We study the linear evolution of a Gaussian pulse injected at different locations along a one-dimensional (1D), hot (T ≥ 6.3 MK) coronal loop, including the dissipative effects of thermal conduction, viscosity, heating, and radiative cooling. We consider both homogeneous and stratified loops of different lengths (50 ≤ L ≤ 400 Mm) and values of the pulse width (or standard deviation, βg/L) between 0.005 and 0.02. We find that a Gaussian velocity pulse can generate propagating waves whose amplitudes increase with increasing width of the pulse. The shape of the waves is quite irregular owing to the superposition of the several harmonics composing the Gaussian pulse. Wave damping due to the combined effects of thermal conduction and viscosity is faster in the shortest and hottest loops. The decay times and periods of the waves are within the observed values of decaying modes of hot SUMER loop oscillations.

We have used far-ultraviolet spectroscopy and broad-band photometry to identify and study dynamically-formed stellar exotica in the core of 47 Tucanane. Here, we present a subset of our main results, including: (i) the spectroscopic confirmation of three cataclysmic variables; (ii) the discovery of stripped sub-giant core in a binary system with a dark primary; (iii) the discovery of a Helium white dwarf; (iv) the discovery of a blue straggler with a white dwarf companion.

Between November 2006 and March 2007, on several occasions, XRT has been taking high-cadence one or two filter observations of prominent active regions on the disk. We took these datasets and conducted a quick search for acoustic brightness oscillation in loops. We concentrated our search on flaring active regions. Here we present the preliminary results of this search. We found one active region - NOAA 10953 from 27 April - 02 May 2007 that had indications of acoustic oscillations with periods around 5 min, as well as multiples of this period, and one 40 min period that we associate with periodic heating of the loops. An interesting result is that all the loops in the active region seemed to oscillate with the same set of periods, only the power in the FFT was different and maybe dependent on the magnetic field strength.

N-body simulations of open cluster evolution with primordial binaries are reviewed. In particular, recent results arising from models with initial N in the range of 20000–100000 bodies are compared to earlier idealized models with N ~ 2000. Efforts to model real clusters are discussed, including how limitations of the models such as simplified initial conditions will be addressed in the near future.

We present sixth- and eighth-order Hermite integrators for astrophysical N-body simulations, which use the derivatives of accelerations up to second order (snap) and third order (crackle). These schemes do not require previous values for the corrector, and require only one previous value to construct the predictor. Thus, they are fairly easy to be implemented. The additional cost of the calculation of the higher order derivatives is not very high. Even for the eighth-order scheme, the number of floating-point operations for force calculation is only about two times larger than that for traditional fourth-order Hermite scheme. The sixth order scheme is better than the traditional fourth order scheme for most cases. When the required accuracy is very high, the eighth-order one is the best.

The existence of complex stellar populations in some star clusters challenges the understanding of star formation. E.g. the ONC or the sigma Orionis cluster host much older stars than the main bulk of the young stars. Massive star clusters (ω Cen, G1, M54) show metallicity spreads corresponding to different stellar populations with large age gaps. We show that (i) during star cluster formation field stars can be captured and (ii) very massive globular clusters can accrete gas from a long-term embedding inter stellar medium and restart star formation.

The generation of parallel electric fields by the propagation of ion cyclotron waves (with frequency 0.3 ωci) in the plasma with a transverse density inhomogeneity was studied. Using two-fluid, cold plasma linearised equations, it was shown for the first time that, in this particular context, E∥ generation can be understood by an analytic equation that couples E∥ to the transverse electric field of the driving ion cyclotron wave. It was proven that the minimal model required to reproduce the previous kinetic simulation results of E∥ generation [Tsiklauri et al 2005, Génot et al 2004] is the two-fluid, cold plasma approximation in the linear regime. By considering the numerical solutions it was also shown that the cause of E∥ generation is the electron and ion flow separation induced by the transverse density inhomogeneity. We also investigate how E∥ generation is affected by the mass ratio and found that amplitude attained by E∥ decreases linearly as inverse of the mass ratio mi/me. For realistic mass ratio of mi/me=1836, such empirical scaling law, within a time corresponding to 3 periods of the driving ion cyclotron wave, is producing E∥=14 Vm−1 for solar coronal parameters. Increase in mass ratio does not have any effect on final parallel (magnetic field aligned) speed attained by electrons. However, parallel ion velocity decreases linearly with inverse of the mass ratio mi/me. These results can be interpreted as following: (i) ion dynamics plays no role in the E∥ generation; (ii) E∥ ∝ 1/mi scaling is caused by the fact that ωd = 0.3 ωci ∝ 1/mi is decreasing with the increase of ion mass, and hence the electron fluid can effectively “short-circuit” (recombine with) the slowly oscillating ions, hence producing smaller E∥.

We have analysed the 6 mHz egression power signatures of some accoustically active X-class solar flares. During the impulsive phase these flares produced conspicuous seismic signatures which have kernel-like structures, mostly aligned with the neutral line of the host active region. The kernel-like structures show the effect of constructive interference of the acoustic waves emanating from the complex sources, suggesting motion of the acoustic sources. The co-aligment between the seismic signatures and the hard X-ray emission observed by RHESSI from the footpoints of the coronal loops suggests a direct link between relativistic particles accelerated during the flare and the hydrodynamic response of the photosphere during flares.

We have developed a new tree-direct hybrid algorithm, “Bridge”. It can simulate small scale systems embedded within large-N systems fully self-consistently. Using this algorithm, we have performed full N-body simulations of star clusters near the Galactic center (GC) and compared the orbital evolutions of the star cluster with those obtained by “traditional” simulations, in which the orbital evolution of the star clusters is calculated from the dynamical friction formula. We found that the inspiral timescale of the star cluster is shorter than that obtained with traditional simulations. Moreover, we investigated the eccentricities of particles escaped from the star cluster. Eccentric orbit of the star cluster can naturally explain the high eccentricities of the observed stars.

Most stars are formed in a cluster or association, where the number density of stars can be high. This means that a large fraction of initially-single stars will undergo close encounters with other stars and/or exchange into binaries. We describe how such close encounters and exchange encounters can affect the properties of a planetary system around a single star. We define a singleton as a single star which has never suffered close encounters with other stars or spent time within a binary system. It may be that planetary systems similar to our own solar system can only survive around singletons. Close encounters or the presence of a stellar companion will perturb the planetary system, often leaving planets on tighter and more eccentric orbits. Thus planetary systems which initially resembled our own solar system may later more closely resemble some of the observed exoplanet systems.

We probe the evolution of globular clusters that could form in giant molecular clouds within high-redshift galaxies. Numerical simulations demonstrate that the large and dense enough gas clouds assemble naturally in current hierarchical models of galaxy formation. These clouds are enriched with heavy elements from earlier stars and could produce star clusters in a similar way to nearby molecular clouds. The masses and sizes of the model clusters are in excellent agreement with the observations of young massive clusters. Do these model clusters evolve into globular clusters that we see in our and external galaxies? In order to study their dynamical evolution, we calculate the orbits of model clusters using the outputs of the cosmological simulation of a Milky Way-sized galaxy. We find that at present the orbits are isotropic in the inner 50 kpc of the Galaxy and preferentially radial at larger distances. All clusters located outside 10 kpc from the center formed in the now-disrupted satellite galaxies. The spatial distribution of model clusters is spheroidal, with a power-law density profile consistent with observations. The combination of two-body scattering, tidal shocks, and stellar evolution results in the evolution of the cluster mass function from an initial power law to the observed log-normal distribution.

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.