High Mass-ratio Binary Population in Open Clusters: Segregation of early type binaries and an increasing binary fraction with mass (original) (raw)
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Stellar Mass Segregation in the Aged Galactic Open Star Cluster Berkeley 17
arXiv: Solar and Stellar Astrophysics, 2016
We present the analysis of the morphology of Berkeley\,17, the oldest known open cluster ($\sim10$ Gyr), using a probabilistic star counting of Pan-STARRS point sources, and confirm its core-tail shape, plus an antitail, previously detected with 2MASS data. The stellar population, as diagnosed by the color-magnitude diagram and theoretical isochrones, shows more massive than lower-mass members in the cluster core, whereas there is a paucity of massive members in both tails. This manifests mass segregation in this aged star cluster with the low-mass members being stripped away from the system. It has been claimed that Berkeley 17 is associated with an excessive number of blue stragglers. Our analysis in comparison of the cluster with nearby reference fields indicates that about half of the blue stragglers may be field contaminations, and some may be confused with the rare blue horizontal-branch stars in this cluster.
The incidence of binaries in globular cluster stellar populations
Astronomy & Astrophysics, 2015
Binary fraction and orbital characteristics provide indications on the conditions of star formation, as they shed light on the environment they were born in. Multiple systems are more common in low density environments than in higher density environments. In the current debate about the formation of globular clusters and their multiple populations, studying the binary incidence in the populations they host offers a crucial piece of information on the environment of their birth and their subsequent dynamical evolution. Through a multiyear observational campaign using FLAMES at VLT, we monitored the radial velocity of 968 red-giant-branch stars located around the half-light radii in a sample of ten Galactic globular clusters. We found a total of 21 radial velocity variables identified as bona fide binary stars, for a binary fraction of 2.2% ± 0.5%. When separating the sample into first generation and second generation stars, we find a binary fraction of 4.9% ± 1.3% and 1.2% ± 0.4%, respectively. Through simulations that take possible sources of bias into account in detecting radial velocity variations in the two populations, we show that the difference is significant and only marginally affected by these effects. This kind of different binary fraction strongly suggests different conditions in the environment of formation and evolution of first and second generations stars, with the latter being born in a much denser environment. Our result hence strongly supports the idea that the second generation forms in a dense subsystem at the center of the loosely distributed first generation, where (loose) binaries are efficiently destroyed.
A highly efficient measure of mass segregation in star clusters
Astronomy & Astrophysics, 2011
Context. Investigations of mass segregation are of vital interest for the understanding of the formation and dynamical evolution of stellar systems on a wide range of spatial scales. A consistent analysis requires a robust measure among different objects and well-defined comparison with theoretical expectations. Various methods have been used for this purpose but usually with limited significance, quantifiability, and application to both simulations and observations. Aims. We aim at developing a measure of mass segregation with as few parameters as possible, robustness against peculiar configurations, independence of mass determination, simple implementation, stable algorithm, and that is equally well adoptable for data from either simulations or observations. Methods. Our method is based on the minimum spanning tree (MST) that serves as a geometry-independent measure of concentration. Compared to previous such approaches we obtain a significant refinement by using the geometrical mean as an intermediate-pass. Results. The geometrical mean boosts the sensitivity compared to previous applications of the MST. It thus allows the detection of mass segregation with much higher confidence and for much lower degrees of mass segregation than other approaches. The method shows in particular very clear signatures even when applied to small subsets of the entire population. We confirm with high significance strong mass segregation of the five most massive stars in the Orion Nebula Cluster (ONC). Conclusions. Our method is the most sensitive general measure of mass segregation so far and provides robust results for both data from simulations and observations. As such it is ideally suited for tracking mass segregation in young star clusters and to investigate the long standing paradigm of primordial mass segregation by comparison of simulations and observations.
Monthly Notices of the Royal Astronomical Society, 2021
We have determined the amount of stellar mass segregation in over 50 globular clusters and ultra-faint dwarf galaxy candidates based on deep HST and ground-based photometry. We find that the amount of mass segregation in globular clusters is strongly correlated with their relaxation time and that all clusters with relaxation times of the order of their ages or longer have little to no mass segregation. For each cluster, the amount of mass segregation seen is fully compatible with the amount expected by dynamical evolution from initially unsegregated clusters, showing that globular clusters formed without primordial mass segregation among their low-mass stars. Ultra-faint dwarf galaxy candidates split into two groups, star clusters which follow the same trend between relaxation time and amount of mass segregation as globular clusters and dark-matter dominated dwarf galaxies that are unsegregated despite having relaxation times smaller than a Hubble time. Stellar abundance and velocit...
Monthly Notices of the Royal Astronomical Society, 2006
We present our findings based on a detailed analysis for the binaries of the Hyades, in which the masses of the components are well known. We fit the models of components of a binary system to the observations so as to give the observed total V and B − V of that system and the observed slope of the main-sequence in the corresponding parts. According to our findings, there is a very definite relationship between the mixing-length parameter and the stellar mass. The fitting formula for this relationship can be given as α = 9.19(M/M ⊙ − 0.74) 0.053 − 6.65, which is valid for stellar masses greater than 0.77M ⊙. While no strict information is gathered for the chemical composition of the cluster, as a result of degeneracy in the colour-magnitude diagram, by adopting Z = 0.033 and using models for the components of 70 Tau and θ 2 Tau we find the hydrogen abundance to be X = 0.676 and the age to be 670 Myr. If we assume that Z = 0.024, then X = 0.718 and the age is 720 Myr. Our findings concerning the mixing length parameter are valid for both sets of the solution. For both components of the active binary system V818 Tau, the differences between radii of the models with Z = 0.024 and the observed radii are only about 4 percent. More generally, the effective temperatures of the models of low mass stars in the binary systems studied are in good agreement with those determined by spectroscopic methods.
Monthly Notices of the Royal Astronomical Society, 2011
We present a homogeneous catalogue for blue straggler, red giant branch, horizontal branch and main-sequence turn-off stars in a sample of 35 clusters taken from the ACS Survey for Globular Clusters. As a result of the superior photometry and relatively large field of view offered by the ACS data, this new catalogue is a significant improvement upon the one presented in Leigh, . Using our catalogue, we study and compare the radial distributions of the different stellar populations.
The Evolution of Binary Populations in Young Star Clusters:From the ONC to OB associations
2012
Observations of the binary populations in young, sparse clusters have shown, that almost all stars are part of a binary system at the end of the star formation process. By contrast, the binary frequency of field stars only ∼ 50%. Most stars, and therefore most binaries, are formed in dense star clusters. This rises the question if the natal environments lead to the observed reduction of the binary frequency. In this thesis this question is addressed using numerical Nbody simulations of binary populations in different star cluster environments. First the evolution of binaries in ONC-like star clusters has been investigated. It was found that there the evolution of the normalised number of binaries is independent from the initial binary frequency. This allows to predict the evolution of binary populations in ONC-like clusters without the need of further numerical simulations. In addition it was found, that dynamical interactions preferentially destroy low-mass binaries resulting in a higher binary frequency for high-mass stars in the simulated clusters, in accordance with observation in the ONC. The combination of dynamical evolution with gas-induced orbital decay of embedded binaries is capable to reshape a log-uniform period distribution, as observed in young star clusters, to a log-normal period distribution as observed in the field today. The modelling has been generalised to clusters with arbitrary densities. Performing simulations for clusters with up to eight times higher densities than before with two different initial binary frequencies, it was shown that the evolution of the normalised number of binaries remains independent of the initial binary frequency. The higher the density in a cluster the more binaries are destroyed as to be expected. However, this effect levels out for clusters with central densities exceeding ≈ 3 × 10 4 pc −3. This means that there is a limit beyond which increasing the binary frequency in the clusters does not lead to significantly more binaries being destroyed. Finally it was investigated how the binary population evolves in star clusters that have undergone instantaneous gas expulsion with a resultant fast decrease in stellar density. It was found that the lower the star formation efficiency of a cluster and therefore the faster the decrease in stellar density, the less binaries are destroyed during the evolution while at the same time the more very wide binaries are formed. Comparison of the evolution of the simulated clusters and the observed leaky cluster sequence shows that clusters including a binary population and a ε = 0.3 match the observations of leaky-star clusters remarkably well.
Astronomy Astrophysics, 2012
Context. Usually, important parameters of young, low-mass star clusters are very difficult to obtain by means of photometry, especially when differential reddening and/or binaries occur in large amounts. Aims. We present a semi-analytical approach (ASAmin) that, applied to the Hess diagram of a young star cluster, is able to retrieve the values of mass, age, star-formation spread, distance modulus, foreground and differential reddening, and binary fraction. Methods. The global optimisation method known as adaptive simulated annealing (ASA) is used to minimise the residuals between the observed and simulated Hess diagrams of a star cluster. The simulations are realistic and take the most relevant parameters of young clusters into account. Important features of the simulations are: a normal (Gaussian) differential reddening distribution, a time-decreasing star-formation rate, the unresolved binaries, and the smearing effect produced by photometric uncertainties on Hess diagrams. Free parameters are: cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening, and binary fraction. Results. Tests with model clusters built with parameters spanning a broad range of values show that ASAmin retrieves the input values with a high precision for cluster mass, distance modulus and foreground reddening, but somewhat lower for the remaining parameters. Given the statistical nature of the simulations, several runs should be performed to obtain significant convergence patterns. Specifically, we find that the retrieved (absolute minimum) parameters converge to mean values with a low dispersion as the Hess residuals decrease. When applied to actual young clusters, the retrieved parameters follow convergence patterns similar to the models. We show how the stochasticity associated with the early phases may affect the results, especially in low-mass clusters. This effect can be minimised by averaging out several twin clusters in the simulated Hess diagrams. Conclusions. Even for low-mass star clusters, ASAmin is sensitive to the values of cluster mass, age, distance modulus, star-formation spread, foreground and differential reddening and, to a lesser degree, binary fraction. Compared with simpler approaches, the inclusion of binaries, a decaying star-formation rate and a normally distributed differential reddening, appear to yield more constrained parameters, especially the mass, age and distance from the Sun. A robust determination of cluster parameters may have a positive impact on many fields. For instance, age, mass and binary fraction are important for establishing the dynamical state of a cluster, or deriving a more precise star-formation rate in the Galaxy.
Revisiting the mass of open clusters with Gaia data
Monthly Notices of the Royal Astronomical Society
The publication of the Gaia catalogue and improvements in methods to determine memberships and fundamental parameters of open clusters has led to major advances in recent years. However, important parameters such as the masses of these objects, although being studied mostly in some isolated cases, have not been addressed in large homogeneous samples based on Gaia data, taking into account details such as binary fractions. Consequently, relevant aspects such as the existence of mass segregation were not adequately studied. Within this context, in this work, we introduce a new method to determine individual stellar masses, including an estimation for the ones in binary systems. This method allows us to study the mass of open clusters, as well as the mass functions of the binary star populations. We validate the method and its efficiency and characterize uncertainties using a grid of synthetic clusters with predetermined parameters. We highlight the application of the method to the Ple...
Monthly Notices of the Royal Astronomical Society, 2011
A new method is presented to describe the evolution of the orbital-parameter distributions for an initially universal binary population in star clusters by means of the currently largest existing library of N -body models. It is demonstrated that a stellar-dynamical operator, Ω M ecl ,r h dyn (t), exists, which uniquely transforms an initial (t = 0) orbital parameter distribution function for binaries, D in , into a new distribution, D M ecl ,r h (t), depending on the initial cluster mass, M ecl , and half-mass radius, r h , after some time t of dynamical evolution. For D in the distribution functions derived by Kroupa (1995a,b) are used, which are consistent with constraints for pre-main sequence and Class I binary populations. Binaries with a lower energy and a higher reduced-mass are dissolved preferentially. The Ω-operator can be used to efficiently calculate and predict binary properties in clusters and whole galaxies without the need for further N -body computations. For the present set of N -body models it is found that the binary populations change their properties on a crossing time-scale such that Ω M ecl ,r h dyn (t) can be well parametrized as a function of the cluster density, ρ ecl . Furthermore it is shown that the binary-fraction in clusters with similar initial velocity dispersions follows the same evolutionary tracks as a function of the passed number of relaxation-times. Present-day observed binary populations in star clusters put constraints on their initial stellar densities, ρ ecl , which are found to be in the range 10 2 ρ ecl ( r h )/M ⊙ pc −3 2 × 10 5 for open clusters and a few×10 3 ρ ecl ( r h )/M ⊙ pc −3 10 8 for globular clusters, respectively.