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Context.Observations indicate that present-day star formation in the Milky Way disk takes place in stellar ensembles or clusters rather than in isolation. Bound, long-lived stellar groups are known as open clusters. They gradually lose stars and are severely disrupted in their final evolutionary stages, leaving an open cluster remnant made up of a few stars.

Context.Fractals are geometric objects with dimensionalities that are not integers. They play a fundamental role in the dynamics of chaotic systems. Observation of fractal structure in both the gas and the star-forming sites in galaxies suggests that the spatial distribution of young open clusters should follow a fractal pattern, too.

Context. Observations indicate that the fraction of potential binary star clusters in the Magellanic Clouds is about 10%. In contrast, it?is widely accepted that the binary cluster frequency in the Galaxy disk is much lower.Aims. Here we investigate the multiplicity of clusters in the Milky?Way disk to either confirm or disprove this dearth of binaries.Methods. We quantify the open cluster multiplicity using complete, volume-limited samples from WEBDA and NCOVOCC.Results. At the Solar Circle, at least 12% of all open clusters appear to be experiencing some type of interaction with another cluster; i.e., are possible binaries. As in the Magellanic Clouds, the pair separation histogram hints at a bimodal distribution. Nearly 40% of identified pairs are probably primordial. Most of the remaining pairs could be undergoing some type of close encounter, perhaps as a result of orbital resonances. Confirming early theoretical predictions, the characteristic time scale for destruction of bound pairs in the disk is 200?Myr, or one galactic orbit.Conclusions. Our results show that the fraction of possible binary clusters in the Galactic disk is comparable to that in the Magellanic Clouds.

Context.Formation of super star-clusters can be triggered during the final stages of galaxy mergers or galactic interactions, when significant numbers of massive stars are formed out of large gas-cloud systems. Giant cloud complexes show a fractal structure due to turbulence and/or self-gravitation; therefore, super clusters formed out of these complexes are expected to show a multifractal spectrum.Aims.Here we investigate the projected spatial distribution of young massive clusters in the ultraluminous infrared galaxy Arp?220 and its underlying fractal geometry.Methods.The projected radial distribution of super clusters is dominated by a prominent ring of star formation. Taking the presence of this annulus into account, the fractal spectrum is determined by using the Minkowski-sausage method to compute the Minkowski-Bouligand dimension as a function of the parameter q.Results.The ring appears to extend from a radial distance of 2.0 to 4.5?kpc. The Arp?220 annulus of star formation could be an outer ring associated with the outer Lindblad resonance/radius of corotation. The average projected fractal dimension in the ring of star formation is found to be 1.7?0.1, which is consistent with values found for non-starburst, star-forming regions in spiral galaxies. However, the fractal dimension appears to be lower in the outer regions of the ring and, for Arp?220, this implies an actual decrease in the fractal dimension over time probably induced by star cluster destruction. This trend is not observed under quiescent star formation.Conclusions.The projected distribution of super clusters in Arp?220 can be described using an annular model, and it exhibits multifractal behaviour. The properties of its fractal geometry are similar to those found in quiescent star-forming regions in disk galaxies although the average fractal dimension evolves differently over time, decreasing instead of increasing. This result implies that, for the short-term evolution of super cluster populations, destruction may be far more important than diffusion-like processes.

Europa's low crater density suggests that geological activity has continued to the present epoch, leading to the possibility that current resurfacing events might be detectable. CCD observations were carried out with a ST-6 camera at the 0.5 m Mons Cassegrain telescope (Izaña Observatory, Tenerife,Canary Islands, Spain) during the night between 2–3 October 1999. Our images show a transient bright feature on the Galilean satellite. These images are analyzed here with the purpose of understanding the nature of the transient phenomena as it could be the result of explosive venting on the surface of the Jovian satellite. By comparison, we use NASA Infrared Telescope Facility images of two Io hot spots taken on12 October 1990. Although we mainly restrict our discussion on apossible eruptive nature of the observed spots, we also consider other alternative mechanisms able to produce bright events. In particular, an interaction between charged material being ejected from Europa and the Jovian magnetosphere cannot be entirely ruled out. If confirmed, this result would lend support for the existence of active resurfacing in Europa.

The star formation process usually leads to the formation of protoplanetary disks. Planets are thought to arise from the material of these disks. Amongst the stars in the solar neighbourhood, single systems like our own are a minority. Most stars are found in binaries or in systems of even higher multiplicity. In this paper, we extend the simulations presented in Paper I (de la Fuente Marcos and de la Fuente Marcos, 1998a) to hierarchical triple systems. As in Paper I, we study the stage of planetary formation during which the particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of the particles, in a protoplanetary disk with radius RD= 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600 AU) orbit. As for this problem no analytic description of the orbital evolution of the particles exists, we perform numerical integrations using a Bulirsch–Stoer integrator. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations with three stars using a direct integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components of the triple system. As in Paper I, the evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative revolution of the stellar companions, and on the temperature and mass of the circumstellar disk. The perturbation of prograde companions induces a trailing spiral structure across the protoplanetary sub-metric dust sub-disk. Metre-sized particles are affected by strong precession. Our results show that the lifetime of particles in a disk in a hierarchical triple system is slightly shorter relative to its valuein Paper I, although the actual value depends on the nature, prograde or retrograde, of the outer companion. The lifetime of particles in a hierarchical triple system including a prograde inner binary and a retrograde outer body is longer than in an equivalent triple with all the companions prograde. Dust disks in hierarchical triple systems with both companions in retrograde motion with respect to the particles in the disk show the shortest lifetimes. Our previous calculations suggest that disk luminosities in binary systems are several orders of magnitude higher than those for single stars. Circumstellar disks in triple systems may be 5–50% more luminous depending on the relative direction of rotation.

It is by now well established that open clusters contain a considerable fraction of brown dwarfs (BDs). This paper investigates the dynamical evolution of this substellar population by using simulations with Aarseth's (1994) NBODY5 code. A noticeable preferential escape of BDs is found, which may influence the determination of the IMF of substellar objects in dynamically evolved open clusters. This small dynamical-in-origin depletion may not explain, however, the scarcity of BDs observed in some evolved clusters, as the Hyades. On the other hand, BD cooling processes are able to reduce our ability to detect BDs in old clusters in a very significant way. Our results confirm that the probability of observing BDs in open clusters is almost the same over the whole cluster area because they are distributed quite uniformly even at late stages of the evolution of the cluster. This is expected to be a general feature as observed for low-mass stars in well studied open clusters (Pleiades, Praesepe). Our present calculations show that clusters as old as the Pleiades may have lost about 10% of their initial BD population but the number ratio of BDs to normal (not substellar) stars must remain almost unchanged. However, the long-term behavior of the relative percentage of BDs depends strongly on the initial mass function (IMF) assumed in the calculations. Clusters with a Salpeterian IMF evolve to reach relative percentages of BDs as low as 40% for a starting value around 70%. Our results suggest that BDs in clusters escape preferentially by evaporation.

Context. Shortly after birth, open clusters start dissolving; gradually losing stars into the surrounding star field. The time scale for complete disintegration depends both on their initial membership and location within the Galaxy. Open clusters undergoing the terminal phase of cluster disruption or open cluster remnants (OCRs) are notoriously difficult to identify. From an observational point, a combination of low number statistics and minimal contrast against the general stellar field conspire to turn them into very challenging objects. To make the situation even worst, random samples of field stars often display features that may induce to classify them erroneously as extremely evolved open clusters.

Context. Shortly after birth, open clusters start dissolving; gradually losing stars into the surrounding star field. The time scale for complete disintegration depends both on their initial membership and location within the Galaxy. Open clusters undergoing the terminal phase of cluster disruption or open cluster remnants (OCRs) are notoriously difficult to identify. From an observational point, a combination of low number statistics and minimal contrast against the general stellar field conspire to turn them into very challenging objects. To make the situation even worst, random samples of field stars often display features that may induce to classify them erroneously as extremely evolved open clusters.Aims. In this paper, we provide a detailed study of the stellar content and kinematics of NGC 6863, a compact group of a few stars located in Aquila and described by the Palomar Observatory Sky Survey as a non existent cluster. Nonetheless, this object has been recently classified as OCR. The aim of the present work is to either confirm or disprove its OCR status by a detailed star-by-star analysis.Methods. The analysis is performed using wide-field photometry in the UBVI pass-band, proper motions from the UCAC3 catalogue, and high resolution spectroscopy as well as results from extensive N-body calculations.Results. The spectra of the four brightest stars in this field clearly indicate that they are part of different populations. Their radial velocities are statistically very different and their spectroscopic parallaxes are inconsistent with them being part of a single, bound stellar system. Out of the four stars, only two of them have similar metallicity. The color magnitude diagram for the field of NGC 6863 does not show any clear signature typical of actual open clusters. Consistently, spatial scan statistics confirms the absence of any statistically significant, kinematically supported over-density at the purported location of NGC 6863.Conclusions. Our results show that the four brightest stars commonly associated to NGC 6863 form an asterism, a group of non-physically associated stars projected together, leading to the conclusion that NGC 6863 is not a real open cluster.