bluuebunny/crossref_metadata_2025 huggingface dataset:
The Early Evolution of Dense Stellar Systems
Authors: Clarke C. J.
Published: May 2008

The early evolution of dense stellar systems is dominated by the majority mass component – the gas – and so any credible modeling of the first Myr or so of a cluster's life inevitably involves hydrodynamical simulations. Such simulations have increased considerably in sophistication over the last few years and are now beginning to incorporate the effects of stellar feedback, thus enabling one, for the first time, to model the formation of populous clusters. In this review I focus on two issues that have arisen from the simulations – the relationship between maximum stellar mass and cluster mass, and the issue of the maximum density that is attainable during the cluster formation process. I also report on the first results of new simulations that model feedback from ionising radiation.
Changing Structures in Galactic Star Clusters
Authors: Schmeja S. , Kumar M. S. N. , Froebrich D. , Klessen R. S.
Published: May 2008

We investigate the structures of embedded and open clusters using statistical methods, in particular the combined parameter $\Q$, 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.
Clustered Star Formation in the Magellanic Clouds
Authors: Gouliermis Dimitrios A.
Published: May 2008

The Large and Small Magellanic Cloud (LMC, SMC) offer an outstanding variety of young stellar associations, in which large samples of low-mass stars (withM≤ 1 M⊙) currently in the act of formation can be resolved and explored sufficiently with theHubble Space Telescope. Previous observations with theWide-Field Planetary Camera 2(WFPC2) provided the first evidence of the existence of low-mass pre-main sequence (PMS) stars in the vicinity of star forming associations in the Magellanic Clouds (MCs) (Gouliermiset al. 2006a), and recent results from deeper observations with theAdvanced Camera for Surveys(ACS) enhanced dramatically the picture of these systems with the discovery of large numbers of PMS stars. The associations LH 95 (Gouliermiset al. 2002, 2007a) in the LMC, and NGC 346 (Gouliermiset al. 2006b) and NGC 602 (Gouliermiset al. 2007b) in the SMC, are currently under investigation with the use of observations from bothHubbleandSpitzer Space Telescope. I present the impact of our recent results in terms of the star formation history and Initial Mass Function (IMF) of these interesting systems, using as example the case of NGC 602.
Imprints of Stellar Encounters in the ONC
Authors: Olczak C. , Pfalzner S.
Published: May 2008

External destruction of protoplanetary discs acts mainly due to two mechanisms: gravitational drag by stellar encounters and evaporation by stellar winds and radiation. It is an important question whether any of these mechanisms is important in the stellar evolution process. We focus on the effect of stellar encounters and investigate if there are any observables that could trace this mechanism in young stellar clusters. An analysis of observational data of the Orion Nebula Cluster (ONC) and accompanying n-body simulations both provide evidence for encounters of star-disc systems in the ONC, eventually leading to substantial disk disruption.
Capture-Induced Binarity of Massive Stars in Young Dense Clusters
Authors: Pfalzner S. , Olczak Ch.
Published: May 2008

Observations show that for massive stars the binary frequency seems to be higher than for lower mass stars in young dense clusters. This suggests that in clusters like the ONC different mechanisms are at work in the formation of high-mass binary or multiple systems than for low-mass stars. We investigate the stellar dynamics in young dense clusters to determine the role of capture in binary formation in high-mass stars. It turns out that in contrast to lower mass stars capture is a frequent process for massive stars. However, this does not necessarily lead to long lasting binary systems but is often of transient nature. Nevertheless, capture processes could account for 15-25% of the observed ‘binaries’ of the OB-stars (75%) in Orion.
On the Origin of Complex Stellar Populations in Star Clusters
Authors: Pflamm-Altenburg J. , Kroupa P.
Published: May 2008

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.
Star Formation in Young Cluster NGC 1893
Authors: Sharma Saurabh , Pandey A. K. , Ojha D. K. , Chen W. P. , Ghosh S. K. , Bhatt B. C. , Maheswar G. , Sagar Ram
Published: May 2008

We have carried out a multi-wavelength study of the star forming region NGC 1893 to make a comprehensive exploration of the effects of massive stars on low mass star formation. Using deep optical U BV RI broad band, Hα narrow band photometry and slit-less spectroscopy along with archival data from the surveys such as 2MASS, MSX, IRAS and NVSS, we have studied the region to understand the star formation scenario in the region.
Open Clusters: Open Windows on Stellar Dynamics
Authors: Mathieu Robert D.
Published: May 2008

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.
N-body Models of Open Clusters
Authors: Hurley Jarrod R.
Published: May 2008

N-body simulations of open cluster evolution with primordial binaries are reviewed. In particular, recent results arising from models with initialNin the range of 20000–100000 bodies are compared to earlier idealized models withN~ 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.
Defining the Binary Star Population in the Young Open Cluster M35 (NGC 2168)
Authors: Braden Ella K. , Mathieu Robert D. , Meibom Sören
Published: May 2008

We present current results from the ongoing WIYN Open Cluster Study radial-velocity survey for 1410 stars in the young (150 Myr) open cluster M35 (NGC 2168) and establish a benchmark for initial conditions in young open clusters. We find for periods ≲ 1000 days a minimum binary frequency of 0.36 – 0.51. We also analyze the spatial, period and eccentricity distributions of the binary systems and find that the period and eccentricity distributions are well approximated by scaled field distributions from Duquennoy & Mayor (1991). With our large sample size and long baseline, we have a unique understanding of the binary population in this young cluster, making it ideal for defining initial conditions for dynamical simulations.
The WIYN Open Cluster Study Photometric Binary Survey: Initial Findings for NGC 188
Authors: Frinchaboy P. M. , Nielsen D.
Published: May 2008

The WIYN open cluster study (WOCS) has been working to yield precise optical (UBRVI) photometry for all stars in the field of a selection of “prototypical” open clusters. Additionally, WOCS has been using radial velocities to obtain orbit solutions for cluster member hard-binary stars (with period less than 1000 days). Recently, WOCS has been expanded to include the near-infrared (JHKs; 2MASS plus new deep ground-based) and mid-infrared ([3.6], [4.5], [5.8], [8.0] micron) photometry from Spitzer/IRAC observations. This multi-wavelength data (0.3–8.0 microns) allows us to identify binaries photometrically, with mass ratios from 1.0–0.3, across a wide range of primary masses. The spectral energy distribution (SED) fitter by Robitaille et al. (2007) is used to fit the fluxes of 10–12 bands to Kurucz stellar models. This technique allows us to explore the soft binary population for the first time. Using this photometric technique, we find that NGC 188 has a binary fraction of 36-49% and provide a star-by-star comparison to the WOCS radial velocity-based hard binary study.
Dynamics of the Open Cluster NGC 188: A Comparison to an N-body Simulation of M67
Authors: Geller Aaron M. , Mathieu Robert D. , Harris Hugh C. , McClure Robert D.
Published: May 2008

We present a detailed dynamical study of the old (7 Gyr) open cluster NGC 188. Our combined radial-velocity data set spans a baseline of 35 years, a magnitude range of 12 ≤ V ≤ 16.5, and a 1° diameter region on the sky. Our magnitude limits include solar-mass main-sequence stars, subgiants, giants, and blue stragglers, and our spatial coverage extends radially to 11.5 core radii. We have measured radial velocities for 1014 stars in the direction of NGC 188 with a precision of 0.4 km s−1, and have calculated radial-velocity membership probabilities for stars with ≥ 3 measurements. We find 420 stars to be high-probability cluster members, including 137 spectroscopic binaries. These detectable binaries all have orbital periods of less than 104 days, and thus are hard. We have derived orbit solutions for 67 member binary stars, and use our 35 main-sequence binaries with orbit solutions to compare the eccentricity and period distributions with simulated observations of the Hurley et al. (2005) model of M67 (4.5 Gyr). We also compare the spatial distributions of cluster member populations.
NIR Spectroscopy of the Most Massive Open Cluster in the Galaxy: Westerlund 1
Authors: Mengel S. , Tacconi-Garman L. E.
Published: May 2008

Using ISAAC/VLT, we have obtained individual spectra of all NIR-bright stars in the central 2′ × 2′ of the cluster Westerlund 1 (Wd 1) with a resolution of R ≈ 9000 at a central wavelength of 2.30 μm. This allowed us to determine radial velocities of ten post-main-sequence stars, and from these values a velocity dispersion. Assuming virial equilibrium, the dispersion of σ = 8.4 km/s leads to a total dynamical cluster mass of 1.25 × 105M⊙, comparable to the photometric mass of the cluster. There is no extra-virial motion which would have to be interpreted as a signature of cluster expansion or dissolution.
Tidal Radii and Masses of Galactic Open Clusters
Authors: Schilbach Elena , Kharchenko Nina V. , Piskunov Anatoly E. , Röser Siegfried , Scholz Ralf-Dieter
Published: May 2008

For 236 of 650 Galactic open clusters identified in the ASCC-2.5 catalogue, we determine tidal radii from a three-parameter fit of King's profiles to the observed integrated density distribution of cluster members. The results are used to calibrate the observed sizes of the remaining clusters to a uniform scale of tidal radii of open clusters in the Solar neighbourhood. The tidal masses are computed from tidal radii. Within a distance of 850 pc where our sample is complete, the observed distributions of cluster masses can be explained by a general mass loss in open clusters with increasing age.
Modelling Individual Globular Clusters
Authors: Heggie Douglas C. , Giersz Mirek
Published: May 2008

Astronomers have constructed models of globular clusters for over 100 years. These models mainly fall into two categories: (i) static models, such as King's model and its variants, and (ii) evolutionary models. Most attention has been given to static models, which are used to estimate mass-to-light ratios and mass segregation, and to combine data from proper motions and radial velocities. Evolutionary models have been developed for a few objects using the gaseous model, the Fokker-Planck model, Monte Carlo models and N-body models. These models have had a significant role in the search for massive black holes in globular clusters, for example.

In this presentation the problems associated with these various techniques will be summarised, and then we shall describe new work with Giersz's Monte Carlo code, which has been enhanced recently to include the stellar evolution of single and binary stars. We describe in particular recent attempts to model the nearby globular cluster M4, including predictions on the spatial distribution of binary stars and their semi-major axis distribution, to illustrate the effects of about 12 Gyr of dynamical evolution. We also discuss work on an approximate way of predicting the “initial” conditions for such modelling.
Why Haven't Loose Globular Clusters Collapsed yet?
Authors: De Marchi Guido , Paresce Francesco , Pulone Luigi
Published: May 2008

We report on the discovery of a surprising observed correlation between the slope of the low-mass stellar global mass function (GMF) of globular clusters (GCs) and their central concentration parameterc= log(rt/rc), i.e. the logarithmic ratio of tidal and core radii. This result is based on the analysis of a sample of twenty Galactic GCs, with solid GMF measurements from deep HST or VLT data, representative of the entire population of Milky Way GCs. While all high-concentration clusters in the sample have a steep GMF, low-concentration clusters tend to have a flatter GMF implying that they have lost many stars via evaporation or tidal stripping. No GCs are found with a flat GMF and high central concentration. This finding appears counter-intuitive, since the same two-body relaxation mechanism that causes stars to evaporate and the cluster to eventually dissolve should also lead to higher central density and possibly core-collapse. Therefore, severely depleted GCs should be in a post core-collapse state, contrary to what is suggested by their low concentration. Several hypotheses can be put forth to explain the observed trend, none of which however seems completely satisfactory. It is likely that GCs with a flat GMF have a much denser and smaller core than suggested by their surface brightness profile and may well be undergoing collapse at present. It is, therefore, likely that the number of post core-collapse clusters in the Galaxy is much larger than thought so far.
Dynamical Evolution of Rotating Globular Clusters with Embedded Black Holes
Authors: Fiestas J. , Porth O. , Spurzem R.
Published: May 2008

Evolution of self-gravitating rotating dense stellar systems (e.g. globular clusters) with embedded black holes is investigated. The interplay between velocity diffusion due to relaxation and black hole star accretion is followed together with cluster differential rotation using 2D+1 Fokker Planck numerical methods. The models can reproduce the Bahcall-Wolf f ∝ E1/4 (∝ r−7/4) cusp inside the zone of influence of the black hole. Angular momentum transport and star accretion processes support the development of central rotation in relaxation time scales, before re-expansion and cluster dissolution due to mass loss in the tidal field of a parent galaxy. Gravogyro and gravothermal instabilities conduce the system to a faster evolution leading to shorter collapse times with respect to models without black hole.
Black Holes and Core Expansion in Massive Star Clusters
Authors: Mackey A. D. , Wilkinson M. I. , Davies M. B. , Gilmore G. F.
Published: May 2008

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.
Dynamical Evolution of Mass-Segregated Clusters
Authors: Vesperini Enrico , McMillan Steve , Zwart Simon Portegies
Published: May 2008

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.
Numerical Modelling of the Tidal Tails of NGC 5466
Authors: Fellhauer M. , Evans N. W. , Belokurov V. , Wilkinson M. I. , Gilmore G.
Published: May 2008

The study of sub-structures in the stellar halo of the Milky Way has made a lot of progress in recent years, especially since surveys like the Sloan Digital Sky Survey became available. In this paper we focus on the newly discovered tidal tails of the Galactic globular cluster NGC 5466. By means of numerical simulations we reproduce the tidal tails, which are the longest tails associated with a globular cluster known (>45°) and hereby finding a possible progenitor of NGC 5466 and analyse its stability. We show that perigalactic passages are the dominant process in the slow dissolution of NGC 5466. Furthermore we use the position of the tails to verify the accuracy of the observationally determined proper motion. The proper motion has to be refined only slightly (within their stated error-margin) to match the location of the tidal tails.
On the Efficiency of Field Star Capture by Star Clusters
Authors: Mieske S. , Baumgardt H.
Published: May 2008

An exciting recent finding regarding scaling relations among globular clusters is the so-called ‘blue tilt’: clusters of the blue sub-population follow a trend of redder colour with increasing luminosity. In this contribution we estimate by means of collisional N-body simulations to which extent this trend can be explained by field star capture occurring over a Hubble time. We investigate star clusters with 103 to 106 stars. We find that the ratio between captured field stars and total number of clusters stars is very low (≲ 10−4), even for co-rotation of the star cluster in a cold disk. This holds for star clusters in the mass range of both open clusters and globular clusters. Therefore, field star capture is not a probable mechanism for creating the colour-magnitude trend of metal-poor globular clusters.
The Problem of Three Stars: Stability Limit
Authors: Valtonen M. , Mylläri A. , Orlov V. , Rubinov A.
Published: May 2008

The problem of three stars arises in many connections in stellar dynamics: three-body scattering drives the evolution of star clusters, and bound triple systems form long-lasting intermediate structures in them. Here we address the question of stability of triple stars. For a given system the stability is easy to determine by numerical orbit calculation. However, we often have only statistical knowledge of some of the parameters of the system. Then one needs a more general analytical formula. Here we start with the analytical calculation of the single encounter between a binary and a single star by Heggie (1975). Using some of the later developments we get a useful expression for the energy change per encounter as a function of the pericenter distance, masses, and relative inclination of the orbit. Then we assume that the orbital energy evolves by random walk in energy space until the accumulated energy change leads to instability. In this way we arrive at a stability limit in pericenter distance of the outer orbit for different mass combinations, outer orbit eccentricities and inclinations. The result is compared with numerical orbit calculations.
Numerical Evolution of Single, Binary and Triple Stars
Authors: Eggleton Peter P.
Published: May 2008

I discuss my stellar evolution code Ev in the context of simulations of large clusters of stars. It has long been able to handle single stars, and also binary stars up to a point. That point is far beyond what other codes are able to do, but well short of what is necessary for believable simulations. A recent version, Ev(Twin), can in principle deal with the contact phase of binary evolution, but it is not yet clear what the physical interaction is that needs to be simulated.

An upgrade, which I hope will be only a few lines, should allow it to follow Kozai cycles with tidal friction, a process that strongly influences the orbital period of close pairs that reside within wide, non-coplanar triples. However, there are many substantial gaps in the physics of even single stars, let alone binaries or triples.
Full Ionisation in Binary-Binary Scattering
Authors: Sweatman W. L.
Published: May 2008

Encounters between binary stars and single stars and between binary stars and other binary stars play a key role in the dynamics of dense stellar systems. In the simple model, in which stars are approximated by point masses, a number of theoretical and numerical results are known. In particular there exist relationships to help to describe the destruction process of binary stars (ionisation) through three-body encounters between binary and single stars. Here we extend these results to the four-body case involving encounters between pairs of binary stars that lead to a disruption of the binaries into single stars.
Effects of Hardness of Primordial Binaries on Evolution of Star Clusters
Authors: Tanikawa A. , Fukushige T.
Published: May 2008

We performed N-body simulations of star clusters with primordial binaries using a new code, GORILLA. It is based on Makino and Aarseth (1992)'s integration scheme on GRAPE, and includes a special treatment for relatively isolated binaries. Using the new code, we investigated effects of hardness of primordial binaries on whole evolution of the clusters. We simulated seven N=16384 equal-mass clusters containing 10% (in mass) primordial binaries whose binding energies are 1, 3, 10, 30, 100, 300, and 1000kT, respectively. Additionally, we also simulated a cluster without primordial binaries and that in which all binaries are replaced by stars with double mass, as references of soft and hard limits, respectively. We found that, in both soft (≤ 3kT) and hard (≥ 1000kT) limits, clusters experiences deep core collapse and shows gravothermal oscillations. On the other hands, in the intermediate hardness (10-300kT), the core collapses halt halfway due an energy releases of the primordial binaries.
Dynamical Evolution of Star Clusters with Intermediate Mass Black Holes and Primordial Binaries
Authors: Trenti Michele
Published: May 2008

The evolution of a star cluster is strongly influenced by the presence of primordial binaries and of a central black hole, as dynamical interactions within the core prevents a deep core collapse under these conditions. We present the results from a large set of direct N-body simulations of star clusters that include an intermediate mass black hole, single and binary stars. We highlight the structural and dynamical differences for the various cases showing in particular that on a timescale of a few relaxation times the density profile of the star cluster does no longer depend on the details of the initial conditions but only on the efficiency of the energy generation due to gravitational encounters at the center of the system.
The Binary Fraction of NGC 6397
Authors: Davis D. Saul , Richer Harvey B. , Anderson Jay , Brewer James
Published: May 2008

The binary fraction, η, of a globular cluster (GC) is a key parameter in determining its dynamical evolution, as well as its content of rare stars, such as cataclysmic variables and blue stragglers. The precise value of η for a GC was historically difficult to constrain due to an inability to obtain reliable photometry for faint objects in dense stellar fields. However, today, the HST allows us to image the main sequence of the nearest GCs to their terminations. Using HST observations we constrain η for NGC 6397. While the necessary computing power is now available to realistically simulate entire GCs, large discrepancies in the assumed primordial binary fraction, ηp, of GCs still exist. Estimates range from 5% (Hurley et al. 2007) to 100% (Ivanova et al. 2005). The N-body models of Hurley et al. (2007) suggest that η beyond the half-mass radius remains close to ηp, while cluster evolution can increase the value in the core. We find η for NGC 6397 is 15.2±0.8% in a field centered on the core, and 1.1±0.3% in a field beyond the half mass radius. These findings suggests ηp ~ 1%.
Binaries and the Dynamical Mass of Star Clusters
Authors: Kouwenhoven M. B. N. , de Grijs R.
Published: May 2008

The total mass of a distant star cluster is often derived from the virial theorem, using line-of-sight velocity dispersion measurements and half-light radii, under the implicit assumption that all stars are single (although it is known that most stars form part of binary systems). The components of binary stars exhibit orbital motion, which increases the measured velocity dispersion, resulting in a dynamical mass overestimation. In these proceedings we quantify the effect of neglecting the binary population on the derivation of the dynamical mass of a star cluster. We find that the presence of binaries plays an important role for clusters with total mass Mcl ≤ 105 M⊙; the dynamical mass can be significantly overestimated (by a factor of two or more). For the more massive clusters, with Mcl ≥ 105 M⊙, binaries do not affect the dynamical mass estimation significantly, provided that the cluster is significantly compact (half-mass radius ≤qslant 5 pc).
Mass Transfer in Binary Systems: A Numerical Approach
Authors: Lajoie C.-P. , Sills A.
Published: May 2008

We present preliminary work on the formation scenario of blue straggler stars by mass transfer in binary systems. More precisely, using Smoothed Particle Hydrodynamics (SPH), we want to model only the outer parts of the stars in order to get a much greater spatial resolution of the mass transfer flow itself. The inner boundary conditions are achieved using the so-called ghost particles and by replacing the inner mass by a central point mass. Stability of this central point mass is crucial, and it is shown that we get reasonable results. These simulations should give us indications on which layers of the donor star are actually transferred to the other star as well as how mass is transferred and how it settles on the accretor. This work is aimed at getting distinct observational signatures which would help identifying the dominant formation mechanism of blue straggler stars.
Is our Sun a Singleton?
Authors: Malmberg D. , Davies M. B. , Chambers J. E. , De Angeli F. , Church R. P. , Mackey D. , Wilkinson M. I.
Published: May 2008

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.
A Search for Spectroscopic Binaries in the Globular Cluster M4
Authors: Sommariva V. , Piotto G. , Rejkuba M. , Bedin L. R. , Heggie D. C. , Milone A. , Mathieu R. D. , Moretti A.
Published: May 2008

We present preliminary results from an observational campaign aimed at the study of the binary fraction and binary radial distribution in Galactic globular clusters. In particular, we concentrate on the ongoing observational campaign for the search of spectroscopic binaries.
Blue Straggler Stars in Galactic Globular Clusters: Tracing the Effect of Dynamics on Stellar Evolution
Authors: Ferraro Francesco R. , Lanzoni Barbara
Published: May 2008

In this contribution we review the main observational properties of Blue Stragglers Stars (BSS) in galactic GCs. A flower of results on the BSS frequency, radial distribution, and chemical composition are presented and discussed.
Pulsars in Globular Clusters
Authors: Ransom Scott M.
Published: May 2008

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.
Observational Evidence for the Origin of X-ray Sources in Globular Clusters
Authors: Verbunt Frank , Pooley Dave , Bassa Cees
Published: May 2008

Low-mass X-ray binaries, recycled pulsars, cataclysmic variables and magnetically active binaries are observed as X-ray sources in globular clusters. We discuss the classification of these systems, and find that some presumed active binaries are brighter than expected. We discuss a new statistical method to determine from observations how the formation of X-ray sources depends on the number of stellar encounters and/or on the cluster mass. We show that cluster mass is not a proxy for the encounter number, and that optical identifications are essential in proving the presence of primordial binaries among the low-luminosity X-ray sources.
Black Hole Motion as Catalyst of Orbital Resonances
Authors: Boily C. M. , Padmanabhan T. , Paiement A.
Published: May 2008

The motion of a black hole about the centre of gravity of its host galaxy induces a strong response from the surrounding stellar population. We consider the case of a harmonic potential and show that half of the stars on circular orbits in that potential shift to an orbit of lower energy, while the other half receive a positive boost. The black hole itself remains on an orbit of fixed amplitude and merely acts as a catalyst for the evolution of the stellar energy distribution function f(E). We then consider orbits in the logarithmic potential and identify the response of stars near resonant energies. The kinematic signature of black hole motion imprints the stellar line-of-sight mean velocity to a magnitude ≃ 13% the local root mean-square velocity dispersion σ. The high velocity dispersion at the 5:2 resonance hints to an observable effect at a distance ≃ 3 times the hole's influence radius.
Stellar Exotica in 47 Tucanae
Authors: Knigge C. , Dieball A. , Maíz-Apellániz J. , Long K. S. , Zurek D. R. , Shara Mike M.
Published: May 2008

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.
The Imprints of IMBHs on the Structure of Globular Clusters: Monte-Carlo Simulations
Authors: Umbreit Stefan , Fregeau John M. , Rasio Frederic A.
Published: May 2008

We present the first results of a series of Monte-Carlo simulations investigating the imprints of a central black hole on the core structure of globular clusters. We investigate the three-dimensional and the projected density profile of the inner regions of idealized as well as more realistic globular cluster models, taking into account a stellar mass spectrum, stellar evolution and allowing for a larger, more realistic, number of stars than was previously possible with direct N-body methods. We compare our results to other N-body simulations published previously in the literature.
The Formation and Evolution of Very Massive Stars in Dense Stellar Systems
Authors: Belkus Houria , Van Bever Joris , Vanbeveren Dany
Published: May 2008

The early evolution of dense stellar systems is governed by massive single star and binary evolution. Core collapse of dense massive star clusters can lead to the formation of very massive objects through stellar collisions (M≥ 1000M⊙). Stellar wind mass loss determines the evolution and final fate of these objects, and determines whether they form black holes (with stellar or intermediate mass) or explode as pair instability supernovae, leaving no remnant. We present a computationally inexpensive evolutionary scheme for very massive stars that can readily be implemented in an N-body code. Using our new N-body code ‘Youngbody’ which includes a detailed treatment of massive stars as well as this new scheme for very massive stars, we discuss the formation of intermediate mass and stellar mass black holes in young starburst regions. A more detailed account of these results can be found in Belkus, Van Bever & Vanbeveren (2007).
Tracing Intermediate-Mass Black Holes in the Galactic Centre
Authors: Löckmann Ulf , Baumgardt Holger
Published: May 2008

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.
Environmental Effects on the Globular Cluster Blue Straggler Population: a Statistical Approach
Authors: Moretti Alessia , De Angeli Francesca , Piotto Giampaolo
Published: May 2008

Blue stragglers stars (BSS) constitute an ubiquitous population of objects whose origin involves both dynamical and stellar evolution. We took advantage of the homogeneous sample of 56 Galactic globular clusters observed with WFPC2/HST by Piotto et al. (2002) to investigate the environmental dependence of the BSS formation mechanisms. We explore possible monovariate relations between the frequency of BSS (divided in different subsamples according to their location with respect to the parent cluster core radius and half mass radius) and the main parameters of their host GC. We also performed a Principal Component Analysis to extract the main parent cluster parameters which characterise the BSS family.
XMM-Newton and Chandra Observations of Neutron Stars and Cataclysmic Variables in the Globular Cluster NGC 2808
Authors: Servillat M. , Webb N. A. , Barret D. , Cornelisse R. , Dieball A. , Knigge C. , Long K. S. , Shara M. M. , Zurek D. R.
Published: May 2008

We report on XMM-Newton and Chandra observations of the globular cluster NGC 2808. We detect one quiescent low mass X-ray binary of the 3±1 expected, if these systems are formed through encounters, and we show evidence for the presence of 20±10 bright cataclysmic variables in the core with a luminosity above 4×1031 erg s−1. We also review the specific nature of cataclysmic variables in globular clusters with reference to recent VLT/FORS1 observations of a cataclysmic variable in M 22.
Giant Elliptical Galaxies: Globular Clusters and UCDs
Authors: Harris William E.
Published: May 2008

Explorations of the globular cluster populations in many nearby galaxies are revealing increasing connections to other dense stellar systems such as UCDs, DGTOs, and nuclear star clusters in dwarf galaxies. The nearest giant elliptical, NGC 5128, is now giving us a much-improved delineation of the GC Fundamental Plane of structural parameters, and indicates as well that the known correlation between GC scale size and metallicity is likely to be at least partly a projection effect coupled with the different spatial distributions of the metal-poor and metal-rich clusters. New photometry of the huge cluster populations around the giant Brightest Cluster Ellipticals, which allows us to study samples of many thousands of GCs at once, are now beginning to turn up surprising examples of “sequences” of high-mass GCs leading up to the UCD regime. Lastly, new modelling of cluster formation through a specially tuned semi-analytic galaxy formation code strongly suggests that the mass-metallicity relation now known to affect the blue GC sequence can arise fairly naturally out of such models, if significant numbers of the massive GCs actually represent the remnant nuclei of stripped dwarf satellites.
Observational Constraints on the Formation and Evolution of Globular Cluster Systems
Authors: Zepf Stephen E.
Published: May 2008

This paper reviews some of the observational properties of globular cluster systems, with a particular focus on those that constrain and inform models of the formation and dynamical evolution of globular cluster systems. I first discuss the observational determination of the globular cluster luminosity and mass function. I show results from new very deep HST data on the M87 globular cluster system, and discuss how these constrain models of evaporation and the dynamical evolution of globular clusters. The second subject of this review is the question of how to account for the observed constancy of the globular cluster mass function with distance from the center of the host galaxy. The problem is that a radial trend is expected for isotropic cluster orbits, and while the orbits are observed to be roughly isotropic, no radial trend in the globular cluster system is observed. I review three extant proposals to account for this, and discuss observations and calculations that might determine which of these is most correct. The final subject is the origin of the very weak mass-radius relation observed for globular clusters. I discuss how this strongly constrains how globular clusters form and evolve. I also note that the only viable current proposal to account for the observed weak mass-radius relation naturally effects the globular cluster mass function, and that these two problems may be closely related.
Clues to Globular Cluster Evolution from Multiwavelength Observations of Extragalactic Systems
Authors: Kundu Arunav , Maccarone Thomas J. , Zepf Stephen E.
Published: May 2008

We present a study of the globular cluster (GC) systems of nearby elliptical and S0 galaxies at a variety of wavelengths from the X-ray to the infrared. Our analysis shows that roughly half of the low mass X-ray binaries (LMXBs), that are the luminous tracers of accreting neutron star or black hole systems, are in clusters. There is a surprisingly strong correlation between the LMXB frequency and the metallicity of the GCs, with metal-rich GCs hosting three times as many LMXBs as metal-poor ones, and no convincing evidence of a correlation with GC age so far. In some of the galaxies the LMXB formation rate varies with GC color even within the red peak of the typical bimodal cluster color distribution, providing some of the strongest evidence to date that there are metallicity variations within the metal-rich GC peak as is expected in a hierarchical galaxy formation scenario. We also note that any analysis of subtler variations in GC color distributions must carefully account for both statistical and systematic errors. We caution that some published GC correlations, such as the apparent ‘blue-tilt’ or mass-metallicity effect might not have a physical origin and may be caused by systematic observational biases.
Slow Evolution of a System of Satellites Induced by Dynamical Friction
Authors: Arena Serena E. , Bertin Giuseppe
Published: May 2008

The evolution induced by dynamical friction on a spherical shell of rigid satellites interacting directly with the particles sampling the host elliptical galaxy is followed by means of N-body simulations for a variety of shell–galaxy configurations.
Dynamical Evolution of the Mass Function of the Galactic Globular Cluster System
Authors: Shin Jihye , Kim Sungsoo S. , Takahashi Koji
Published: May 2008

Using the most advanced anisotropic (2D) Fokker-Planck (FP) models, we calculate the evolution of the mass functions of the Galactic globular cluster system (GCMF). Our models include two-body relaxation, binary heating, tidal shocks, dynamical friction, stellar evolution, and realistic cluster orbits. We perform 2D-FP simulations for a large number of virtual globular clusters and synthesize these results to study the relation between the initial and present GCMFs. We found two probable IGCMFs that eventually evolve into the Milky Way GCMF : truncated power-law, and log-normal model with higher initial low mass limit and peak mass than the earlier studies.
TKira—A Hybrid N-Body Code
Authors: Mamikonyan Ernest N. , McMillan Stephen L. W. , Portegies Zwart Simon F. , Vesperini Enrico
Published: May 2008

Accurately modeling the evolution of a star cluster in a strong tidal field poses unique computational challenges. We present a hybrid code that combines the strengths of two different approaches to computing gravitational forces. The internal, collisional, dynamics of the cluster is followed with a direct N-body integrator, Kira, while the galactic tidal field is modeled with a cosmological code, GADGET, that uses a Barnes-Hut tree to evaluate gravitational forces in O(N log N) time. The quadrupole moment at the center of mass of the cluster is used to compute the external potential and provides a mechanism for mass loss. This forms a robust, bidirectional interaction. The advantages of combining two highly-developed and well-established software packages at such high level are obvious and many; not the least of the these is the ability to include other physical processes, e.g., stellar evolution. One problem to which we applied this technique is the evolution of a dense star cluster near the Galactic Center. We are also using this code to explore the effects of the strong time variation in the tidal field of merging galaxies on the evolution of young star clusters forming during the merger.
6th and 8th Order Hermite Integrator Using Snap and Crackle
Authors: Nitadori Keigo , Iwasawa Masaki , Makino Junichiro
Published: May 2008

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.
Inkonsistenz als differenzielles Indikationskriterium in der Psychotherapie
Authors: Grosse Holtforth Martin , Grawe Klaus , Fries Alexander , Znoj Hansjörg
Published: May 2008

Zusammenfassung. Theoretischer Hintergrund: Es wird angenommen, dass Inkonsistenz im psychischen Geschehen zur Entstehung und Aufrechterhaltung psychischer Störungen beiträgt. Fragestellung: Sagt das Ausmaß an Inkonsistenz den Erfolg einer klärungs- und bewältigungsorientierten Psychotherapie sowie einer nur bewältigungsorientierten Psychotherapie differenziell vorher? Methode: In einer randomisierten-kontrollierten Interventionsstudie mit 67 ambulanten Patienten verschiedener Diagnosen werden Allgemeine Psychotherapie und eine bewältigungsorientierte Therapie verglichen sowie differenzielle Effekte von Inkonsistenz in zwei verschiedenen Operationalisierungen untersucht. Ergebnisse: Während sich nach 20 Sitzungen keine Erfolgsunterschiede zwischen den Gruppen zeigen, ergeben sich die vorhergesagten differenziellen Effekte für ein Inkonsistenzmaß (Stärke der Vermeidungsziele) und zwei Erfolgsmasse (Veränderung interpersonaler Probleme, retrospektive Veränderungseinschätzung). Schlussfolgerungen: Es gibt erste Hinweise auf differentielle Therapieeffekte der Inkonsistenz auf einzelne Aspekte des Therapieerfolgs, die in weiteren Studien repliziert werden sollten.
Ein Überblick über experimentelle Befunde zur Wirkung von Alkohol auf klinisch relevante Angst
Authors: Stevens Stephan , Rist Fred , Gerlach Alexander L.
Published: May 2008

Zusammenfassung. Hintergrund: Ein überzufälliger Zusammenhang zwischen Angststörungen und Alkoholproblemen ist belegt. Trotz zahlreicher Studien mit subklinischen Populationen ist die Befundlage zur anxiolytischen Wirkung von Alkohol jedoch uneindeutig. Fragestellung: Wirkt Alkohol bei klinisch relevanten Ängsten anxiolytisch? Welche Einflussfaktoren existieren und welche Mechanismen sind beteiligt? Methode: Ein Review empirischer Studien zur anxiolytischen Wirkung von Alkohol bei pathologischer Angst. Ergebnisse: Die Ergebnisse der 11 publizierten Studien werden dargestellt. Der Fokus liegt auf der Wirkung von Alkohol bei Sozialphobie (N = 7), Panikstörung (N = 3) und spezifischer Phobie (N = 1). Zusammenfassend ist eine anxiolytische Wirkung an eine Mindestmenge Alkohol (BAK: > 0.05%), positive Wirkungserwartungen sowie andere psychologische Faktoren gebunden. Diskussion: Die Selbstmedikationshypothese hat nur unter Berücksichtigung verschiedener biologischer und psychologischer Faktoren Bestand. Mögliche Implikationen für die Behandlung werden diskutiert.