Massive star clusters in the Magellanic Clouds are observed to follow a striking trend in size with age – older clusters exhibit a much greater spread in core radius than do younger clusters, which are generally compact. We present results from realistic N-body modelling of massive star clusters, aimed at investigating a dynamical origin for the radius-age trend. We find that stellar-mass black holes, formed as remnants of the most massive stars in a cluster, can constitute a dynamically important population. If retained, these objects rapidly form a dense core where interactions are common, resulting in the scattering of black holes into the cluster halo, and the ejection of black holes from the cluster. These processes heat the stellar component, resulting in prolonged core expansion of a magnitude matching the observations. Core expansion at early times does not result from the action of black holes, but can be reproduced by the effects of rapid mass-loss due to stellar evolution in a primordially mass segregated cluster.

By means of high-resolution and wide-field observations in the UV and optical bands we have derived the radial distribution of the Blue Stragglers Star (BSS) population in a number of galactic globular clusters. Monte-Carlo dynamical simulations have then been used to interpret the observed radial distributions in terms of percentage of collisional and mass-transfer BSS populating each cluster. I will present the main results thus obtained and an overall cluster–to–cluster comparison for the whole sample collected so far, mainly focusing on the clues that such an approach provides about the BSS formation mechanisms.

Recent observations of the Galactic center revealed a nuclear disk of young OB stars, in addition to many similar outlying stars with higher eccentricities and/or high inclinations relative to the disk (some of them possibly belonging to a second disk). Binaries in such nuclear disks, if they exist in non-negligible fractions, could have a major role in the evolution of the disks through binary heating of this stellar system. We suggest that interactions with/in binaries may explain some (or all) of the observed outlying young stars in the Galactic center. Such stars could have been formed in a disk, and later on kicked out from it through binary related interactions, similar to ejection of high velocity runaway OB stars in young clusters throughout the galaxy.

In this work wave propagation in a 3-D magnetic flux tube is solved numerically. The aim is to find interaction between kink waves and higher order modes (flute modes).

A 60x60x60 cube is set up, containing a vertically oriented uniform magnetic flux tube, to solve numerically. Waves are observed propagating after triggering them with solution to the linearized system.

The waves propagate acquiring a distinctive shape (seen in the crosscut of the tube at an arbitrary height showing the radial velocity maps). It is discarded that this is caused by the existence of higher order modes and is found that the radial dependence of the phase speed creates the motion.

It was also found that the study of the profile of the radial velocity map of a slice of the system is a very intuitive way of analysing the modes of waves propagating through the flux tube.

We have conducted an extensive photometric search for dwarf nova (DN) outbursts in 16 Galactic globular clusters (GCs). The survey was based on the rich photometric data collected by the Cluster AgeS Experiment (CASE) team. We have identified two new DNe. Together with previously known systems this gives the total number of 12 known DNe in 7 Galactic GCs. Inserting artificial light curves of “DNe” into frames of investigated clusters allowed us to assess completeness of the search. Our results clearly show that outbursting cataclysmic variables (CVs) are very rare in GCs in comparison to field CVs where half of the systems belongs to DNe. Recent X-ray observations of GCs lead to identification of hundreds of compact binaries. Many of them are promising candidates for CVs. The theory also predicts that dozens of white/red dwarf binaries should form in the cores of GCs via dynamical processes or internal evolution of the binaries. Our results rises the question about possible causes of paucity of outbursts in GCs.

The existence of the solar tachocline, a thin differentially rotating layer at the base of the convection zone which is inferred from helioseismology, leads to the concept of an interface dynamo. The tachocline is magnetically coupled to the radiative interior and the overlying convection zone. A multilayered interface dynamo is required to describe the dynamo process involved. We first discuss a two-dimensional multilayered interface dynamo model in cartesian geometry consisting of four horizontal layers with different magnetic diffusivities magnetically coupled by the three sets of interface matching conditions for the generated magnetic field. Exact solutions of the coupled dynamo system are obtained in this model. We then discuss a fully three-dimensional and multi-layered spherical dynamic interface dynamo using a finite element method based on the three-dimensional tetrahedralization of the whole spherical system. The spherical dynamic interface dynamo also consists of four magnetically coupled zones. In the convection zone, the fully three-dimensional dynamic feedback of Lorentz forces is taken into account. It is shown that the dynamo is characterized by a strong toroidal magnetic field, selects dipolar symmetry and propagates equatorward.

The dissolution time (tdis) of clusters in a tidal field does not scale with the “classical” expression for the relaxation time. First, the scaling with N, and hence cluster mass, is shallower due to the finite escape time of stars. Secondly, the cluster half-mass radius is of little importance. This is due to a balance between the relative tidal field strength and internal relaxation, which have an opposite effect on tdis, but of similar magnitude. When external perturbations, such as encounters with giant molecular clouds (GMC) are important, tdis for an individual cluster depends strongly on radius. The mean dissolution time for a population of clusters, however, scales in the same way with mass as for the tidal field, due to the weak dependence of radius on mass. The environmental parameters that determine tdis are the tidal field strength and the density of molecular gas. We compare the empirically derived tdis of clusters in six galaxies to theoretical predictions and argue that encounters with GMCs are the dominant destruction mechanism. Finally, we discuss a number of pitfalls in the derivations of tdis from observations, such as incompleteness, with the cluster system of the SMC as particular example.

Globular clusters produce orders of magnitude more millisecond pulsars per unit mass than the Galactic disk. Since the first cluster pulsar was uncovered 20 years ago, at least 138 have been identified – most of which are binary millisecond pulsars. Because their origins involve stellar encounters, many of the systems are exotic objects that would never be observed in the Galactic disk. Examples include pulsar-main sequence binaries, extremely rapid rotators (including the current record holder), and millisecond pulsars in highly eccentric orbits. These systems are allowing new probes of the interstellar medium, the equation of state of material at supra-nuclear density, the masses of neutron stars, and globular cluster dynamics.

Observations in EUV lines of the solar corona revealed large scale propagating waves generated by eruptive events able to travel across the solar disk for large distances. In the low corona, CMEs are known to generate, e.g. EIT waves which can be used to sample the coronal local and global magnetic field. This contribution presents theoretical models for finding values of magnetic field in the quiet Sun and coronal loops based on the interaction of global waves and local coronal loops as well as results on the generation and propagation of EIT waves. The physical connection between local and global solar coronal events (e.g. flares, EIT waves and coronal loop oscillations) will also be explored.

The damping of standing slow mode oscillations in hot (T > 6 MK) coronal loops is described in the linear limit. The effects of energy dissipation by thermal conduction, viscosity, and radiative losses and gains are examined for both stratified and nonstratified loops. We find that thermal conduction acts on the way of increasing the period of the oscillations over the sound crossing time, whereas the decay times are mostly determined by viscous dissipation. Thermal conduction alone results in slower damping of the density and velocity waves compared to the observations. Only when viscosity is added do these waves damp out at the same rate of the observed SUMER loop oscillations. In the linear limit, the periods and decay times are barely affected by gravity.

We study the formation, growth, and co-evolution of single and multiple supermassive black holes (SMBHs) and compact objects like neutron stars, white dwarfs, and stellar mass black holes in galactic nuclei and star clusters, focusing on the role of stellar dynamics. In this paper we focus on one exemplary topic out of a wider range of work done, the study of orbital parameters of binary black holes in galactic nuclei (binding energy, eccentricity, relativistic coalescence) as a function of initial parameters. In some cases the classical evolution of black hole binaries in dense stellar systems drives them to surprisingly high eccentricities, which is very exciting for the emission of gravitational waves and relativistic orbit shrinkage. Such results are interesting to the emerging field of gravitational wave astronomy, in relation to a number of ground and space based instruments designed to measure gravitational waves from astrophysical sources (VIRGO, Geo600, LIGO, LISA). Our models self-consistently cover the entire range from Newtonian dynamics to the relativistic coalescence of SMBH binaries.

We discuss results from numerical simulations of star cluster formation in the turbulent interstellar medium (ISM). The thermodynamic behavior of the star-forming gas plays a crucial role in fragmentation and determines the stellar mass function as well as the dynamic properties of the nascent stellar cluster. This holds for star formation in molecular clouds in the solar neighborhood as well as for the formation of the very first stars in the early universe. The thermodynamic state of the ISM is a result of the balance between heating and cooling processes, which in turn are determined by atomic and molecular physics and by chemical abundances. Features in the effective equation of state of the gas, such as a transition from a cooling to a heating regime, define a characteristic mass scale for fragmentation and so set the peak of the initial mass function of stars (IMF). As it is based on fundamental physical quantities and constants, this is an attractive approach to explain the apparent universality of the IMF in the solar neighborhood as well as the transition from purely primordial high-mass star formation to the more normal low-mass mode observed today.

We investigate the structures of embedded and open clusters using statistical methods, in particular the combined parameter , which permits to quantify the cluster structure. Star clusters build up from several subclusters evolving from a structured to a more centrally concentrated stage. The evolution is not only a function of time, but also of the mass of the objects. Massive stars are usually centrally concentrated, while lower-mass stars are more widespread, reflecting the effect of mass segregation. Using this method we find that in IC 348 and the Orion Nebula Cluster the spatial distribution of brown dwarfs does not follow the central clustering of stars, giving important clues to their formation mechanism by supporting the ejected embryo scenario.

We present the results of a survey of N-body simulations aimed at exploring the implications of primordial mass segregation on the dynamical evolution of star clusters. We show that, in a mass-segregated cluster, the effect of early mass loss due to stellar evolution is, in general, more destructive than for an unsegregated cluster with the same density profile and leads to shorter lifetimes, a faster initial evolution toward less concentrated structure and flattening of the stellar initial mass function.

The detection of rapidly damped transverse oscillations in coronal loops by Aschwanden et al. (1999) and Nakariakov et al. (1999) gave a strong impetus to the study of MHD waves and their damping. The common interpretation of the observations of these oscillations is based on kink modes. This paper reviews how the observed period and damping time can be reproduced by MHD wave theory when non-uniform equilibrium models are considered that have a transversal variation of the local Alfven velocity. The key point here is that resonant absorption cannot be avoided and occurs as natural damping mechanism for kink waves in non-uniform equilibrium models. The present paper starts with work by Hollweg & Yang (1988) and discusses subsequent developments in theory and their applications to seismology of coronal loops. It addresses the consistent use of observations of periods and damping times as seismological tools within the framework of resonant absorption. It shows that within the framework of resonant absorption infinitely many equilibrium models can reproduce the observed values of periods and damping times.

The extensive stellar radial-velocity surveys of the WIYN Open Cluster Study now allow comprehensive studies of the solar-type hard-binary populations in open clusters as a function of age. We first describe an empirical “initial” hard-binary population as derived from the young open cluster NGC 2168 (M35). Given the limited analyses so far, the cluster binary population is indistinguishable from that of the field. We then compare the hard-binary population in the old open cluster NGC 188 to the binary population in the sophisticated N-body simulations of the old cluster M67 by Hurley et al. The binary populations in the cluster and the simulation show significant differences in binary frequency and fraction of circularized binaries, while otherwise showing similar orbital eccentricity distributions. Since the simulations were designed to match the encounter products in M67, such as blue stragglers, the large reduction in binary fraction indicated by the empirical results likely will also require changes in the simulation physics producing blue stragglers and other anomalous stars arising from stellar dynamics. We present three case studies of stars in open clusters which very likely are products of dynamical encounters between binaries and either single stars or other binaries: the M67 blue straggler S1082, the M67 sub-subgiant S1113, and the horizontal branch star 6819-3002 in the intermediate-age open cluster NGC 6819. Finally, we remind the reader of recent empirical results on the rates of tidal interactions, using tidal circularization periods in open clusters. Every indication is that current theories underestimate the effectiveness of tidal circularization, a result that need to be incorporated into dynamical simulations of dense stellar systems.

Recent high-resolution observations have pointed out that prominences are made of small threads (also named fibrils) piled up to form the body of the prominence. These fine structures also seem to support their own oscillatory modes, while their effect on the global modes of the prominences are less certain. We study the effect of adding a smooth transition layer between the prominence material and the corona along the magnetic field line, since previous studies have considered a jump in density in this interface. Then we compare the results with previous models and check that these transition layers do not affect significantly the periods of the modes.

We have developed a new method for post-Newtonian, high-precision integration of stellar systems containing a super-massive black hole (SMBH), splitting the forces on a particle between a dominant central force and perturbations. We used this method to perform fully collisional N-body simulations of inspiralling intermediate-mass black holes (IMBHs) in the centre of the Milky Way.

The analysis of an 11-hour series of high resolution white light observations of a large pore in the sunspot group NOAA 7519, observed on 5 June 1993 with the Swedish Vacuum Solar Telescope at La Palma on Canary Islands, has been recently described by Dorotovič et al. (2002). Special attention was paid to the evolution of a filamentary region attached to the pore, to horizontal motions around the pore, and to small-scale morphological changes. One of the results, relevant to out work here, was the determination of temporal area evolution of the studied pore where the area itself showed a linear trend of decrease with time at an average rate of −0.23 Mm2h−1 during the entire observing period. Analysing the time series of the are of the pore, there is strong evidence that coupling between the solar interior and magnetic atmosphere can occur at various scales and that the referred decrease of the area may be connected with a decrease of the magnetic field strength according to the magnetic field-to-size relation. Periods of global acoustic, e.g. p-mode, driven waves are usually in the range of 5–10 minutes, and are favourite candidates for the coupling of interior oscillations with atmospheric dynamics. However, by assuming that magneto-acoustic gravity waves may be there too, and may act as drivers, the observed periodicities (frequencies) are expected to be much longer (smaller), falling well within the mMHz domain. In this work we determine typical periods of such range in the area evolution of the pore using wavelet analysis. The resulted periods are in the range of 20–70 minutes, suggesting that periodic elements of the temporal evolution of the area of this studied pore could be linked to, and considered as, observational evidence of linear low-frequency slow sausage (magneto-acoustic gravity) waves in magnetic pores. This would give us further evidence on the coupling of global solar oscillations to the overlaying magnetic atmosphere.

We explore the possibility that all close binaries, i.e. those with periods < 3 days, including contact binaries, are produced from initially wider binaries by the action of a triple companion through the medium of Kozai Cycles with Tidal Friction (KCTF).