The diagnostic age versus mass-to-light ratio diagram is often used in attempts to constrain the shape of the stellar initial mass function (IMF), and the potential longevity of extragalactic young to intermediate-age massive star clusters. Here, we explore its potential for Galactic open clusters. On the basis of a small, homogenised cluster sample we provide useful constraints on the presence of significant binary fractions. Using the massive young Galactic cluster Westerlund 1 as a key example, we caution that stochasticity in the IMF introduces significant additional uncertainties. We conclude that for an open cluster to survive for any significant length of time, and in the absence of substantial external perturbations, it is a necessary but not a sufficient condition to be located close to or (in the presence of a significant binary population) somewhat BELOW the predicted photometric evolutionary sequences for "normal" simple stellar populations (although such a location may be dominated by a remaining "bound" cluster core and thus not adequately reflect the overall cluster dynamics).
We used deep observations collected with ACS@HST to derive the fraction of binary systems in a sample of thirteen low-density Galactic globular clusters. By analysing the color distribution of Main Sequence stars we derived the minimum fraction of binary systems required to reproduce the observed color-magnitude diagram morphologies. We found that all the analysed globular clusters contain a minimum binary fraction larger than 6% within the core radius. The estimated global fractions of binary systems range from 10% to 50% depending on the cluster. A dependence of the relative fraction of bina…
We study the evolution of binary stars in globular clusters using a novel approach combining a state-of-the-art population synthesis code with a simple treatment of dynamical interactions in the dense cluster core. We find that the combination of stellar evolution and dynamical interactions (binary-single and binary-binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of 100%, is only about 5%. We show that this is in good agreement with recent H…
We report on an extensive series of numerical experiments of binary--binary scattering, analysing the cross--section for close approach during interactions for a range of hard binary parameters of interest in globular cluster cores. We consider the implied rate for tidal interactions for different globular clusters and compare our results with previous, complementary estimates of stellar collision rates in globular clusters. We find that the collision rate for binary--binary encounters dominates in low density clusters if the binary fraction in the cluster is larger than $0.2$ for wide main--s…
This paper investigates the long-term orbital stability of small bodies near the central binary of the Alpha Centauri system. Test particles on circular orbits are integrated in the field of this binary for 32000 binary periods or approximately 2.5 Myr. In the region exterior to the binary, particles with semi-major axes less than roughly three times the binary's semi-major axis are unstable. Inside the binary, particles are unstable if further than 0.2 binary semimajor axes from the primary, with stablility closer in a strong function of inclination: orbits inclined near 90 degrees are unstab…
We study the evolution of binary stars in globular clusters using a new Monte Carlo approach combining a population synthesis code (StarTrack), and a simple treatment of dynamical interactions in the dense cluster core using a new tool for computing 3-body and 4-body interactions (Fewbody). We find that the combination of stellar evolution and dynamical interactions (binary-single and binary-binary) leads to a rapid depletion of the binary population in the cluster core. The maximum binary fraction today in the core of a typical dense cluster like 47 Tuc, assuming an initial binary fraction of…
The dynamics of a close binary system of globular clusters is considered. It is shown that the star transfer process from one of the components to the other should lead to the decrease of dimension of the first cluster with simultaneous increase of the dimension of the second cluster. The rates of variations of clusters' sizes due to the mutual tidal interaction between the components, as well as of the orbital rotation of the binary system on the evolution of clusters' sizes, are estimated. The results may be attributed to the double nuclei observed in certain galaxies.
Using Hubble Space Telescope (HST) observations of the globular cluster NGC6397, we constrain the cluster's binary fraction. The observations consist of two fields: the primary science field, a single ACS pointing centered approximately 5' from the cluster center; and the parallel field, a single WFPC2 field centered on the cluster center. Using the exquisite photometric precision of these observations, we determine the binary fraction in these regions of the cluster by examining stars lying off the main sequence. The binary fraction is constrained to be 0.012 +/- 0.004 in the ACS field, and t…
About half of all known stellar systems with Sun-like stars consist of two or more stars, significantly affecting the orbital stability of any planet in these systems. This observational evidence has prompted a large array of theoretical research, including the derivation of mathematically stringent criteria for the orbital stability of planets in stellar binary systems, valid for the "coplanar circular restricted three-body problem". In the following, we use these criteria to explore the validity of results from previous theoretical studies.
I present a simplified analytical model that simulates the evolution of the binary population in a dynamically evolving globular cluster. A number of simulations have been run spanning a wide range in initial cluster and environmental conditions by taking into account the main mechanisms of formation and destruction of binary systems. Following this approach, I investigate the evolution of the fraction, the radial distribution, the distribution of mass ratios and periods of the binary population. According to these simulations, the fraction of surviving binaries appears to be dominated by the …
About half of all known stellar systems with Sun-like stars consist of two or more stars, significantly affecting the orbital stability of any planet in these systems. Here we study the onset of instability for an Earth-type planet that is part of a binary system. Our investigation makes use of previous analytical work allowing to describe the permissible region of planetary motion. This allows us to establish a criterion for the orbital stability of planets that may be useful in the context of future observational and theoretical studies.
A CCD V,V-I colour-magnitude diagram (CMD) of the Galactic globular cluster NGC 2808 has been obtained with the ESO-NTT, reaching down to V ~ 24. The highly populated Main Sequence (MS) presents a significant broadening redward to the MS ridge line, larger than expected from photometric errors alone, which could be interpreted as due to binary candidates.
We present a detailed study of the cluster pair SL 353 & SL 349. This candidate binary cluster is located at the northwestern rim of the LMC bar. Based on photometric data we find that both clusters are coeval with an age of 550 +/- 100 Myr. We use the Ca II triplet in the spectra of individual red giants to derive radial velocities. Both components of the binary cluster candidate show very similar mean velocities (approx. 274 +/- 10 km/sec for SL 349 and approx. 279 +/- 4 km/sec for SL 353) while the field stars show lower velocities (approx. 240 +/- 19 km/sec). These findings suggest a commo…
Close to 70 radio pulsars have now been discovered in 23 globular clusters, with a record 22 pulsars observed in 47 Tuc alone. Accurate timing solutions, including positions in the cluster, are known for many of the pulsars. These recent observations provide a unique opportunity to re-examine theoretically the formation and evolution of recycled pulsars in globular clusters. This brief review focuses on dynamical exchange interactions between neutron stars and primordial binaries, through which neutron stars can acquire intermediate-mass binary companions. The later evolution of these intermed…
We investigate the dynamical formation and evolution of binaries containing neutron stars in dense globular clusters. Our numerical simulations combine a simple Monte Carlo prescription for stellar dynamics, a sophisticated binary population synthesis code, and a small-N-body integrator for computing 3-body and 4-body interactions. Our results suggest that there is no ``retention problem,'' i.e., that, under standard assumptions, globular clusters can retain enough neutron stars to produce the observed numbers of millisecond pulsars. We also identify the dominant evolutionary and dynamical cha…
In this paper we push forward and exploit an analogy between the morphologies of the X-ray cavities observed in some galaxy clusters, and the optically deficient point-symmetric bubbles occurring in some planetary nebulae (PNe). Point-symmetric PNe are thought to be shaped by stellar binary interactions; namely, the presence of a companion to the PN's progenitor star is required. We suggest that similar point-symmetric structures in the X-ray cavities of galaxy clusters might be associated with the presence of massive binary black holes. A systematic cataloguing of high-resolution images of th…
With the aim of investigating the binary population of the 700 Myr old Praesepe cluster, we have observed 149 G and K-type cluster members using adaptive optics. We detected 26 binary systems with an angular separation ranging from less than 0.08 to 3.3 arcsec (15-600 AU). After correcting for detection biases, we derive a binary frequency (BF) in the logP (days) range from 4.4 to 6.9 of 25.3 +/- 5.4%, which is similar to that of field G-type dwarfs (23.8%, Duquennoy & Mayor 1991). This result, complemented by similar ones obtained for the 2 Myr old star forming cluster IC 348 (Paper II) and t…
The frequency of low-mass pre-main sequence binary systems is significantly lower in the Trapezium Cluster than in Taurus-Auriga. We investigate if this difference can be explained through stellar encounters in dense clusters. To this effect, a range of possible models of the well observed Trapezium Cluster are calculated using Aarseth's direct N-body code, which treats binaries accurately. The results are confronted with observational constraints. The range of models include clusters in virial equilibrium, expanding clusters as a result of instantaneous mass loss, as well as collapsing cluste…
A simple question of celestial mechanics is investigated: in what regions of phase space near a binary system can planets persist for long times? The planets are taken to be test particles moving in the field of an eccentric binary system. A range of values of the binary eccentricity and mass ratio is studied, and both the case of planets orbiting close to one of the stars, and that of planets outside the binary orbiting the system's center of mass, are examined. From the results, empirical expressions are developed for both 1) the largest orbit around each of the stars, and 2) the smallest or…
We show how the second sequence seen lying above the main sequence in cluster colour magnitude diagrams results from binaries with a large range of mass ratios and not just from those with equal masses. We conclude that the presence of a densely populated second sequence, with only sparse filling in between it and the single star main sequence, does not necessarily imply that binary mass ratios are close to unity.
We investigate the evolution of binary fractions in star clusters using N-body models of up to 100000 stars. Primordial binary frequencies in these models range from 5% to 50%. Simulations are performed with the NBODY4 code and include a full mass spectrum of stars, stellar evolution, binary evolution and the tidal field of the Galaxy. We find that the overall binary fraction of a cluster almost always remains close to the primordial value, except at late times when a cluster is near dissolution. A critical exception occurs in the central regions where we observe a marked increase in binary fr…
Context. Blue Stragglers Stars (BSSs) are thought to form in globular clusters by two main formation channels: i) mergers induced by stellar collisions and ii) coalescence or mass-transfer between companions in binary systems. The detailed study of the BSS properties is therefore crucial for understanding the binary evolution mechanisms, and the complex interplay between dynamics and stellar evolution in dense stellar systems. Aims. We present the first comparison between the BSS specific frequency and the binary fraction in the core of a sample of Galactic globular clusters, with the aim of i…
We study the dynamical evolution of globular clusters using our 2D Monte Carlo code with the inclusion of primordial binary interactions for equal-mass stars. We use approximate analytical cross sections for energy generation from binary-binary and binary-single interactions. After a brief period of slight contraction or expansion of the core over the first few relaxation times, all clusters enter a much longer phase of stable "binary burning" lasting many tens of relaxation times. The structural parameters of our models during this phase match well those of most observed globular clusters. At…
The stellar-dynamical evolution of bound star clusters during the first few Myr is dominated by binary-binary and binary-star interactions, the rapid sinking of the most massive stars to the centre of the clusters and mass loss from evolving stars. The consequences of these processes for the binary and stellar population in clusters, and for the star clusters as a whole, are studied by following the evolution over 150 Myr of a library of compact cluster models containing up to 10^4 stars.
We consider the effect of binary stars on the main-sequence luminosity functions observed in the core of globular clusters, with specific reference to NGC 6752. We find that mass segregation results in an increased binary fraction at fainter magnitudes along the main-sequence. If this effect is not taken into account when analyzing luminosity functions, erroneous conclusions can be drawn regarding the distribution of single stars, and the dynamical state of the cluster. In the core of NGC 6752, our HST data reveal a flat luminosity function, in agreement with previous results. However, when we…
We first summarize work that has been done on the effects of binaries on theoretical population synthesis of stars and stellar phenomena. Next, we highlight the influence of stellar dynamics in young clusters by discussing a few candidate UFOs (unconventionally formed objects) like intermediate mass black holes, Eta Carinae, Zeta Puppis, Gamma Velorum and WR 140.
We explore the signatures that a binary intermediate mass black hole (IMBH) imprints on the velocity and on the angular momentum of globular cluster stars. Simulating 3-body encounters between a star and a binary IMBH, we find that the binary IMBH generates a family of few hundreds of stars (~100-300) which remain bound to the globular cluster (GC) and have velocity significantly higher than the dispersion velocity. For this reason we term them ''supra-thermal'' stars. We also notice that, after the interaction, a considerable fraction (55-70%) of stars tend to align their orbital angular mome…
The preponderance of binary systems in all known stellar populations makes them exciting dynamical agents for research on topics as varied as star formation, star-cluster dynamics and the interiors of young and old stars. Today we know that the Galactic-field binary population is probably a dynamically evolved version of the Taurus-Auriga pre-main sequence population, and that the initial distributions of binary-star orbital elements are probably universal. Furthermore, N-body calculations tentatively suggest that OB stars form in energetic binaries near cluster cores, and that binaries with '…
We study the evolution of bound pairs of star clusters by means of direct N-body simulations. Our simulations include mass loss by stellar evolution. The initial conditions are selected to mimic the observed binary star cluster NGC 2136 and NGC 2137 in the Large Magellanic Cloud. Based on the rather old ages ($\sim 100$ Myr), masses, sizes of the two clusters and their projected separation, we conclude that the cluster pair must have been born with an initial separation of 15--20 pc. Clusters with a smaller initial separation tend to merge in $\aplt 60$ Myr due to loss of angular momentum from…
We explore a Boltzmann scheme for studying the evolution of compact binary populations of globular clusters. We include processes of compact-binary formation by tidal capture and exchange encounters, binary destruction by dissociation and other mechanisms, and binary hardening by encounters, gravitational radiation and magnetic braking, as also the orbital evolution during mass transfer, following Roche lobe contact. For the encounter processes which are stochastic in nature, we study the probabilistic, continuous limit in this introductory work, deferring the specific handling of the stochast…
The five millisecond pulsars that inhabit NGC 6752 display locations or accelerations that are quite unusual compared to all other pulsars known in globular clusters. In particular PSR-A, a binary pulsar, lives in the cluster halo, while PSR-B and PSR-E, located in the core, show remarkably high negative spin derivatives. This is suggestive that some uncommon dynamical process is at play in the cluster core that we attribute to the presence of a massive perturber. We here investigate whether a single intermediate-mass black hole, lying on the extrapolation of the Mass versus Sigma relation obs…
I present light curves for two detached eclipsing binary stars in the region of the LMC cluster NGC 1850, which is possibly a young globular cluster still in formation. One, a likely spectral type O star, is a newly detected eclipsing binary in the region of the very young subcluster NGC 1850A. This binary is among a small number of highly massive O-type stars in binary systems found in LMC clusters. These two eclipsing binaries are the first discovered in the well studied NGC 1850, and the O-type star is the first eclisping binary found in NGC 1850A. Light curves for two NGC 1850 region Cephe…
We derive a variational principle for the dynamical stability of a cluster as a gas sphere in a box. Newtonian clusters are always dynamically stable and, for relativistic clusters, the relation between dynamical and thermodynamical instabilities is analyzed. The boundaries between dynamically and thermodynamically stable and unstable models are found numerically for relativistic stellar systems with different cut off parameters. A criterion based on binding energy curve is used for determination of the boundary of dynamical stability.
We report the discovery using the Parkes radio telescope of binary millisecond pulsars in four clusters for which no associated pulsars were previously known. The four pulsars have pulse periods lying between 3 and 6 ms. All are in circular orbits with low-mass companions and have orbital periods of a few days or less. One is in a 1.7-hour orbit with a companion of planetary mass. Another is eclipsed by a wind from its companion for 40% of the binary period despite being in a relatively wide orbit. These discoveries result from the use of improved technologies and prove that many millisecond p…
We report on new WFPC2 observations of the globular cluster NGC 288, focusing our attention on peculiar stars. A very pronounced binary sequence, paralleling the ordinary Main Sequence (MS) is clearly observed in the Color Magnitude Diagram (CMD) and a huge relative fraction of Blue Straggler Stars is measured. The dataset offers the opportunity of studying the evolution of a large population of binaries (and binary evolution by-products) in a low density environment, where the evolution of such systems is not dominated by collisions and encounters. Three (very) Extreme Horizontal Branch Stars…
Precision radial velocity measurements of the Gamma Cephei (HR8974) binary system suggest the existence of a planetary companion with a minimum mass of 1.7 Jupiter-mass on an elliptical orbit with a ~2.14 AU semimajor axis and 0.12 eccentricity (hatzes et al. 2003). I present in this paper a summary of the results of an extensive numerical study of the orbital stability of this three-body system for different values of the semimajor axis and orbital eccentricity of the binary, and also the orbital inclination of the planet. Numerical integrations indicate that the system is stable for the plan…
We have investigated different scenarios for the origin of the binary millisecond pulsar PSR J1911-5958A in NGC 6752, the most distant pulsar discovered from the core of a globular cluster to date. The hypothesis that it results from a truly primordial binary born in the halo calls for accretion-induced collapse and negligible recoil speed at the moment of neutron star formation. Scattering or exchange interactions off cluster stars are not consistent with both the observed orbital period and its offset position. We show that a binary system of two black holes with (unequal) masses in the rang…
We carried out a radial velocity survey for spectroscopic binaries in the low density globular cluster NGC5053. Our sample contains a total of 77 cluster member giant and subgiant stars with visual magnitudes of 14.5-18.6. Of these 77 stars, 66 stars have on average of 3-4 measurements with a total of 236 velocities. A typical velocity error per measurement is about 3 km/s. The stars in our sample are spatially distributed from the cluster center out to 10 arcminutes in radius (4.5 core radii). Among these 66 stars with multiple velocity measurements, we discovered 6 spectroscopic binary candi…
This chapter reviews the dynamical processes in young stellar clusters. The accretion of gas by individual stars affects the dynamics of the cluster, and the masses of the stars. Dynamical mass segregation cannot explain the degree of mass segregation observed in clusters such as the Trapezium Cluster in Orion, implying that the location of the massive stars is an indication of where they formed. This can, however, be explained by the competitive accretion model. If most, if not all, stars form in binary systems, then the high proportion of pre-main sequence binary systems can evolve in cluste…
The study of detached eclipsing binaries in open clusters can provide stringent tests of theoretical stellar evolutionary models, which must simultaneously fit the masses, radii, and luminosities of the eclipsing stars and the radiative properties of every other star in the cluster. We review recent progress in such studies and discuss two unusually interesting objects currently under analysis. GV Carinae is an A0m + A8m binary in the Southern open cluster NGC 3532; its eclipse depths have changed by 0.1 mag between 1990 and 2001, suggesting that its orbit is being perturbed by a relatively cl…
We analyze the stability of relativistic, quasi-equilibrium binary neutron stars in synchronous circular orbit. We explore stability against radial collapse to black holes prior to merger, and against orbital plunge. We apply theorems based on turning points along uniformly rotating sequences of constant angular momentum and rest mass to locate the onset of secular instabilities. We find that inspiraling binary neutron stars are stable against radial collapse to black holes all the way down to the innermost stable circular orbit.
We perform simulations of relativistic binary stars in post-Newtonian gravity to investigate their dynamical stability prior to merger against gravitational collapse in a tidal field. In general, our equations are only strictly accurate to first post-Newtonian order, but they recover full general relativity for spherical, static stars. We study both corotational and irrotational binary configurations of identical stars in circular orbits. We adopt a soft, adiabatic equation of state with $Γ= 1.4$, for which the onset of instability occurs at a sufficiently small value of the compaction $M/R$ t…
