Showing papers on "Mass segregation published in 2016"

Dynamical formation signatures of black hole binaries in the first detected mergers by ligo

Abstract: The dynamical formation of stellar-mass black hole-black hole binaries has long been a promising source of gravitational waves for the Laser Interferometer Gravitational-Wave Observatory (LIGO). Mass segregation, gravitational focusing, and multibody dynamical interactions naturally increase the interaction rate between the most massive black holes in dense stellar systems, eventually leading them to merge. We find that dynamical interactions, particularly three-body binary formation, enhance the merger rate of black hole binaries with total mass M_tot roughly as ~M_tot^beta, with beta >~ 4. We find that this relation holds mostly independently of the initial mass function, but the exact value depends on the degree of mass segregation. The detection rate of such massive black hole binaries is only further enhanced by LIGO's greater sensitivity to massive black hole binaries with M_tot <~ 80 solar masses. We find that for power-law BH mass functions dN/dM ~ M^-alpha with alpha <~ 2, LIGO is most likely to detect black hole binaries with a mass twice that of the maximum initial black hole mass and a mass ratio near one. Repeated mergers of black holes inside the cluster result in about ~5% of mergers being observed between two and three times the maximum initial black hole mass. Using these relations, one may be able to invert the observed distribution to the initial mass function with multiple detections of merging black hole binaries.

Dynamical ejections of massive stars from young star clusters under diverse initial conditions

Abstract: We study the effects that initial conditions of star clusters and their massive star population have on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N -body calculations for moderately massive star clusters (M ecl ≈ 103.5 M ⊙ ). We vary the initial conditions of the calculations, such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius r h (0) of 0.1 (0.3) pc can eject up to 50% (30)% of their O-star systems on average, while initially larger (r h (0) = 0.8 pc) clusters, that is, lower density clusters, eject hardly any OB stars (at most ≈ 4.5%). When the binaries are composed of two stars of similar mass, the ejections are most effective. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters that are efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N -body models agree well with the mean mass function of young intermediate-mass clusters in M 31, as reported previously. This implies that the IMF of the observed young clusters is the canonical IMF. We show that the multiplicity fraction of the ejected massive stars can be as high as ≈ 60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. Clusters that are initially not mass-segregated begin ejecting massive stars after a time delay that is caused by mass segregation. When a large kinematic survey of massive field stars becomes available, for instance through Gaia , our results may be used to constrain the birth configuration of massive stars in star clusters. The results presented here, however, already show that the birth mass-ratio distribution for O-star primaries must be near uniform for mass ratios q ≳ 0.1.

A novel look at energy equipartition in globular clusters

Abstract: Two-body interactions play a major role in shaping the structural and dynamical properties of globular clusters (GCs) over their long-term evolution. In particular, GCs evolve toward a state of partial energy equipartition that induces a mass-dependence in their kinematics. By using a set of Monte Carlo cluster simulations evolved in quasi isolation, we show that the stellar mass dependence of the velocity dispersion σ(m) can be described by an exponential function σ 2 ∝ exp(−m/meq), with the parameter meq quantifying the degree of partial energy equipartition of the systems. This simple parametrization successfully captures the behaviour of the velocity dispersion at lower as well as higher stellar masses, that is, the regime where the system is expected to approach full equipartition. We find a tight correlation between the degree if equipartition reached by a GC and its dynamical state, indicating that clusters that are more than about 20 core relaxation times old, have reached a maximum degree of equipartition. This equipartition−dynamical state relation can be used as a tool to characterize the relaxation condition of a cluster with a kinematic measure of the meq parameter. Vice versa, the mass-dependence of the kinematics can be predicted knowing the relaxation time solely on the basis of photometric measurements. Moreover, any deviations from this tight relation could be used as a probe of a peculiar dynamical history of a cluster. Finally, our novel approach is important for the interpretation of state-of-the-art Hubble Space Telescope proper motion data, for which the mass dependence of kinematics can now be measured, and for the application of modeling techniques which take into consideration multi-mass components and mass segregation.

A stellar-mass black hole population in the globular cluster NGC 6101?

Abstract: Dalessandro et al. observed a similar distribution for blue straggler stars and main-sequence turn-off stars in the Galactic globular cluster NGC 6101, and interpreted this feature as an indication that this cluster is not mass-segregated. Using direct N-body simulations, we find that a significant amount of mass segregation is expected for a cluster with the mass, radius and age of NGC 6101. Therefore, the absence of mass segregation cannot be explained by the argument that the cluster is not yet dynamically evolved. By varying the retention fraction of stellar-mass black holes, we show that segregation is not observable in clusters with a high black hole retention fraction (>50 per cent after supernova kicks and >50 per cent after dynamical evolution). Yet all model clusters have the same amount of mass segregation in terms of the decline of the mean mass of stars and remnants with distance to the centre. We also discuss how kinematics can be used to further constrain the presence of a stellar-mass black hole population and distinguish it from the effect of an intermediate-mass black hole. Our results imply that the kick velocities of black holes are lower than those of neutron stars. The large retention fraction during its dynamical evolution can be explained if NGC 6101 formed with a large initial radius in a Milky Way satellite.

CLASH-VLT: Environment-driven evolution of galaxies in the z = 0.209 cluster Abell 209

Abstract: Context. The analysis of galaxy properties, such as stellar masses, colors, sizes and morphologies, and the relations among them and the environment, in which the galaxies reside, can be used to investigate the physical processes driving galaxy evolution.Aims. We conduct a thorough study of the cluster A209 with a new large spectro-photometric dataset to investigate possible environmental effects on galaxy properties that can provide information on galaxy evolution in cluster hostile environments.Methods. We use the dataset obtained as part of the CLASH-VLT spectroscopic survey, supplemented with Subaru/SuprimeCam high-quality imaging in BVRIz -bands, which yields 1916 cluster members (50% of them spectroscopically confirmed) down to a stellar mass M ⋆ = 108.6 M ⊙ . We determine the stellar mass function of these galaxies in different regions of the cluster, by separating the sample into star-forming and passive cluster members. We then determine the intra-cluster light and its properties. We also derive the orbits of low- (M ⋆ ≤ 1010.0 M ⊙ ) and high-mass (M ⋆ > 1010.0 M ⊙ ) passive galaxies and study the effect of the environment on the mass-size relation of early-type galaxies, selected according to their Sersic index; the effects are studied separately for the galaxies in each mass range. Finally, we compare the cluster stellar mass density profile with the number density and total-mass density profiles.Results. The stellar mass function of the star-forming cluster galaxies does not depend on the environment. The slope found for passive galaxies becomes flatter in the densest cluster region, which implies that the low-mass component starts to dominate when moving away from the cluster center. The color distribution of the intra-cluster light is consistent with the color of passive cluster members. The analysis of the dynamical orbits of passive galaxies shows that low-mass galaxies have tangential orbits, avoiding small pericenters around the BCG. The mass-size relation of low-mass passive early-type galaxies is flatter than that of high-mass galaxies, and its slope is consistent with the slope of the relation of field star-forming galaxies. Low-mass galaxies are also more compact within the scale radius of 0.65 Mpc. The ratio between the stellar and number density profiles shows a mass segregation effect in the cluster center. The comparative analysis of the stellar and total density profiles indicates that this effect is due to dynamical friction.Conclusions. Our results are consistent with a scenario in which the “environmental quenching” of low-mass galaxies is due to mechanisms such as harassment out to r 200 , starvation, and ram-pressure stripping at smaller radii. This scenario is supported by the analysis of the mass function, of the dynamical orbits and of the mass-size relation of passive early-type galaxies in different cluster regions. Moreover, our analyses support the idea that the intra-cluster light is formed through the tidal disruption of subgiant (M ⋆ ~ 109.5−10.0 M ⊙ ) galaxies. In fact, our results suggest that low-mass galaxies are destroyed by tidal interactions, and that those that avoid small pericenters around the BCG are influenced by tidal interactions that reduce their sizes. We suggest dynamical friction as the process responsible for the observed mass segregation.

Investigating mass segregation process in globular clusters with Blue Straggler Stars: the impact of dark remnants

Abstract: We present the results of a set of N-body simulations aimed at exploring how the process of mass segregation (as traced by the spatial distribution of blue straggler stars, BSSs) is affected by the presence of a population of heavy dark remnants (as neutron stars and black holes). To this end, clusters characterized by different initial concentrations and different fractions of dark remnants have been modeled. We find that an increasing fraction of stellar-mass black holes significantly delays the mass segregation of BSSs and the visible stellar component. In order to trace the evolution of BSS segregation, we introduce a new parameter ($A^+$) that can be easily measured when the cumulative radial distribution of these stars and a reference population are available. Our simulations show that $A^+$ might also be used as an approximate indicator of the time remaining to the core collapse of the visible component.

Ultra-deep gemini near-infrared observations of the bulge globular cluster ngc 6624*

Abstract: We used ultra-deep $J$ and $K_s$ images secured with the near-infrared GSAOI camera assisted by the multi-conjugate adaptive optics system GeMS at the GEMINI South Telescope in Chile, to obtain a ($K_s$, $J-K_s$) color-magnitude diagram (CMD) for the bulge globular cluster NGC 6624. We obtained the deepest and most accurate near-infrared CMD from the ground for this cluster, by reaching $K_s$ $\sim$ 21.5, approximately 8 magnitudes below the horizontal branch level. The entire extension of the Main Sequence (MS) is nicely sampled and at $K_s$ $\sim$ 20 we detected the so-called MS "knee" in a purely near-infrared CMD. By taking advantage of the exquisite quality of the data, we estimated the absolute age of NGC 6624 ($t_{age}$ = 12.0 $\pm$ 0.5 Gyr), which turns out to be in good agreement with previous studies in the literature. We also analyzed the luminosity and mass functions of MS stars down to M $\sim$ 0.45 M$_{\odot}$ finding evidence of a significant increase of low-mass stars at increasing distances from the cluster center. This is a clear signature of mass segregation, confirming that NGC 6624 is in an advanced stage of dynamical evolution.

A Survey for New Members of the Taurus Star-Forming Region with the Sloan Digital Sky Survey

Abstract: Previous studies have found that ~1 deg2 fields surrounding the stellar aggregates in the Taurus star-forming region exhibit a surplus of solar-mass stars relative to denser clusters like IC~348 and the Orion Nebula Cluster To test whether this difference reflects mass segregation in Taurus or a variation in the IMF, we have performed a survey for members of Taurus across a large field (~40 deg2) that was imaged by the Sloan Digital Sky Survey (SDSS) We obtained optical and near-infrared spectra of candidate members identified with those images and the Two Micron All Sky Survey, as well as miscellaneous candidates that were selected with several other diagnostics of membership We have classified 22 of the candidates as new members of Taurus, which includes one of the coolest known members (M975) Our updated census of members within the SDSS field shows a surplus of solar-mass stars relative to clusters, although it is less pronounced than in the smaller fields towards the stellar aggregates that were surveyed for previously measured mass functions in Taurus In addition to spectra of our new members, we include in our study near-IR spectra of roughly half of the known members of Taurus, which are used to refine their spectral types and extinctions We also present an updated set of near-IR standard spectra for classifying young stars and brown dwarfs at M and L types

On the formation of compact, massive subsystems in stellar clusters and its relation with intermediate-mass black holes

Abstract: During their evolution, star clusters undergo mass segregation, by which the orbits of the most massive stars shrink, while the lighter stars move outwards from the cluster centre. In this context, recent observations and dynamical modelling of several galactic and extra-galactic globular clusters (GCs) suggest that most of them show, close to their centre, an overabundance of mass whose nature is still matter of debate. For instance, many works show that orbitally segregated stars may collide with each other in a runaway fashion, leading to the formation of a very massive star or an intermediate mass black hole (IMBH) with a mass comparable to the observed mass excess. On the other hand, segregated stars can form a dense system if the IMBH formation fails. In this paper we study the early formation phase of a dense, massive sub-system (MSS) in several GCs models using a recently developed semi-analytical treatment of the mass segregation process. In order to investigate how the MSS properties depend on the host cluster properties, we varied initial mass function (IMF), total mass, spatial distribution and metallicity of our models. Our results show how the IMF contributes to determine the final mass of the MSS, while the metallicity and the spatial distribution play a minor role. The method presented in this paper allowed us to provide scaling relations that connect the MSS mass and the host cluster mass in agreement with the observed correlation. In order to follow the early formation stage of the MSSs and improve our statistical results, we performed several $N$-body simulations of stellar clusters with masses between $10^3$ and $2\times 10^5$ solar masses.

Mass segregation in star clusters is not energy equipartition

Abstract: Mass segregation in star clusters is often thought to indicate the onset of energy equipartition, where the most massive stars impart kinetic energy to the lower-mass stars and brown dwarfs/free floating planets. The predicted net result of this is that the centrally concentrated massive stars should have significantly lower velocities than fast-moving low-mass objects on the periphery of the cluster. We search for energy equipartition in initially spatially and kinematically substructured N-body simulations of star clusters with N = 1500 stars, evolved for 100 Myr. In clusters that show significant mass segregation we find no differences in the proper motions or radial velocities as a function of mass. The kinetic energies of all stars decrease as the clusters relax, but the kinetic energies of the most massive stars do not decrease faster than those of lower-mass stars. These results suggest that dynamical mass segregation -- which is observed in many star clusters -- is not a signature of energy equipartition from two-body relaxation.

Do open star clusters evolve towards energy equipartition

Abstract: We investigate whether open clusters (OCs) tend to energy equipartition, by means of direct N-body simulations with a broken power-law mass function. We nd that the simulated OCs become strongly mass segregated, but the local velocity dispersion does not depend on the stellar mass for most of the mass range: the curve of the velocity dispersion as a function of mass is nearly at even after several half-mass relaxation times, regardless of the adopted stellar evolution recipes and Galactic tidal eld model. This result holds both if we start from virialized King models and if we use clumpy sub-virial initial conditions. The velocity dispersion of the most massive stars and stellar remnants tends to be higher than the velocity dispersion of the lighter stars. This trend is particularly evident in simulations without stellar evolution. We interpret this result as a consequence of the strong mass segregation, which leads to Spitzer’s instability. Stellar winds delay the onset of the instability. Our simulations strongly support the result that OCs do not attain equipartition, for a wide range of initial conditions.

The JCMT Gould Belt Survey : Dense Core Clusters in Orion A

Abstract: The Orion A molecular cloud is one of the most well-studied nearby star-forming regions, and includes regions of both highly clustered and more dispersed star formation across its full extent. Here, we analyze dense, star-forming cores identified in the 850 and 450 μm SCUBA-2 maps from the JCMT Gould Belt Legacy Survey. We identify dense cores in a uniform manner across the Orion A cloud and analyze their clustering properties. Using two independent lines of analysis, we find evidence that clusters of dense cores tend to be mass segregated, suggesting that stellar clusters may have some amount of primordial mass segregation already imprinted in them at an early stage. We also demonstrate that the dense core clusters have a tendency to be elongated, perhaps indicating a formation mechanism linked to the filamentary structure within molecular clouds.

The bound fraction of young star clusters

Abstract: The residual gas within newly formed star clusters is expelled through stellar feedback on timescales ~ 1 Myr. The subsequent expansion of the cluster results in an unbinding of a fraction of stars before the remaining cluster members can re-virialize and form a surviving cluster. We investigate the bound fraction after gas expulsion as a function of initial cluster mass in stars and gauge the influence of primordial mass segregation, stellar evolution and the tidal field at the solar distance. We also assess the impact of the star-formation efficiency and gas expulsion velocity. We perform N-body simulations using Sverre Aarseth's NBODY7 code, starting with compact clusters in their embedded phase and approximate the gas expulsion by means of an exponentially depleting external gravitational field. We follow the process of re-virialization through detailed monitoring of different Lagrange radii over several Myr, examining initial half-mass radii of 0.1 pc, 0.3 pc and 0.5 pc and initial masses usually ranging from $5\times10^3 M_\odot$ to $5\times10^4 M_\odot$. The adopted star-formation efficiency of 0.33 in the cluster volume results in a distinct sensitivity to the gas expulsion velocity over a wide mass range, while a variation of the star-formation efficiency can make the cluster robust to the rapidly decreasing external potential. We confirm that primordial mass segregation leads to a smaller bound fraction, its influence possibly decreasing with mass. Stellar evolution has a higher impact on lower mass clusters, but heating through dynamical friction could expand the cluster to a similar extent. The examined clusters expand well within their tidal radii and would survive gas expulsion even in a strong tidal field.

The JCMT Gould Belt Survey: Dense Core Clusters in Orion B

Abstract: The JCMT Gould Belt Legacy Survey obtained SCUBA-2 observations of dense cores within three sub-regions of Orion B: LDN 1622, NGC 2023/2024, and NGC 2068/2071, all of which contain clusters of cores. We present an analysis of the clustering properties of these cores, including the two-point correlation function and Cartwright's Q parameter. We identify individual clusters of dense cores across all three regions using a minimal spanning tree technique, and find that in each cluster, the most massive cores tend to be centrally located. We also apply the independent M-Sigma technique and find a strong correlation between core mass and the local surface density of cores. These two lines of evidence jointly suggest that some amount of mass segregation in clusters has happened already at the dense core stage.

Reversed radial distribution trend of subpopulations in the globular clusters NGC 362 and NGC 6723

Abstract: Most globular clusters (GCs) are now known to host multiple stellar populations with different light element abundances. Here we use narrow-band photometry and low-resolution spectroscopy for NGC 362 and NGC 6723 to investigate their chemical properties and radial distributions of subpopulations. We confirm that NGC 362 and NGC 6723 are among the GCs with multiple populations showing bimodal CN distribution and CN-CH anti-correlation without a significant spread in calcium abundance. These two GCs show more centrally concentrated CN-weak earlier generation stars compared to the later generation CN-strong stars. These trends are reversed with respect to those found in previous studies for many other GCs. Our findings, therefore, seem contradictory to the current scenario for the formation of multiple stellar populations, but mass segregation acting on the two subpopulations might be a possible solution to explain this reversed radial trend.

Evolution of binary stars in multiple-population globular clusters – II. Compact binaries

Abstract: We present the results of a survey of N-body simulations aimed at exploring the evolution of compact binaries in multiple-population globular clusters.We show that as a consequence of the initial differences in the structural properties of the first-generation (FG) and the second-generation (SG) populations and the effects of dynamical processes on binary stars, the SG binary fraction decreases more rapidly than that of the FG population. The difference between the FG and SG binary fraction is qualitatively similar to but quantitatively smaller than that found for wider binaries in our previous investigations.The evolution of the radial variation of the binary fraction is driven by the interplay between binary segregation, ionization and ejection. Ionization and ejection counteract in part the effects of mass segregation but for compact binaries the effects of segregation dominate and the inner binary fraction increases during the cluster evolution. We explore the variation of the difference between the FG and the SG binary fraction with the distance from the cluster centre and its dependence on the binary binding energy and cluster structural parameters. The difference between the binary fraction in the FG and the SG populations found in our simulations is consistent with the results of observational studies finding a smaller binary fraction in the SG population.

A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function

Abstract: Context. NGC 6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Aims. Our main aim is to clarify global properties of NGC 6231 down to low masses through a detailed membership assessment, and to study the cluster stars’ spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. Methods. We use X-ray data, complemented by optical and IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modelling of group-stacked X-ray source spectra. Results. We find a large cluster population down to ~0.3 M ⊙ (complete to ~1 M ⊙ ), with minimal non-member contamination, with a definite age spread (1−8 Myr) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be ~5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong H α emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over ~2 dex in mass, has a slope Γ ~ −1.14. The total mass of cluster members above 1 M ⊙ is 2.28 × 10 3 M ⊙ , and the inferred total mass is 4.38 × 10 3 M ⊙ . We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR 79.

Radial Variation in the Stellar Mass Functions of Star Clusters

Abstract: A number of recent observational studies of Galactic globular clusters have measured the variation in the slope of a cluster's stellar mass function $\alpha$ with clustercentric distance $r$. In order to gather a deeper understanding of the information contained in such observations, we have explored the evolution of $\alpha(r)$ for star clusters with a variety of initial conditions using a large suite of $N$-body simulations. We have specifically studied how the time evolution of $\alpha(r)$ is affected by initial size, mass, binary fraction, primordial mass segregation, black hole retention, an external tidal field, and the initial mass function itself. Previous studies have shown that the evolution of $\alpha_G$ is closely related to the amount of mass loss suffered by a cluster. Hence for each simulation we have also followed the evolution of the slope of the cluster's global stellar mass function, $\alpha_G$, and have shown that clusters follow a well-defined track in the $\alpha_G$-$d\alpha(r)/d(ln(r/r_m))$ plane. The location of a cluster on the $\alpha_G-d\alpha(r)/d(ln(r/r_m))$ plane can therefore constrain its dynamical history and, in particular, constrain possible variations in the stellar initial mass function. The $\alpha_G$-$d\alpha(r)/d(ln(r/r_m))$ plane thus serves as a key tool for fully exploiting the information contained in wide field studies of cluster stellar mass functions.

The binary fraction and mass segregation in Alpha Persei open cluster

Abstract: We have obtained membership probabilities of stars within a field of similar to 3 degrees from the centre of the open cluster Alpha Persei using proper motions and photometry from the PPMXL and Wide-field Infrared Survey Explorer catalogues. We have identified 810 possible stellar members of Alpha Persei. We derived the global and radial present-day mass function (MF) of the cluster and found that they are well matched by two-stage power-law relations with different slopes at different radii. The global MF of Alpha Persei shows a turnover at m = 0.62 M-circle dot with low-and high-mass slopes of alpha(low) = 0.50 +/- 0.09 (0.1 < m/M-circle dot < 0.62) and alpha(high) = 2.32 +/- 0.14 (0.62 <= m/M-circle dot < 4.68), respectively. The high-mass slope of the cluster increases from 2.01 inside 1 degrees.10 to 2.63 outside 2 degrees.2, whereas the mean stellar mass decreases from 0.95 to 0.57 M-circle dot in the same regions, signifying clear evidence of mass segregation in the cluster. From an examination of the high-quality colour-magnitude data of the cluster and performing a series of Monte Carlo simulations, we obtained a binary fraction of f(bin) = 34 +/- 12 per cent for stars with 0.70 < m/M-circle dot < 4.68. This is significantly larger than the observed binary fraction, indicating that this open cluster contains a large population of unresolved binaries. Finally, we corrected the MF slopes for the effect of unresolved binaries and found low-and high-mass slopes of alpha(low) = 0.89 +/- 0.11 and alpha(high) = 2.37 +/- 0.09 and a total cluster mass of 352 M-circle dot for Alpha Persei.

Dynamical ejections of massive stars from young star clusters under diverse initial conditions

Abstract: We study the effects of initial conditions of star clusters and their massive star population on dynamical ejections of massive stars from star clusters up to an age of 3 Myr. We use a large set of direct N-body calculations for moderately massive star clusters (Mecl=$10^{3.5}$ Msun). We vary the initial conditions of the calculations such as the initial half-mass radius of the clusters, initial binary populations for massive stars and initial mass segregation. We find that the initial density is the most influential parameter for the ejection fraction of the massive systems. The clusters with an initial half-mass radius of 0.1 (0.3) pc can eject up to 50% (30%) of their O-star systems on average. Most of the models show that the average ejection fraction decreases with decreasing stellar mass. For clusters efficient at ejecting O stars, the mass function of the ejected stars is top-heavy compared to the given initial mass function (IMF), while the mass function of stars that remain in the cluster becomes slightly steeper (top-light) than the IMF. The top-light mass functions of stars in 3 Myr old clusters in our N-body models agree well with the mean mass function of young intermediate-mass clusters in M31, as reported previously. We show that the multiplicity fraction of the ejected massive stars can be as high as 60%, that massive high-order multiple systems can be dynamically ejected, and that high-order multiples become common especially in the cluster. We also discuss binary populations of the ejected massive systems. When a large kinematic survey of massive field stars becomes available, for instance through Gaia, our results may be used to constrain the birth configuration of massive stars in star clusters. (Abridged)

A Chandra X-ray study of the young star cluster NGC 6231: low-mass population and initial mass function

Abstract: NGC6231 is a massive young star cluster, near the center of the Sco OB1 association. While its OB members are well studied, its low-mass population has received little attention. We present high-spatial resolution Chandra ACIS-I X-ray data, where we detect 1613 point X-ray sources. Our main aim is to clarify global properties of NGC6231 down to low masses through a detailed membership assessment, and to study the cluster stars' spatial distribution, the origin of their X-ray emission, the cluster age and formation history, and initial mass function. We use X-ray data, complemented by optical/IR data, to establish cluster membership. The spatial distribution of different stellar subgroups also provides highly significant constraints on cluster membership, as does the distribution of X-ray hardness. We perform spectral modeling of group-stacked X-ray source spectra. We find a large cluster population down to ~0.3 Msun (complete to ~1 Msun), with minimal non-member contamination, with a definite age spread (1-8 Myrs) for the low-mass PMS stars. We argue that low-mass cluster stars also constitute the majority of the few hundreds unidentified X-ray sources. We find mass segregation for the most massive stars. The fraction of circumstellar-disk bearing members is found to be ~5%. Photoevaporation of disks under the action of massive stars is suggested by the spatial distribution of the IR-excess stars. We also find strong Halpha emission in 9% of cluster PMS stars. The dependence of X-ray properties on mass, stellar structure, and age agrees with extrapolations based on other young clusters. The cluster initial mass function, computed over ~2 dex in mass, has a slope Gamma~-1.14. The total mass of cluster members above 1 Msun is 2280 Msun, and the inferred total mass is 4380 Msun. We also study the peculiar, hard X-ray spectrum of the Wolf-Rayet star WR79.

A comprehensive photometric study of dynamically evolved small van den Bergh–Hagen open clusters

Abstract: We present results from Johnson $UBV$, Kron-Cousins $RI$ and Washington $CT_1T_2$ photometries for seven van den Bergh-Hagen (vdBH) open clusters, namely, vdBH\,1, 10, 31, 72, 87, 92, and 118. The high-quality, multi-band photometric data sets were used to trace the cluster stellar density radial profiles and to build colour-magnitude diagrams (CMDs) and colour-colour (CC) diagrams from which we estimated their structural parameters and fundamental astrophysical properties. The clusters in our sample cover a wide age range, from $\sim$ 60 Myr up to 2.8 Gyr, are of relatively small size ($\sim$ 1 $-$ 6 pc) and are placed at distances from the Sun which vary between 1.8 and 6.3 kpc, respectively. We also estimated lower limits for the cluster present-day masses as well as half-mass relaxation times ($t_r$). The resulting values in combination with the structural parameter values suggest that the studied clusters are in advanced stages of their internal dynamical evolution (age/$t_r$ $\sim$ 20 $-$ 320), possibly in the typical phase of those tidally filled with mass segregation in their core regions. Compared to open clusters in the solar neighbourhood, the seven vdBH clusters are within more massive ($\sim$ 80 $-$ 380$M_\odot$), with higher concentration parameter values ($c$ $\sim$ 0.75$-$1.15) and dynamically evolved ones.

A class of spherical, truncated, anisotropic models for application to globular clusters

Abstract: Recently, a class of non-truncated, radially anisotropic models (the so-called f (ν ) -models), originally constructed in the context of violent relaxation and modelling of elliptical galaxies, has been found to possess interesting qualities in relation to observed and simulated globular clusters. In view of new applications to globular clusters, we improve this class of models along two directions. To make them more suitable for the description of small stellar systems hosted by galaxies, we introduce a “tidal” truncation by means of a procedure that guarantees full continuity of the distribution function. The new f T (ν ) -models are shown to provide a better fit to the observed photometric and spectroscopic profiles for a sample of 13 globular clusters studied earlier by means of non-truncated models; interestingly, the best-fit models also perform better with respect to the radial-orbit instability. Then, we design a flexible but simple two-component family of truncated models to study the separate issues of mass segregation and multiple populations. We do not aim at a fully realistic description of globular clusters to compete with the description currently obtained by means of dedicated simulations. The goal here is to try to identify the simplest models, that is, those with the smallest number of free parameters, but still have the capacity to provide a reasonable description for clusters that are evidently beyond the reach of one-component models. With this tool, we aim at identifying the key factors that characterize mass segregation or the presence of multiple populations. To reduce the relevant parameter space, we formulate a few physical arguments based on recent observations and simulations. A first application to two well-studied globular clusters is briefly described and discussed.

Uncovering mass segregation with galaxy analogues in dark-matter simulations

Abstract: We investigate mass segregation in group and cluster environments by identifying galaxy analogues in high-resolution dark matter simulations. Subhalos identified by the AHF and ROCKSTAR halo finders have similar mass functions, independent of resolution, but different radial distributions due to significantly different subhalo hierarchies. We propose a simple way to classify subhalos as galaxy analogues. The radial distributions of galaxy analogues agree well at large halo-centric radii for both AHF and ROCKSTAR but disagree near parent halo centres where the phase-space information used by ROCKSTAR is essential. We see clear mass segregation at small radii (within $0.5\,r_{vir}$) with average galaxy analogue mass decreasing with radius. Beyond the virial radius, we find a mild trend where the average galaxy analogue mass increases with radius. These mass segregation trends are strongest in small groups and dominated by the segregation of low mass analogues. The lack of mass segregation in massive galaxy analogues suggests that the observed trends are driven by the complex accretion histories of the parent halos rather than dynamical friction.

Globular clusters hosting intermediate-mass black-holes: no mass-segregation based candidates

Abstract: Recently, both stellar mass-segregation and binary-fractions were uniformly measured on relatively large samples of Galactic Globular Clusters (GCs). Simulations show that both sizeable binary-star populations and Intermediate-Mass Black Holes (IMBHs) quench mass-segregation in relaxed GCs. Thus mass-segregation in GCs with a reliable binary-fraction measurement is a valuable probe to constrain IMBHs. In this paper we combine mass-segregation and binary-fraction measurements from the literature to build a sample of 33 GCs (with measured core-binary fractions), and a sample of 43 GCs (with a binary fraction measurement in the area between the core radius and the half-mass radius). Within both samples we try to identify IMBH-host candidates. These should have relatively low mass-segregation, a low binary fraction (< 5%), and short (< 1 Gyr) relaxation time. Considering the core binary fraction sample, no suitable candidates emerge. If the binary fraction between the core and the half-mass radius is considered, two candidates are found, but this is likely due to statistical fluctuations. We also consider a larger sample of 54 GCs where we obtained an estimate of the core binary fraction using a predictive relation based on metallicity and integrated absolute magnitude. Also in this case no suitable candidates are found. Finally, we consider the GC core- to half-mass radius ratio, that is expected to be larger for GCs containing either an IMBH or binaries. We find that GCs with large core- to half-mass radius ratios are less mass-segregated (and show a larger binary fraction), confirming the theoretical expectation that the energy sources responsible for the large core are also quenching mass-segregation

Dynamical estimate of post main sequence stellar masses in 47 Tucanae

Abstract: We use the effects of mass segregation on the radial distribution of different stellar populations in the core of 47 Tucanae to find estimates for the masses of stars at different post main sequence evolutionary stages. We take samples of main sequence (MS) stars from the core of 47 Tucanae, at different magnitudes (i.e. different masses), and use the effects of this dynamical process to develop a relation between the radial distance (RD) at which the cumulative distribution reaches the 20th and 50th percentile, and stellar mass. From these relations we estimate the masses of different post MS populations. We find that mass remains constant for stars going through the evolutionary stages between the upper MS up to the horizontal branch (HB). By comparing RDs of the HB stars with stars of lower masses, we can exclude a mass loss greater than 0.09M during the red giant branch (RGB) stage at nearly the 3{\sigma} level. The slightly higher mass estimates for the asymptotic giant branch (AGB) are consistent with the AGB having evolved from somewhat more massive stars. The AGB also exhibits evidence of contamination by more massive stars, possibly blue stragglers (BSS), going through the RGB phase. We do not include the BSS in this paper due to the complexity of these objects, instead, the complete analysis of this population is left for a companion paper. The process to estimate the masses described in this paper are exclusive to the core of 47 Tuc.

Globular clusters hosting intermediate-mass black holes: no mass-segregation based candidates

Abstract: Recently, both stellar mass segregation and binary fractions were uniformly measured on relatively large samples of Galactic globular clusters (GCs). Simulations show that both sizable binary-star populations and intermediate-mass black holes (IMBHs) quench mass segregation in relaxed GCs. Thus mass segregation in GCs with a reliable binary-fraction measurement is a valuable probe to constrain IMBHs. In this paper we combine mass-segregation and binary-fraction measurements from the literature to build a sample of 33 GCs (with measured core binary fractions), and a sample of 43 GCs (with binary-fraction measurements in the area between the core radius and the half-mass radius). Within both samples we try to identify IMBH-host candidates. These should have relatively low mass segregation, a low binary fraction (<5%), and a short (<1 Gyr) relaxation time. Considering the core-binary-fraction sample, no suitable candidates emerge. If the binary fraction between the core and the half-mass radius is considered, two candidates are found, but this is likely due to statistical fluctuations. We also consider a larger sample of 54 GCs where we obtained an estimate of the core binary fraction using a predictive relation based on metallicity and integrated absolute magnitude. Also in this case no suitable candidates are found. Finally, we consider the GC core- to half-mass radius ratio, which is expected to be larger for GCs containing either an IMBH or binaries. We find that GCs with large core- to half-mass radius ratios are less mass-segregated (and show a larger binary fraction), confirming the theoretical expectation that the energy sources responsible for the large core are also quenching mass segregation.

Dynamical estimate of post main sequence stellar masses in 47 Tucanae

Abstract: We use the effects of mass segregation on the radial distribution of different stellar populations in the core of 47 Tucanae to find estimates for the masses of stars at different post main sequence evolutionary stages. We take samples of main sequence (MS) stars from the core of 47 Tucanae, at different magnitudes (i.e. different masses), and use the effects of this dynamical process to develop a relation between the radial distance (RD) at which the cumulative distribution reaches the 20th and 50th percentile, and stellar mass. From these relations we estimate the masses of different post MS populations. We find that mass remains constant for stars going through the evolutionary stages between the upper MS up to the horizontal branch (HB). By comparing RDs of the HB stars with stars of lower masses, we can exclude a mass loss greater than 0.09M during the red giant branch (RGB) stage at nearly the 3{\sigma} level. The slightly higher mass estimates for the asymptotic giant branch (AGB) are consistent with the AGB having evolved from somewhat more massive stars. The AGB also exhibits evidence of contamination by more massive stars, possibly blue stragglers (BSS), going through the RGB phase. We do not include the BSS in this paper due to the complexity of these objects, instead, the complete analysis of this population is left for a companion paper. The process to estimate the masses described in this paper are exclusive to the core of 47 Tuc.

GeMs/GSAOI observations of La Serena 94: an old and far open cluster inside the solar circle

Abstract: Physical properties were derived for the candidate open cluster La Serena 94, recently unveiled by the VVV collaboration. Thanks to the exquisite angular resolution provided by GeMS/GSAOI, we could characterize this system in detail, for the first time, with deep photometry in JHK$_{s}$ - bands. Decontaminated JHK$_{s}$ diagrams reach about 5 mag below the cluster turnoff in H. The locus of red clump giants in the colour - colour diagram, together with an extinction law, was used to obtain an average extinction of $A_V =14.18 \pm 0.71$. The same stars were considered as standard - candles to derive the cluster distance, $8.5 \pm 1.0$ kpc. Isochrones were matched to the cluster colour - magnitude diagrams to determine its age, $\log{t(yr)}=9.12\pm 0.06$, and metallicity, $Z=0.02\pm0.01$. A core radius of $r_{c}=0.51\pm 0.04$ pc was found by fitting King models to the radial density profile. By adding up the visible stellar mass to an extrapolated mass function, the cluster mass was estimated as $M=(2.65\pm0.57) \times 10^3$ M$_{\odot}$, consistent with an integrated magnitude of $M_{K}=-5.82\pm0.16$ and a tidal radius of $r_{t}=17.2\pm2.1$ pc. The overall characteristics of La Serena 94 confirm that it is an old open cluster located in the Crux spiral arm towards the fourth Galactic quadrant and distant $7.30\pm 0.49$ kpc from the Galactic centre. The cluster distorted structure, mass segregation and age indicate that it is a dynamically evolved stellar system.