starting material, previously interpreted as having solar isotopic composition, is uncertain. The constraint on ¹³C pocket efficiencies in parent stars of these grains may help uncover the mechanism responsible for formation of ¹³C, the primary neutron source for s‐process nucleosynthesis in low‐mass stars.     

Submillimeter Studies of Prestellar Cores and Protostars: Probing the Initial Conditions for Protostellar Collapse

Improving our understanding of the earliest stages of star formation is crucial to gain insight into the origin of stellar masses, multiple systems, and protoplanetary disks. We discuss recent advances made in this area, thanks to submillimeter mapping observations with large single-dish telescopes and interferometers. Although ambipolar diffusion appears to be too slow cores, there is nevertheless good evidence that the gravitational collapse of isolated protostellar cores is strongly magnetically controlled. We also argue that the beginning of protostellar collapse is much more violent in cluster-forming clouds than in regions of distributed star formation.     

On the possibility of disk-fed formation in supergiant high-mass X-ray binaries

We consider the existence of a neutron star magnetic field by the detected cyclotron lines. We collected data on nine sources of high-mass X-ray binaries with supergiant companions as a test case for our model, to demonstrate their distribution and evolution. The wind velocity, spin period and magnetic field strength are studied under different mass loss rates. In our model, correlations between mass-loss rate and wind velocity are found and can be tested in further observations. We examine the parameter space where wind accretion is allowed, avoiding the barrier of rotating magnetic fields, with robust data on the magnetic field of neutron stars. Our model shows that most sources(six of nine systems) can be fed by the wind with relatively slow velocity, and this result is consistent with previous predictions. In a few sources,our model cannot fit the standard wind accretion scenario. In these peculiar cases, other scenarios(disk formation, partial Roche lobe overflow) should be considered. This would provide information about the evolutionary tracks of various types of binaries, and thus exhibit a clear dichotomy behavior in wind-fed X-ray binary systems.