Showing papers on "Mass segregation published in 2009"
TL;DR: In this article, the authors discuss the observations and theory of star cluster formation and argue that clusters form dynamically cool (subvirial) and with substructure, and perform an ensemble of simulations of cool, clumpy (fractal) clusters and show that they often dynamically mass segregation on timescales far shorter than expected from simple models.
Abstract: We discuss the observations and theory of star cluster formation to argue that clusters form dynamically cool (subvirial) and with substructure. We then perform an ensemble of simulations of cool, clumpy (fractal) clusters and show that they often dynamically mass segregate on timescales far shorter than expected from simple models. The mass segregation comes about through the production of a short-lived, but very dense core. This shows that in clusters like the Orion Nebula Cluster the stars ≥ 4 M ☉ can dynamically mass segregate within the current age of the cluster. Therefore, the observed mass segregation in apparently dynamically young clusters need not be primordial, but could be the result of rapid and violent early dynamical evolution.
275 citations
TL;DR: In this paper, the authors discuss the observations and theory of star cluster formation and argue that clusters form dynamically cool (subvirial) and with substructure, and perform an ensemble of simulations of cool, clumpy (fractal) clusters and show that they often dynamically mass segregate on timescales far shorter than expected from simple models.
Abstract: We discuss the observations and theory of star cluster formation to argue that clusters form dynamically cool (subvirial) and with substructure. We then perform an ensemble of simulations of cool, clumpy (fractal) clusters and show that they often dynamically mass segregate on timescales far shorter than expected from simple models. The mass segregation comes about through the production of a short-lived, but very dense core. This shows that in clusters like the Orion Nebula Cluster the stars >4 Msun can dynamically mass segregate within the current age of the cluster. Therefore, the observed mass segregation in apparently dynamically young clusters need not be primordial, but could be the result of rapid and violent early dynamical evolution.
237 citations
TL;DR: The mass-segregation solution for the steady state distribution of stars around a massive black hole (MBH) has two branches: the known weak segregation solution (Bahcall & Wolf 1977), and a newly discovered strong segregation solution, presented in this paper.
Abstract: We show that the mass-segregation solution for the steady state distribution of stars around a massive black hole (MBH) has two branches: the known weak segregation solution (Bahcall & Wolf 1977), and a newly discovered strong segregation solution, presented here. The nature of the solution depends on the heavy-to-light stellar mass ratio M_H/M_L and on the unbound population number ratio N_H/N_L, through the relaxational coupling parameter \Delta=4 N_H M_H^2 /[N_L M_L^2(3+M_H/M_L)]. When the heavy stars are relatively common (\Delta>>1), they scatter frequently on each other. This efficient self-coupling leads to weak mass segregation, where the stars form n \propto r^{-\alpha_M} mass-dependent cusps near the MBH, with indices \alpha_H=7/4 for the heavy stars and 3/2<\alpha_L<7/4 for the light stars (i.e. \max(\alpha_H-\alpha_L)~=1/4). However, when the heavy stars are relatively rare (\Delta<<1), they scatter mostly on light stars, sink to the center by dynamical friction and settle into a much steeper cusp with 2~<\alpha_H<11/4, while the light stars form a 3/2<\alpha_L<7/4 cusp, resulting in strong segregation (i.e. \max(\alpha_H-\alpha_L)~=1). We show that the present-day mass function of evolved stellar populations (coeval or continuously star forming) with a universal initial mass function, separate into two distinct mass scales, ~1 Mo of main sequence and compact dwarfs, and ~10 Mo of stellar black holes (SBHs), and have \Delta<0.1. We conclude that it is likely that many relaxed galactic nuclei are strongly segregated. We review indications of strong segregation in observations of the Galactic Center and in results of numeric simulations, and briefly list some possible implications of a very high central concentration of SBHs around a MBH.
208 citations
TL;DR: In this paper, the authors proposed a method to detect and quantify mass segregation in star clusters by comparing the minimum spanning tree (MST) of massive stars with that of random stars.
Abstract: We present a new method to detect and quantify mass segregation in star clusters. It compares the minimum spanning tree (MST) of massive stars with that of random stars. If mass segregation is present, the MST length of the most massive stars will be shorter than that of random stars. This difference can be quantified (with an associated significance) to measure the degree of mass segregation. We test the method on simulated clusters in both 2D and 3D and show that the method works as expected. We apply the method to the Orion Nebula Cluster (ONC) and show that the method is able to detect the mass segregation in the Trapezium with a 'mass segregation ratio (MSR)' Lambda(MSR) = 8.0 +/- 3.5 (where Lambda(MSR) = 1 is no mass segregation) down to 16M(circle dot), and also that the ONC is mass segregated at a lower level (similar to 2.0 +/- 0.5) down to 5M(circle dot). Below 5M(circle dot) we find no evidence for any further mass segregation in the ONC.
206 citations
TL;DR: The structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants is reported in this article.
Abstract: We report on the structure of the nuclear star cluster in the innermost 0.16 pc of the Galaxy as measured by the number density profile of late-type giants. Using laser guide star adaptive optics in conjunction with the integral field spectrograph, OSIRIS, at the Keck II telescope, we are able to differentiate between the older, late-type ($\sim$ 1 Gyr) stars, which are presumed to be dynamically relaxed, and the unrelaxed young ($\sim$ 6 Myr) population. This distinction is crucial for testing models of stellar cusp formation in the vicinity of a black hole, as the models assume that the cusp stars are in dynamical equilibrium in the black hole potential. Based on the late-type stars alone, the surface stellar number density profile, $\Sigma(R) \propto R^{-\Gamma}$, is flat, with $\Gamma = -0.27\pm0.19$. Monte Carlo simulations of the possible de-projected volume density profile, n(r) $\propto r^{-\gamma}$, show that $\gamma$ is less than 1.0 at the 99.73 % confidence level. These results are consistent with the nuclear star cluster having no cusp, with a core profile that is significantly flatter than predicted by most cusp formation theories, and even allows for the presence of a central hole in the stellar distribution. Of the possible dynamical interactions that can lead to the depletion of the red giants observable in this survey -- stellar collisions, mass segregation from stellar remnants, or a recent merger event -- mass segregation is the only one that can be ruled out as the dominant depletion mechanism. The lack of a stellar cusp around a supermassive black hole would have important implications for black hole growth models and inferences on the presence of a black hole based upon stellar distributions.
128 citations
TL;DR: In this paper, the authors proposed a method to detect and quantify mass segregation in star clusters by comparing the minimum spanning tree (MST) of massive stars with that of random stars.
Abstract: We present a new method to detect and quantify mass segregation in star clusters. It compares the minimum spanning tree (MST) of massive stars with that of random stars. If mass segregation is present, the MST length of the most massive stars will be shorter than that of random stars. This difference can be quantified (with an associated significance) to measure the degree of mass segregation. We test the method on simulated clusters in both 2D and 3D and show that the method works as expected.
We apply the method to the Orion Nebula Cluster (ONC) and show that the method is able to detect the mass segregation in the Trapezium with a `mass segregation ratio' \Lambda_{MSR}=8.0 \pm 3.5 (where \Lambda_{MSR}=1 is no mass segregation) down to 16 \Msun, and also that the ONC is mass segregated at a lower level (~2.0 \pm 0.5) down to 5 \Msun. Below 5 \Msun we find no evidence for any further mass segregation in the ONC.
122 citations
TL;DR: In this paper, the authors use N-body simulations to study the evolution of the orbital eccentricities of stars deposited near the Milky Way massive black hole (MBH), starting from initial conditions motivated by two competing models for their origin: formation in a disk followed by inward migration; and exchange interactions involving a binary star.
Abstract: We use N-body simulations to study the evolution of the orbital eccentricities of stars deposited near (<0.05 pc) the Milky Way massive black hole (MBH), starting from initial conditions motivated by two competing models for their origin: formation in a disk followed by inward migration; and exchange interactions involving a binary star. The first model predicts modest eccentricities, lower than those observed in the S-star cluster, while the second model predicts higher eccentricities than observed. The N-body simulations include a dense cluster of 10 M_sun stellar black holes (SBHs), expected to accumulate near the MBH by mass segregation. Perturbations from the SBHs tend to randomize the stellar orbits, partially erasing the dynamical signatures of their origin. The eccentricities of the initially highly eccentric stars evolve, in 20 Myr (the S-star lifespan), to a distribution that is consistent at the ~95 % level with the observed eccentricity distribution. In contrast, the eccentricities of the initially more circular orbits fail to evolve to the observed values in 20 Myr, arguing against the disk migration scenario. We find that 20 % - 30 % of the S-stars are tidally disrupted by the MBH over their lifetimes, and that the S-stars are not likely to be ejected as hypervelocity stars outside the central 0.05 pc by close encounters with stellar black holes.
117 citations
TL;DR: In this article, it was shown that the Galactic globular cluster NGC 1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500 arcsec (>250 pc).
Abstract: Using imaging that shows 4 mag of main-sequence stars, we have discovered that the Galactic globular cluster NGC 1851 is surrounded by a halo that is visible from the tidal radius of 700 arcsec (41 pc) to more than 4500 arcsec (>250 pc). This halo is symmetric and falls in density as a power law of r {sup -1.24}. It contains approximately 0.1% of the dynamical mass of NGC 1851. There is no evidence for tidal tails. Current models of globular cluster evolution do not explain this feature, although simulations of tidal influences on dwarf spheroidal galaxies qualitatively mimic these results. Given the state of published models, it is not possible to decide between creation of this halo from either isolated cluster evaporation or from tidal or disk shocking, or from destruction of a dwarf galaxy in which this object may have once been embedded.
113 citations
TL;DR: In this paper, the authors derived dissolution timescales and used these to predict the M/L_V ratios of the 24 Galactic GCs for which orbital parameters and dynamical M/l_V are known, and also predicted the slopes of their low-mass stellar mass functions.
Abstract: The observed dynamical mass-to-light (M/L) ratios of globular clusters (GCs) are systematically lower than those expected from `canonical' simple stellar population models, which do not account for the preferential loss of low-mass stars due to energy equipartition. It was recently shown that low-mass star depletion can qualitatively explain the M/L discrepancy. To verify whether it is indeed the driving mechanism, we derive dissolution timescales and use these to predict the M/L_V ratios of the 24 Galactic GCs for which orbital parameters and dynamical M/L_V are known. We also predict the slopes of their low-mass stellar mass functions (MFs). We use the SPACE cluster models, which include dynamical dissolution, low-mass star depletion, stellar evolution, stellar remnants and various metallicities. The predicted M/L_V are in 1 sigma agreement with the observations for 12 out of 24 GCs. The discrepancy for the other GCs probably arises because our predictions give global M/L ratios, while the observations represent extrapolated central values that are different from global ones in case of mass segregation and a long dissolution timescale. GCs in our sample which likely have dissimilar global and central M/L ratios can be excluded by imposing limits on the dissolution timescale and King parameter. For the remaining GCs, the observed and predicted average M/L_V are 78^+9_-11% and 78+/-2% of the canonically expected values, while for the entire sample the values are 74^+6_-7% and 85+/-1%. The predicted correlation between the slope of the low-mass stellar MF and M/L_V is qualitatively consistent with observed MF slopes. It is concluded that the variation of M/L ratio due to dissolution and low-mass star depletion is a plausible explanation for the discrepancy between the observed and canonically expected M/L ratios of GCs. (Abridged)
109 citations
TL;DR: In this article, the stellar initial mass function (IMF) of the starburst cluster 30Dor (NGC 2070) in the Large Magellanic Cloud (LMC) was derived using the deep Hubble Space Telescope NICMOS 2 F160W band observations.
Abstract: We present deep Hubble Space Telescope NICMOS 2 F160W band observations of the central 56 '' x 57 '' (14 pc x 14.25 pc) region around R136 in the starburst cluster 30Dor (NGC 2070) located in the Large Magellanic Cloud. Our aim is to derive the stellar initial mass function (IMF) down to similar to 1M(circle dot) in order to test whether the IMF in a massive metal-poor cluster is similar to that observed in nearby young clusters and the field in our Galaxy. We estimate the mean age of the cluster to be 3 Myr by combining our F160W photometry with previously obtained HST WFPC2 optical F555W and F814W band photometry and comparing the stellar locus in the color-magnitude diagram with main sequence and pre-main sequence isochrones. The color-magnitude diagrams show the presence of differential extinction and possibly an age spread of a few megayear. We convert the magnitudes into masses adopting both a single mean age of 3 Myr isochrone and a constant star formation history from 2 to 4 Myr. We derive the IMF after correcting for incompleteness due to crowding. The faintest stars detected have a mass of 0.5 M-circle dot and the data are more than 50% complete outside a radius of 5 pc down to amass limit of 1.1M(circle dot) for 3 Myr old objects. We find an IMF of dN/dlogM alpha M-1.20 +/- 0.2 over the mass range 1.1-20 M-circle dot only slightly shallower than a Salpeter IMF. In particular, we find no strong evidence for a flattening of the IMF down to 1.1 M-circle dot at a distance of 5 pc from the center, in contrast to a flattening at 2 M-circle dot at a radius of 2 pc, reported in a previous optical HST study. We examine several possible reasons for the different results including the possible presence of mass segregation and the effects of differential extinction, particularly for the pre-main sequence sources. If the IMF determined here applies to the whole cluster, the cluster would be massive enough to remain bound and evolve into a relatively low-mass globular cluster.
102 citations
TL;DR: In this paper, the dissolution timescales due to two-body relaxation and disc shocking were derived from the orbital parameters of a sample of 24 Galactic GCs for which orbital parameters and dynamical V-band mass-to-light ratios M/LV are known.
Abstract: Context. The observed dynamical mass-to-light (M/L) ratios of globular clusters (GCs) are systematically lower than the value expected from “canonical” simple stellar population models, which do not account for dynamical effects such as the preferential loss of low-mass stars due to energy equipartition. It has recently been shown that low-mass star depletion can qualitatively explain this discrepancy for globular clusters in several galaxies. Aims. To verify whether low-mass star depletion is indeed the driving mechanism behind the M/L decrease, we aim to predict the M/LV ratios of individual GCs for which orbital parameters and dynamical V-band mass-to-light ratios M/LV are known. There is a sample of 24 Galactic GCs for which this is possible. Methods. We used the SPACE cluster models, which include dynamical dissolution, low-mass star depletion, stellar evolution, stellar remnants, and various metallicities. We derived the dissolution timescales due to two-body relaxation and disc shocking from the orbital parameters of our GC sample and used these to predict the M/LV ratios of the individual GCs. To verify our findings, we also predicted the slopes of their low-mass stellar mass functions. Results. The computed dissolution timescales agree well with earlier empirical studies. The predicted M/LV are in 1σ agreement with the observations for 12 out of 24 GCs. The discrepancy for the other GCs probably arises because our predictions give global M/L ratios, while the observations represent extrapolated central values that are different from global ones in the case of mass segregation and a long dissolution timescale. The GCs in our sample that likely have dissimilar global and central M/L ratios can be excluded by imposing limits on the dissolution timescale and King parameter. For the remaining GCs, the observed and predicted average M/LV are 78 +9 −11 % and 78 ± 2% of the canonically expected values, while the values are 74 +6 −7 % and 85 ± 1% for the entire sample. The predicted correlation between the slope of the low-mass stellar mass function and M/LV drop is found to be qualitatively consistent with observed mass function slopes. Conclusions. The dissolution timescales of Galactic GCs are such that the ∼20% gap between canonically expected and observed M/LV ratios is bridged by accounting for the preferential loss of low-mass stars, also when considering individual clusters. It is concluded that the variation in M/L ratio due to dissolution and low-mass star depletion is a plausible explanation for the discrepancy between the observed and canonically expected M/L ratios of GCs.
TL;DR: In this paper, a simple physical model for the evolution of the stellar mass function in star clusters for a large range of the parameter space is derived from the basic principles of two-body encounters and energy considerations.
Abstract: (Abridged) The dynamical ejection of stars from star clusters affects the shape of the stellar mass function (MF) in these clusters, because the escape probability of a star depends on its mass. The objective of this paper is to provide and to apply a simple physical model for the evolution of the MF in star clusters for a large range of the parameter space. The model is derived from the basic principles of two-body encounters and energy considerations. It is independent of the adopted mass loss rate or initial mass function (IMF), and contains stellar evolution, stellar remnant retention, dynamical dissolution in a tidal field, and mass segregation. It is found that the MF evolution in star clusters depends on the disruption time, remnant retention fraction, initial-final stellar mass relation, and IMF. Low-mass stars are preferentially ejected after t~400 Myr. Before that time, masses around 15-20% of the maximum stellar mass are lost. The degree of low-mass star depletion grows for increasing disruption times, but can be quenched when the retained fraction of massive remnants is large. The highly depleted MFs of certain Galactic globular clusters are explained by the enhanced low-mass star depletion that occurs for low remnant retention fractions. Unless the retention fraction is exceptionally large, dynamical evolution always decreases the mass-to-light ratio. The modeled evolution of the MF is consistent with N-body simulations when adopting identical boundary conditions. However, it is found that the results from N-body simulations only hold for their specific boundary conditions and should not be generalised to all clusters. It is concluded that the model provides an efficient method do understand the evolution of the stellar MF in star clusters under widely varying conditions.
TL;DR: In this paper, the authors used N-body simulations to study the effects of primordial mass segregation on the early and long-term evolution of star clusters and showed that in segregated clusters early mass loss due to stellar evolution triggers a stronger expansion than for unsegregated clusters.
Abstract: In this paper, we use N-body simulations to study the effects of primordial mass segregation on the early and long-term evolution of star clusters. Our simulations show that in segregated clusters early mass loss due to stellar evolution triggers a stronger expansion than for unsegregated clusters. Tidally limited, strongly segregated clusters may dissolve rapidly as a consequence of this early expansion, while segregated clusters initially underfilling their Roche lobe can survive the early expansion and have a lifetime similar to that of unsegregated clusters. Long-lived initially segregated clusters tend to have looser structure and reach core collapse later in their evolution than initially unsegregated clusters. We have also compared the effects of dynamical evolution on the global stellar mass function (MF) of low-mass main-sequence stars. In all cases, the MF flattens as the cluster loses stars. The amount of MF flattening induced by a given amount of mass loss in a rapidly dissolving initially segregated cluster is less than for an unsegregated cluster. The evolution of the MF of a long-lived segregated cluster, on the other hand, is very similar to that of an initially unsegregated cluster.
TL;DR: The findings suggest that the current hard binary fraction near the half-mass radius is a good indicator of the hard primordial binary fraction, but the relationship between the true binary fraction and the fraction of main-sequence stars in binaries is nonlinear and rather complicated.
Abstract: Using our recently improved Monte Carlo evolution code, we study the evolution of the binary fraction in globular clusters. In agreement with previous N-body simulations, we find generally that the hard binary fraction in the core tends to increase with time over a range of initial cluster central densities for initial binary fractions 90%. The dominant processes driving the evolution of the core binary fraction are mass segregation of binaries into the cluster core and preferential destruction of binaries there. On a global scale, these effects and the preferential tidal stripping of single stars tend to roughly balance, leading to overall cluster binary fractions that are roughly constant with time. Our findings suggest that the current hard binary fraction near the half-mass radius is a good indicator of the hard primordial binary fraction. However, the relationship between the true binary fraction and the fraction of main-sequence stars in binaries (which is typically what observers measure) is nonlinear and rather complicated. We also consider the importance of soft binaries, which not only modify the evolution of the binary fraction, but can also drastically change the evolution of the cluster as a whole. Finally, we briefly describe the recent addition of single and binary stellar evolution to our cluster evolution code.
TL;DR: In this paper, a simple, physically self-contained model for the evolution of the stellar mass function in star clusters is derived from the basic principles of two-body encounters and energy considerations.
Abstract: Context. The dynamical escape of stars from star clusters affects the shape of the stellar mass function (MF) in these clusters, because the escape probability of a star depends on its mass. This is found in N-body simulations and has been approximated in analytical cluster models by fitting the evolution of the MF. Both approaches are naturally restricted to the set of boundary conditions for which the simulations were performed. Aims. The objective of this paper is to provide and to apply a simple physical model for the evolution of the MF in star clusters for a large range of the parameter space. It should also offer a new perspective on the results from N-body simulations. Methods. A simple, physically self-contained model for the evolution of the stellar MF in star clusters is derived from the basic principles of two-body encounters and energy considerations. It is independent of the adopted mass loss rate or initial mass function (IMF), and contains stellar evolution, stellar remnant retention, dynamical dissolution in a tidal field, and mass segregation. Results. The MF evolution in star clusters depends on the disruption time, remnant retention fraction, initial-final stellar mass relation, and IMF. Low-mass stars are preferentially ejected after t ∼ 400 Myr. Before that time, masses around 15–20% of the maximum stellar mass are lost due to their rapid two-body relaxation with the massive stars that still exist at young ages. The degree of low-mass star depletion grows for increasing disruption times, but can be quenched when the retained fraction of massive remnants is large. The highly depleted MFs of certain Galactic globular clusters are explained by the enhanced low-mass star depletion that occurs for low remnant retention fractions. Unless the retention fraction is exceptionally large, dynamical evolution always decreases the mass-tolight ratio. The retention of black holes reduces the fraction of the cluster mass in remnants because white dwarfs and neutron stars have masses that are efficiently ejected by black holes. Conclusions. The modeled evolution of the MF is consistent with N-body simulations when adopting identical boundary conditions. However, it is found that the results from N-body simulations only hold for their specific boundary conditions and should not be generalised to all clusters. It is concluded that the model provides an efficient method to understand the evolution of the stellar MF in star clusters under widely varying conditions.
TL;DR: In this article, the authors investigate the validity of the mass segregation indicators commonly used in analysing young stellar clusters and find that commonly used indicators are highly sensitive to sample incompleteness in observational data and that radial completeness determinations do not provide satisfactory corrections.
Abstract: Aims. We investigate the validity of the mass segregation indicators commonly used in analysing young stellar clusters. Methods. We simulate observations by constructing synthetic seeing-limited images of a 1000 massive clusters (10 4 M� ) with a standard IMF and a King-density distribution function. Results. We find that commonly used indicators are highly sensitive to sample incompleteness in observational data and that radial completeness determinations do not provide satisfactory corrections, rendering the studies of radial properties highly uncertain. On the other hand, we find that, under certain conditions, the global completeness can be estimated accurately, allowing for the correction of the global luminosity and mass functions of the cluster. Conclusions. We argue that there is currently no observational evidence of mass segregation in young compact clusters since there is no robust way to differentiate between true mass segregation and sample incompleteness effects. Caution should then be exercised when interpreting results from observations as evidence of mass segregation.
TL;DR: For example, this paper showed that for reasonable ages and cluster sizes, the merger of subclusters can indeed lead to compact configurations of the most massive stars, a signal seen both in nature and in large-scale hydrodynamic simulations of star formation from collapsing molecular clouds, and that subvirial initial conditions can make an unmerged cluster display a similar type of mass segregation.
Abstract: The nearest site of massive star formation in Orion is dominated by the Trapezium subsystem, with its four OB stars and numerous companions. The question of how these stars came to be in such close proximity has implications for our understanding of massive star formation and early cluster evolution. A promising route towards rapid mass segregation was proposed by McMillan et al., who showed that the merger product of faster evolving subclusters can inherit their apparent dynamical age from their progenitors. In this paper, we briefly consider this process at a size and time-scale more suited for local and perhaps more typical star formation, with stellar numbers from hundreds to thousands. We find that for reasonable ages and cluster sizes, the merger of subclusters can indeed lead to compact configurations of the most massive stars, a signal seen both in nature and in large-scale hydrodynamic simulations of star formation from collapsing molecular clouds, and that subvirial initial conditions can make an unmerged cluster display a similar type of mass segregation. Additionally, we discuss a variation of the minimum spanning tree mass-segregation technique introduced by Allison et al.
INAF1, University of Toulouse2, Max Planck Society3, Tokai University4, University of Chicago5, University of Padua6, ETH Zurich7, Space Telescope Science Institute8, Lawrence Berkeley National Laboratory9, Institut d'Astrophysique de Paris10, Columbia University11, California Institute of Technology12
TL;DR: In this article, the authors used the first ~10000 galaxies of the zCOSMOS-bright survey and its group catalogue to study the complex interplay between group environment and galaxy properties.
Abstract: We took advantage of the wealth of information provided by the first ~10000 galaxies of the zCOSMOS-bright survey and its group catalogue to study the complex interplay between group environment and galaxy properties. The classical indicator F_blue (fraction of blue galaxies) proved to be a simple but powerful diagnostic tool. We studied its variation for different luminosity and mass selected galaxy samples. Using rest-frame B-band selected samples, the groups galaxy population exhibits significant blueing as redshift increases, but maintains a lower F_blue with respect both to the global and the isolated galaxy population. However moving to mass selected samples it becomes apparent that such differences are largely due to the biased view imposed by the B-band luminosity selection, being driven by the population of lower mass, bright blue galaxies for which we miss the redder, equally low mass, counterparts. By focusing the analysis on narrow mass bins such that mass segregation becomes negligible we find that only for the lowest mass bin explored (logMass = 10.8 are already in place at z ~ 1 and do not exhibit any strong environmental dependence, possibly originating from so-called 'nature'/internal mechanisms. In contrast, for lower galaxy masses and redshifts lower than z ~ 1, we observe the emergence in groups of a population of 'nurture' red galaxies: slightly deviating from the trend of the downsizing scenario followed by the global galaxy population, and more so with cosmic time. These galaxies exhibit signatures of group-related secular physical mechanisms directly influencing galaxy evolution.
TL;DR: In this article, the dynamical evolution of embedded stellar clusters from the protocluster stage, through the embedded star-forming phase, and out to ages of 10 Myr was investigated.
Abstract: This paper investigates the dynamical evolution of embedded stellar clusters from the protocluster stage, through the embedded star-forming phase, and out to ages of 10 Myr—after the gas has been removed from the cluster. The relevant dynamical properties of young stellar clusters are explored over a wide range of possible star formation environments using N-body simulations. Many realizations of equivalent initial conditions are used to produce robust statistical descriptions of cluster evolution including the cluster bound fraction, radial probability distributions, as well as the distributions of close encounter distances and velocities. These cluster properties are presented as a function of parameters describing the initial configuration of the cluster, including the initial cluster membership N, initial stellar velocities, cluster radii, star formation efficiency, embedding gas dispersal time, and the degree of primordial mass segregation. The results of this parameter space survey, which includes ~25,000 simulations, provide a statistical description of cluster evolution as a function of the initial conditions. We also present a compilation of the FUV radiation fields provided by these same cluster environments. The output distributions from this study can be combined with other calculations, such as disk photoevaporation models and planetary scattering cross sections, to ascertain the effects of the cluster environment on the processes involved in planet formation.
TL;DR: For example, this article showed that for reasonable ages and cluster sizes, the merger of sub-clusters can indeed lead to compact configurations of the most massive stars, a signal seen both in Nature and in large-scale hydrodynamic simulations of star formation from collapsing molecular clouds, and that sub-virial initial conditions can make an unmerged cluster display a similar type of mass segregation.
Abstract: The nearest site of massive star formation in Orion is dominated by the Trapezium subsystem, with its four OB stars and numerous companions. The question of how these stars came to be in such close proximity has implications for our understanding of massive star formation and early cluster evolution. A promising route toward rapid mass segregation was proposed by McMillan et al. (2007), who showed that the merger product of faster-evolving sub clusters can inherit their apparent dynamical age from their progenitors. In this paper we briefly consider this process at a size and time scale more suited for local and perhaps more typical star formation, with stellar numbers from the hundreds to thousands. We find that for reasonable ages and cluster sizes, the merger of sub-clusters can indeed lead to compact configurations of the most massive stars, a signal seen both in Nature and in large-scale hydrodynamic simulations of star formation from collapsing molecular clouds, and that sub-virial initial conditions can make an un-merged cluster display a similar type of mass segregation. Additionally, we discuss a variation of the minimum spanning tree mass-segregation technique introduced by Allison et al. (2009).
TL;DR: In this article, the authors present a clear $N$-body realization of the strong mass segregation solution for the stellar distribution around a massive black hole and show that quasi-steady, mass segregated, stellar cusps may be common around MBHs in this mass range.
Abstract: We present, for the first time, a clear $N$-body realization of the {\it strong mass segregation} solution for the stellar distribution around a massive black hole. We compare our $N$-body results with those obtained by solving the orbit-averaged Fokker-Planck (FP) equation in energy space. The $N$-body segregation is slightly stronger than in the FP solution, but both confirm the {\it robustness} of the regime of strong segregation when the number fraction of heavy stars is a (realistically) small fraction of the total population. In view of recent observations revealing a dearth of giant stars in the sub-parsec region of the Milky Way, we show that the time scales associated with cusp re-growth are not longer than $(0.1-0.25) \times T_{rlx}(r_h)$. These time scales are shorter than a Hubble time for black holes masses $\mbul \lesssim 4 \times 10^6 M_\odot$ and we conclude that quasi-steady, mass segregated, stellar cusps may be common around MBHs in this mass range. Since EMRI rates scale as $\mbul^{-\alpha}$, with $\alpha \in [1\4,1]$, a good fraction of these events should originate from strongly segregated stellar cusps.
TL;DR: In this article, the authors analyzed the distribution of stars of arbitrary mass function ξ(m) around a massive black hole (MBH) and showed that unless ξ is strongly dominated by light stars, the steady state distribution function approaches a power law in specific energy x ≡ −E/mσ2 < x max with index p = m/4M 0, where E is the energy, σ is the typical velocity dispersion of unbound stars, and M 0 is the mass averaged over mξxp max.
Abstract: We analyze the distribution of stars of arbitrary mass function ξ(m) around a massive black hole (MBH). Unless ξ is strongly dominated by light stars, the steady-state distribution function approaches a power law in specific energy x ≡ –E/mσ2 < x max with index p = m/4M 0, where E is the energy, σ is the typical velocity dispersion of unbound stars, and M 0 is the mass averaged over mξxp max. For light-dominated ξ, p can grow as large as 3/2—much steeper than previously thought. A simple prescription for the stellar density profile around MBHs is provided. We illustrate our results by applying them to stars around the MBH in the Milky Way.
TL;DR: In this paper, the authors investigated the spatial distribution and clustering behavior of the pre-main-sequence (PMS) stellar population in the Small Magellanic Cloud, using data obtained with the Advanced Camera for Surveys.
Abstract: We present evidence that the star-forming region NGC 346/N66 in the Small Magellanic Cloud is the product of hierarchical star formation, probably from more than one star formation event. We investigate the spatial distribution and clustering behavior of the pre-main-sequence (PMS) stellar population in the region, using data obtained with Hubble Space Telescope's Advanced Camera for Surveys. By applying the nearest neighbor and minimum spanning tree methods on the rich sample of PMS stars previously discovered in the region, we identify 10 individual PMS clusters in the area and quantify their structures. The clusters show a wide range of morphologies from hierarchical multipeak configurations to centrally condensed clusters. However, only about 40% of the PMS stars belong to the identified clusters. The central association NGC 346 is identified as the largest stellar concentration, which cannot be resolved into subclusters. Several PMS clusters are aligned along filaments of higher stellar density pointing away from the central part of the region. The PMS density peaks in the association coincide with the peaks of [O III] and 8 μm emission. While more massive stars seem to be concentrated in the central association when considering the entire area, we find no evidence for mass segregation within the system itself.
TL;DR: In this article, the dynamical evolution of embedded stellar clusters from the protocluster stage, through the embedded star-forming phase, and out to ages of 10 Myr was investigated.
Abstract: This paper investigates the dynamical evolution of embedded stellar clusters from the protocluster stage, through the embedded star-forming phase, and out to ages of 10 Myr -- after the gas has been removed from the cluster. The relevant dynamical properties of young stellar clusters are explored over a wide range of possible star formation environments using N-body simulations. Many realizations of equivalent initial conditions are used to produce robust statistical descriptions of cluster evolution including the cluster bound fraction, radial probability distributions, as well as the distributions of close encounter distances and velocities. These cluster properties are presented as a function of parameters describing the initial configuration of the cluster, including the initial cluster membership N, initial stellar velocities, cluster radii, star formation efficiency, embedding gas dispersal time, and the degree of primordial mass segregation. The results of this parameter space survey, which includes about 25,000 simulations, provide a statistical description of cluster evolution as a function of the initial conditions. We also present a compilation of the FUV radiation fields provided by these same cluster environments. The output distributions from this study can be combined with other calculations, such as disk photoevaporation models and planetary scattering cross sections, to ascertain the effects of the cluster environment on the processes involved in planet formation.
TL;DR: In this paper, the authors presented 5201 radial-velocity measurements of 1144 stars, as part of an ongoing study of the young (150 Myr) open cluster M35 (NGC 2168).
Abstract: We present 5201 radial-velocity measurements of 1144 stars, as part of an ongoing study of the young (150 Myr) open cluster M35 (NGC 2168). We have observed M35 since 1997, using the Hydra Multi-Object Spectrograph on the WIYN 3.5m telescope. Our stellar sample covers main-sequence stars over a magnitude range of 13.0 =3 measurements, we derive radial-velocity membership probabilities and identify radial-velocity variables, finding 360 cluster members, 55 of which show significant radial- velocity variability. Using these cluster members, we construct a color-magnitude diagram for our stellar sample cleaned of field star contamination. We also compare the spatial distribution of the single and binary cluster members, finding no evidence for mass segregation in our stellar sample. Accounting for measurement precision, we place an upper limit on the radial-velocity dispersion of the cluster of 0.81 +/- 0.08 km/s. After correcting for undetected binaries, we derive a true radial-velocity dispersion of 0.65 +/- 0.10 km/s.
TL;DR: In this article, the authors show that star formation scenarios predicting general primordial mass segregation are inconsistent with observed segregation levels, and discuss the implications of their results for theories of star formation.
Abstract: Mass segregation is observed in many star clusters, including several that are less than a few Myr old. Time-scale arguments are frequently used to argue that these clusters must be displaying primordial segregation, because they are too young to be dynamically relaxed. Looking at this argument from the other side, the youth of these clusters and the limited time available to mix spatially distinct populations of stars can provide constraints on the amount of initial segregation that is consistent with current observations. We present n-body experiments testing this idea, and discuss the implications of our results for theories of star formation. For system ages less than a few crossing times, we show that star formation scenarios predicting general primordial mass segregation are inconsistent with observed segregation levels.
TL;DR: In this paper, the authors measured the radial profile of mass segregation using main-sequence stars for the globular cluster NGC 2298 from resolved source photometry based on HST/ACS data.
Abstract: Theoretical investigations have suggested the presence of intermediate mass black holes (IMBHs, with masses in the 100-10000 Mrange) in the cores of some globular clusters (GCs). In this paper, we present the first application of a new technique to determine the presence or absence of a central IMBH in globular clusters that have reached energy equipartition via two-body relaxation. The method is based on the measurement of the radial profile for the average mass of stars in the system, using the fact that a quenching of mass segregation is expected when an IMBH is present. Here, we measure the radial profile of mass segregation using main-sequence stars for the globular cluster NGC 2298 from resolved source photometry based on Hubble Space Telescope (HST/ACS) data. NGC 2298 is one of the smallest galactic globular clusters, thus not only it is dynamically relaxed but also a single ACS field of view extends to about twice its half-light radius, providing optimal radial coverage. The observations are compared to expectations from direct N-body simulations of the dynamics of star clusters with and without an IMBH. The mass segregation profile for NGC 2298 is quantitatively matched to that inferred from simulations without a central massive object over all the radial range probed by the observations, that is from the center to about two half-mass radii. Profiles from simulations containing an IMBH more massive than ≈300-500 M� (depending on the assumed total mass of NGC 2298) are instead inconsistent with the data at about 3σ confidence, irrespective of the initial mass function and binary fraction chosen for these runs. Our finding is consistent with the currently favored formation scenarios for IMBHs in GCs, which are not likely to apply to NGC 2298 due to its modest total mass. While providing a null result in the quest of detecting a central black hole in globular clusters, the data-model comparison carried out here demonstrates the feasibility of the method which can also be applied to other globular clusters with resolved photometry in their cores.
TL;DR: In this article, the authors analyzed the distribution of stars of arbitrary mass function xi(m) around a massive black hole (MBH) and showed that unless xi is strongly dominated by light stars, the steady-state distribution function approaches a power-law in specific energy x=-E/(m*sigma^2)
Abstract: We analyze the distribution of stars of arbitrary mass function xi(m) around a massive black hole (MBH). Unless xi is strongly dominated by light stars, the steady-state distribution function approaches a power-law in specific energy x=-E/(m*sigma^2)
TL;DR: In this article, structural parameters for ~2000 globular clusters in the giant Virgo elliptical Messier 87 (M87) using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC.
Abstract: We derive structural parameters for ~2000 globular clusters in the giant Virgo elliptical Messier 87 (M87) using extremely deep Hubble Space Telescope images in F606W (V) and F814W (I) taken with the ACS/WFC. The cluster scale sizes (half-light radii rh ) and ellipticities are determined from point-spread-function -convolved King-model profile fitting. We find that the rh distribution closely resembles the inner Milky Way clusters, peaking at rh 2.5 pc and with virtually no clusters more compact than rh 1 pc. The metal-poor clusters have on average an rh 24% larger than the metal-rich ones. The cluster scale size shows a gradual and noticeable increase with galactocentric distance. Clusters are very slightly larger in the bluer waveband V, a possible hint that we may be beginning to see the effects of mass segregation within the clusters. We also derived a color magnitude diagram for the M87 globular cluster system which shows a striking bimodal distribution.
TL;DR: In this paper, a color-magnitude diagram for the bulge cluster NGC 6366 was constructed using observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs).
Abstract: We have used observations obtained as part of the Hubble Space Telescope/ACS Survey of Galactic globular clusters (GCs) to construct a color-magnitude diagram for the bulge cluster, NGC 6366 The luminosity function derived from those data extends to M F606W ~ 9, or masses of ~03 M ☉ Unlike most GCs, the mass function peaks near the main-sequence turnoff with significantly fewer low-mass stars even after correction for completeness and mass segregation Using a multimass King model, we extrapolate the global cluster behavior and find the global mass function to be poorly matched by a power law, with a particular deficit of stars with masses between 05 and 07 M ☉ We briefly discuss this interesting anomaly within the context of tidal stripping