Showing papers on "Mass segregation published in 2012"

Importance of the initial conditions for star formation - III. Statistical properties of embedded protostellar clusters

Abstract: We investigate the formation of protostellar clusters during the collapse of dense molecular cloud cores with a focus on the evolution of potential and kinetic energy, the degree of substructure and the early phase of mass segregation. Our study is based on a series of hydrodynamic simulations of dense cores, where we vary the initial density profile and the initial turbulent velocity. In the three-dimensional adaptive mesh refinement simulations, we follow the dynamical formation of filaments and protostars until a star formation efficiency of 20 per cent. Despite the different initial configurations, the global ensemble of all protostars in a setup shows a similar energy evolution and forms sub-virial clusters with an energy ratio Ekin/|Epot|∼ 0.2. Concentrating on the innermost central region, the clusters show a roughly virialized energy balance. However, the region of virial balance only covers the innermost ∼10–30 per cent of all the protostars. In all simulations with multiple protostars, the total kinetic energy of the protostars is higher than the kinetic energy of the gas cloud, although the protostars only contain 20 per cent of the total mass. The clusters vary significantly in size, mass and number of protostars, and show different degrees of substructure and mass segregation. Flat density profiles and compressive turbulent modes produce more subclusters than centrally concentrated profiles and solenoidal turbulence. We find that dynamical relaxation and hence dynamical mass segregation is very efficient in all cases from the very beginning of the nascent cluster, i.e. during a phase when protostars constantly form and accrete.

The evolution of embedded star clusters

Abstract: We study the evolution of embedded clusters. The equations of motion of the stars in the cluster are solved by direct N-body integration while taking the effects of stellar evolution and the hydrodynamics of the natal gas content into account. The gravity of the stars and the surrounding gas are coupled self-consistently to allow the realistic dynamical evolution of the cluster. While the equations of motion are solved, a stellar evolution code keeps track of the changes in stellar mass, luminosity and radius. The gas liberated by the stellar winds and supernovae deposits mass and energy into the gas reservoir in which the cluster is embedded. We examine cluster models with 1000 stars, but we varied the star formation efficiency (between 0.05 and 0.5), cluster radius (0.1–1.0 pc), the degree of virial support of the initial population of stars (0–100 per cent) and the strength of the feedback. We find that an initial star fraction M★/Mtot > 0.05 is necessary for cluster survival. Survival is more likely if gas is not blown out violently by a supernova and if the cluster has time to approach virial equilibrium during outgassing. While the cluster is embedded, dynamical friction drives early and efficient mass segregation in the cluster. Stars of m≳ 2 M⊙ are preferentially retained, at the cost of the loss of less massive stars. We conclude that the degree of mass segregation in open clusters such as the Pleiades is not the result of secular evolution but a remnant of its embedded stage.

Limits on intermediate-mass black holes in six Galactic globular clusters with integral-field spectroscopy

Abstract: The formation of supermassive black holes at high redshift still remains a puzzle to astronomers. Their growth becomes reasonable only when starting from a massive seed black hole with mass of the order of 10^2 - 10^5 M_SUN. Intermediate-mass black holes (IMBHs) are therefore an important field of research. Especially the possibility of finding them in the centers of globular clusters has recently drawn attention. The search for IMBHs in the centers of globular clusters could therefore shed light on the process of black-hole formation and cluster evolution. We are investigating six galactic globular clusters for the presence of an IMBH at their centers. Based on their kinematic and photometric properties, we selected the globular clusters NGC 1851, NGC 1904 (M79), NGC 5694, NGC 5824, NGC 6093 (M80) and NGC 6266 (M62). We use integral field spectroscopy in order to obtain the central velocity-dispersion profile of each cluster. We compute the cluster photometric center and the surface brightness profile using HST data. After combining these datasets we compare them to analytic Jeans models. We use varying M/L_V profiles for clusters with enough data points in order to reproduce their kinematic profiles in an optimal way. Finally, we vary the mass of the central black hole and test whether the cluster is better fitted with or without an IMBH. We present the statistical significance, including upper limits, of the black-hole mass for each cluster. NGC 1904 and NGC 6266 provide the highest significance for a black hole. Jeans models in combination with a M/L_V profile obtained from N-body simulations (in the case of NGC 6266) predict a central black hole of M_BH = (3 +- 1) x 10^3 M_SUN for NGC 1904 and M_BH = (2 +- 1) x 10^3 M_SUN for NGC 6266. Furthermore, we discuss the possible influence of dark remnants and mass segregation at the center of the cluster on the detection of an IMBH.

Long-term evolution of massive black hole binaries - IV. Mergers of galaxies with collisionally relaxed nuclei

Abstract: We simulate mergers between galaxies containing collisionally relaxed nuclei around massive black holes (MBHs). Our galaxies contain four mass groups, representative of old stellar populations; a primary goal is to understand the distribution of stellar-mass black holes (BHs) after the merger. Mergers are followed using direct-summation N-body simulations, assuming a mass ratio of 1:3 and two different orbits. Evolution of the binary MBH is followed until its separation has shrunk by a factor of 20 below the hard-binary separation. During the galaxy merger, large cores are carved out in the stellar distribution, with radii several times the influence radius of the massive binary. Much of the pre-existing mass segregation is erased during this phase. We follow the evolution of the merged galaxies for approximately three central relaxation times after coalescence of the massive binary; both standard and top-heavy mass functions are considered. The cores that were formed in the stellar distribution persist, and the distribution of the stellar-mass BHs evolves against this essentially fixed background. Even after one central relaxation time, these models look very different from the relaxed, multi-mass models that are often assumed to describe the distribution of stars and stellar remnants near a massive BH. While the stellar BHs do form a cusp on roughly a relaxation timescale, the BH density can be much smaller than in those models. We discuss the implications of our results for the extreme-mass-ratio inspiral problem and for the existence of Bahcall-Wolf cusps.

Characterizing the dynamical state of star clusters from snapshots of their spatial distributions

Abstract: We determine the distribution of stellar surface densities, Σ, from models of static and dynamically evolving star clusters with different morphologies, including both radially smooth and substructured clusters. We find that the Σ distribution is degenerate, in the sense that many different cluster morphologies (smooth or substructured) produce similar cumulative distributions. However, when used in tandem with a measure of structure, such as the Q-parameter, the current spatial and dynamical state of a star cluster can be inferred. The effect of cluster dynamics on the Σ distribution and the Q-parameter is investigated using N-body simulations and we find that, depending on the assumed initial conditions, the Σ distribution can rapidly evolve from high to low densities in less than 5 Myr. This suggests that the Σ distribution can only be used to assess the current density of a star-forming region, and provides little information on its initial density. However, if the Σ distribution is used together with the Q-parameter, then information on the amount of substructure can be used as a proxy to infer the amount of dynamical evolution that has taken place. Substructure is erased quickly through dynamics, which can disrupt binary star systems and planets, as well as facilitate dynamical mass segregation. Therefore, dynamical processing in young star-forming regions could still be significant, even without currently observed high densities.

Mergers and ejections of black holes in globular clusters

Abstract: We report on results of fully consistent N-body simulations of globular cluster models with N= 100 000 members containing neutron stars and black holes (BHs). Using the improved ‘algorithmic regularization’ method of Hellstrom & Mikkola for compact subsystems, the new code nbody7 enables for the first time general relativistic coalescence to be achieved for post-Newtonian terms and realistic parameters. Following an early stage of mass segregation, a few BHs form a small dense core which usually leads to the formation of one dominant binary. The subsequent evolution by dynamical shrinkage involves the competing processes of ejection and mergers by radiation energy loss. Unless the binary is ejected, long-lived triple systems often exhibit Kozai cycles with extremely high inner eccentricity (e > 0.999) which may terminate in coalescence at a few Schwarzschild radii. A characteristic feature is that ordinary stars as well as BHs and even BH binaries are ejected with high velocities. On the basis of the models studied so far, the results suggest a limited growth of a few remaining stellar mass BHs in globular clusters.

Luminosity Functions of Spitzer Identified Protostars in Nine Nearby Molecular Clouds

Abstract: We identify protostars in Spitzer surveys of nine star-forming molecular clouds within 1 kpc: Serpens, Perseus, Ophiuchus, Chamaeleon, Lupus, Taurus, Orion, Cep OB3, and Mon R2, which combined host over 700 protostar candidates. Our diverse cloud sample allows us to compare protostar luminosity functions in these varied environments. We combine photometry from 2MASS J, H, and Ks bands and Spitzer IRAC and MIPS 24 micron bands to create 1 - 24 micron spectral energy distributions (SEDs). Using protostars from the c2d survey with well-determined bolometric luminosities (Lbol), we derive a relationship between Lbol, L_MIR (integrated from 1 - 24 microns), and SED slope. Estimations of Lbol for protostar candidates are combined to create luminosity functions for each cloud. Contamination due to edge-on disks, reddened Class II sources, and galaxies is estimated and removed from the luminosity functions. We find that luminosity functions for high mass star forming clouds peak near 1 Lsun and show a tail extending toward luminosities above 100 Lsun. The luminosity functions of the low mass star forming clouds do not exhibit a common peak, however the combined luminosity function of these regions peaks below 1 Lsun. Finally, we examine the luminosity functions as a function of the local surface density of YSOs. In the Orion molecular cloud, we find a significant difference between the luminosity functions of protostars in regions of high and low stellar density, the former of which is biased toward more luminous sources. This may be the result of primordial mass segregation, although this interpretation is not unique. We compare our luminosity functions to those predicted by models and find that our observed luminosity functions are best matched by models which invoke competitive accretion, although we do not find strong agreement of the high mass star forming clouds with any of the models.

A Comparison between the Stellar and Dynamical Masses of Six Globular Clusters

Abstract: We present the results of a comprehensive analysis of the structure and kinematics of six Galactic globular clusters. By comparing the results of the most extensive photometric and kinematical surveys available to date with suitable dynamical models, we determine the stellar and dynamical masses of these stellar systems taking into account the effect of mass segregation, anisotropy, and unresolved binaries. We show that the stellar masses of these clusters are on average smaller than those predicted by canonical integrated stellar evolution models because of the shallower slopes of their mass functions. The derived stellar masses are found to be also systematically smaller than the dynamical masses by ~40%, although the presence of systematics affecting our estimates cannot be excluded. If confirmed, this evidence can be linked to an increased fraction of retained dark remnants or to the presence of a modest amount of dark matter.

Is the massive young cluster Westerlund I bound

Abstract: Context. Westerlund I is the richest young cluster currently known in our Galaxy, making it one of the most massive clusters for which we can resolve the individual stars even in the crowded centre. This makes it an ideal target to assess whether massive clusters formed currently will remain bound or will disperse and contribute significantly to the stellar field population.Aims. We measure the radial velocity dispersion of Westerlund I to explore whether the cluster is either currently in virial equilibrium, in the process of collapse, or expanding and dispersing into the field.Methods. We obtained MIKE/Magellan high resolution optical spectra of 22 post main-sequence stars in Westerlund I for 2 or 3 epochs with a maximum baseline of about one year. Radial velocities variations between these spectra are measured by means of cross-correlation.Results. We calculate the velocity dispersion from the cross-correlation of five yellow hypergiants and one luminous blue variable, that show little radial velocity variations between epochs and have many spectral features in common. After taking into account the effect of small number statistics and undetected binaries, we estimate the velocity dispersion for the massive stars in Westerlund I to be 2.1-2.1 +3.3 km s-1 . For several different assumptions concerning possible mass segregation and the elongation of the cluster, we find that Westerlund I is subvirial at the 90% confidence level. Conclusions. We can rule out that the cluster is significantly supervirial at the 97% confidence level, indicating that Westerlund I is currently bound. This implies that Westerlund I has survived past the point where any gas expulsion has taken place and is expected to survive for billions of years.

The formation of young dense star clusters through mergers

Abstract: Young star clusters such as NGC 3603 and Westerlund 1 and 2 in the Milky Way and R136 in the Large Magellanic Cloud are dynamically more evolved than expected based on their current relaxation times. In particular, the combination of a high degree of mass segregation, a relatively low central density, and the large number of massive runaway stars in their vicinity are hard to explain with the monolithic formation of these clusters. Young star clusters can achieve such a mature dynamical state if they formed through the mergers of a number of less massive clusters. The shorter relaxation times of less massive clusters cause them to dynamically evolve further by the time they merge, and the merger product preserves the memory of the dynamical evolution of its constituent clusters. With a series of N-body simulations, we study the dynamical evolution of single massive clusters and those that are assembled through merging smaller clusters together. We find that the formation of massive star clusters through the mergers of smaller clusters can reproduce the currently observed spatial distribution of massive stars, the density, and the characteristics (number and mass distribution) of the stars ejected as runaways from young dense clusters. Wemore » therefore conclude that these clusters and possibly other young massive star clusters formed through the mergers of smaller clusters.« less

The formation of young dense star clusters through mergers

Abstract: Young star clusters like R136 in the Large Magellanic Cloud and NGC 3603, Westerlund 1, and 2 in the Milky Way are dynamically more evolved than expected based on their current relaxation times. In particular, the combination of a high degree of mass segregation, a relatively low central density, and the large number of massive runaway stars in their vicinity are hard to explain with the monolithic formation of these clusters. Young star clusters can achieve such a mature dynamical state if they formed through the mergers of a number of less massive clusters. The shorter relaxation times of less massive clusters cause them to dynamically evolve further by the time they merge, and the merger product preserves the memory of the dynamical evolution of its constituent clusters. With a series of $N$-body simulations, we study the dynamical evolution of single massive clusters and those that are assembled through merging smaller clusters together. We find that the formation of massive star clusters through the mergers of smaller clusters can reproduce the currently observed spatial distribution of massive stars, the density, and the characteristics (number and mass distribution) of the stars ejected as runaways from young dense clusters. We therefore conclude that these clusters and possibly other young massive star clusters formed through the mergers of smaller clusters.

Central kinematics of the globular cluster NGC 2808: upper limit on the mass of an intermediate-mass black hole ,

Abstract: Context. Globular clusters are an excellent laboratory for stellar population and dynamical research. Recent studies have shown that these stellar systems are not as simple as previously assumed. With multiple stellar populations as well as outer rotation and mass segregation they turn out to exhibit high complexity. This includes intermediate-mass black holes (IMBHs) which are proposed to sit at the centers of some massive globular clusters. Today's high angular resolution ground based spectrographs allow velocity-dispersion measurements at a spatial resolution comparable to the radius of influence for plausible IMBH masses, and to detect changes in the inner velocity-dispersion profile. Together with high quality photometric data from HST, it is possible to constrain black-hole masses by their kinematic signatures. Aims. We determine the central velocity-dispersion profile of the globular cluster NGC 2808 using VLT/FLAMES spectroscopy. In combination with HST/ACS data our goal is to probe whether this massive cluster hosts an IMBH at its center and constrain the cluster mass to light ratio as well as its total mass. Methods. We derive a velocity-dispersion profile from integral field spectroscopy in the center and Fabry Perot data for larger radii. High resolution HST data are used to obtain the surface brightness profile. Together, these data sets are compared to dynamical models with varying parameters such as mass to light ratio profiles and black-hole masses. Results. Using analytical Jeans models in combination with variable M/L V profiles from N-body simulations we find that the best fit model is a no black hole solution. After applying various Monte Carlo simulations to estimate the uncertainties, we derive an upper limit of the back hole mass of M BH < 1 × 10 4 M · (with 95% confidence limits) and a global mass-to-light ratio of M/L V = (2.1 ± 0.2) M ·/L ·.

On the Origin of Mass Segregation in NGC 3603

Abstract: We present deep Hubble Space Telescope/Wide Field and Planetary Camera 2 photometry of the young HD 97950 star cluster in the giant H {\sc ii} region NGC 3603. The data were obtained in 1997 and 2007 permitting us to derive membership based on proper motions of the stars. Our data are consistent with an age of 1 Myr for the HD 97950 cluster. A possible age spread, if present in the cluster, appears to be small. The global slope of the incompleteness-corrected mass function for member stars within 60$"$ is $\rm \Gamma=-0.88\pm0.15$, which is flatter than the value of a Salpeter slope of -1.35. The radially varying mass function shows pronounced mass segregation ranging from slopes of $-0.26 \pm 0.32$ in the inner $5"$ to $-0.94\pm 0.36$ in the outermost annulus ($40"$ -- $60"$). Stars more massive than 50 M$_{\odot}$ are found only in the cluster center. The $\Lambda$ minimum spanning tree technique confirms significant mass segregation down to 30 M$_{\odot}$. The dependence of $\Lambda$ on mass, i.e., that high-mass stars are more segregated than low mass stars, and the (weak) dependence of the velocity dispersion on stellar mass might imply that the mass segregation is dynamical in origin. While primordial segregation cannot be excluded, the properties of the mass segregation indicate that dynamical mass segregation may have been the dominant process for segregation of high-mass stars.

A star disrupted by a stellar black hole as the origin of the cloud falling toward the galactic center

Abstract: We propose that the cloud moving on a highly eccentric orbit near the central black hole in our Galaxy, reported by Gillessen et al., is formed by a photoevaporation wind originating in a disk around a star that is tidally perturbed and shocked at every peribothron passage. The disk is proposed to have formed when a stellar black hole flew by the star, tidally disrupted its envelope, and placed the star on its present orbit with some of the tidal debris forming a disk. A disrupting encounter at the location of the observed cloud is most likely to be caused by a stellar black hole because of the expected dynamical mass segregation; the rate of these disk-forming encounters may be as high as ~10–6 per year. The star should also be spun up by the encounter, so the disk may subsequently expand by absorbing angular momentum from the star. Once the disk expands up to the tidal truncation radius, the tidal perturbation of the outer disk edge at every peribothron may place gas streams on larger orbits, which can give rise to a photoevaporation wind that forms the cloud at every orbit. This model predicts that, after the cloud is disrupted at the next peribothron passage in 2013, a smaller unresolved cloud will gradually grow around the star on the same present orbit. An increased infrared luminosity from the disk may also be detectable when the peribothron is reached. We also note that this model revives the encounter theory for planet formation.

The velocity dispersion and mass function of the outer halo globular cluster Palomar 4

Abstract: We obtained precise line-of-sight radial velocities of 23 member stars of the remote halo globular cluster Palomar 4 (Pal 4) using the High Resolution Echelle Spectrograph (HIRES) at the Keck I telescope. We also measured the mass function of the cluster down to a limiting magnitude of V 28 mag using archival HST /WFPC2 imaging. We derived the cluster’s surface brightness prole based on the WFPC2 data and on broad-band imaging with the Low-Resolution Imaging Spectrometer (LRIS) at the Keck II telescope. We nd a mean cluster velocity of 72 :55 0:22 km s 1 and a velocity dispersion of 0:87 0:18 km s 1 . The global mass function of the cluster, in the mass range 0:55 6 M 6 0:85 M , is shallower than a Kroupa mass function and the cluster is signicantly depleted in low-mass stars in its center compared to its outskirts. Since the relaxation time of Pal 4 is of the order of a Hubble time, this points to primordial mass segregation in this cluster. Extrapolating the measured mass function towards lower-mass stars and including the contribution of compact remnants, we derive a total cluster mass of 29,800 M . For this mass, the measured velocity dispersion is consistent with the expectations of Newtonian dynamics and below the prediction of MOND. Pal 4 adds to the growing body of evidence that the dynamics of star clusters in the outer Galactic halo can hardly be explained by MOND.

Observations of dark and luminous matter: the radial distribution of satellite galaxies around massive red galaxies

Abstract: We study the projected radial distribution of satellite galaxies around more than 28,000 luminous red galaxies (LRGs) at 0.28 25 kpc whereas baryons account for more than 50% of the mass at smaller radii. We calculate the total dark-to-baryonic mass ratio and show that it is consistent with measurements from weak lensing for environments dominated by massive early-type galaxies. Finally, we divide the satellite galaxies in our sample into three luminosity bins and show that the satellite light profiles of all brightness levels are consistent with each other outside of roughly 25 kpc. At smaller radii we find evidence for a mild mass segregation with an increasing fraction of bright satellites close to the central LRG.

WIYN Open Cluster Study : Wide-Field CCD Photometry of the Old Open Cluster NGC 6819

Abstract: We present a comprehensive photometric study of the old open cluster, NGC 6819 using 1x1 degree field VI MOSAIC CCD imaging taken with the WIYN 0.9m telescope. The resultant color-magnitude diagram (CMD) shows a well developed main sequence (MS) extending from V~14.5 mag down to our photometric limit of V~21 mag. Fitting theoretical isochrones with adopted values of the reddening and metallicity (E(B-V)=0.14, [Fe/H]=+0.09 dex) to the observed CMD yields a distance modulus of (m-M)_{0}=11.93+/-0.10 and an age of ~2.6 Gyr for NGC 6819. Our wide-field imaging reveals that NGC 6819 is larger in areal extent (R=13') than previously thought. The wide-field also benefits our estimate of the degree of field star contamination, and ultimately yields improved measurements of the structural parameters (r_c=2.80', r_t=38.2', and r_h=7') and tidal mass of the cluster (M_{tid}=3542.4 M(sun)). The flattened luminosity and mass functions indicate that NGC 6819 has experienced mass segregation as a result of its dynamical evolution. Our variability study of the cluster blue straggler star (BSS) population using the Welch-Stetson variability index (I_{WS}) has revealed a number of variable BSS candidates.

N-body models of globular clusters: metallicities, half-light radii and mass-to-light ratios

Abstract: Size differences of ≈20 per cent between red (metal-rich) and blue (metal-poor) subpopulations of globular clusters have been observed, generating an ongoing debate as to whether these originate from projection effects or the difference in metallicity. We present direct N-body simulations of metal-rich and metal-poor stellar populations evolved to study the effects of metallicity on cluster evolution. The models start with N = 100 000 stars and include primordial binaries. We also take metallicity-dependent stellar evolution and an external tidal field into account. We find no significant difference for the half-mass radii of those models, indicating that the clusters are structurally similar. However, utilizing observational tools to fit half-light (or effective) radii confirms that metallicity effects related to stellar evolution combined with dynamical effects such as mass segregation produce an apparent size difference of 17 per cent on average. The metallicity effect on the overall cluster luminosity also leads to higher mass-to-light ratios for metal-rich clusters.

A direct N-body model of core-collapse and core oscillations

Abstract: We report on the results of a direct N-body simulation of a star cluster that started with N = 200000, comprising 195000 single stars and 5000 primordial binaries. The code used for the simulation includes stellar evolution, binary evolution, an external tidal field and the effects of two-body relaxation. The model cluster is evolved to 12Gyr, losing more than 80% of its stars in the process. It reaches the end of the main core-collapse phase at 10.5Gyr and experiences core oscillations from that point onwards – direct numerical confirmation of this phenomenon. However, we find that after a further 1Gyr the core oscillations are halted by the ejection of a massive binary comprised of two black holes from the core, producing a core that shows no signature of the prior core-collapse. We also show that the results of previous studies with N ranging from 500 to 100000 scale well to this new model with larger N. In particular, the timescale to core-collapse (in units of the relaxation timescale), mass segregation, velocity dispersion, and the energies of the binary population all show similar behaviour at different N.

Central kinematics of the globular cluster NGC 2808: Upper limit on the mass of an intermediate-mass black hole

Abstract: Globular clusters are an excellent laboratory for stellar population and dynamical research. Recent studies have shown that these stellar systems are not as simple as previously assumed. With multiple stellar populations as well as outer rotation and mass segregation they turn out to exhibit high complexity. This includes intermediate-mass black holes which are proposed to sit at the centers of some massive globular clusters. Today's high angular resolution ground based spectrographs allow velocity-dispersion measurements at a spatial resolution comparable to the radius of influence for plausible IMBH masses, and to detect changes in the inner velocity-dispersion profile. Together with high quality photometric data from HST, it is possible to constrain black-hole masses by their kinematic signatures. We determine the central velocity-dispersion profile of the globular cluster NGC 2808 using VLT/FLAMES spectroscopy. In combination with HST/ACS data our goal is to probe whether this massive cluster hosts an intermediate-mass black hole at its center and constrain the cluster mass to light ratio as well as its total mass. We derive a velocity-dispersion profile from integral field spectroscopy in the center and Fabry Perot data for larger radii. High resolution HST data are used to obtain the surface brightness profile. Together, these data sets are compared to dynamical models with varying parameters such as mass to light ratio profiles and black-hole masses. Using analytical Jeans models in combination with variable M/L profiles from N-body simulations we find that the best fit model is a no black hole solution. After applying various Monte Carlo simulations to estimate the uncertainties, we derive an upper limit of the back hole mass of M_BH < 1 x 10^4 M_SUN (with 95 % confidence limits) and a global mass-to-light ratio of M/L_V = (2.1 +- 0.2) M_SUN/L_SUN.

Statistics of substructures in dark matter haloes

Abstract: We study the amount and distribution of dark matter substructures within dark matter haloes, using a large set of high-resolution simulations ranging from group-size to cluster-size haloes, and carried out within a cosmological model consistent with Wilkinson Microwave Anisotropy Probe (WMAP) 7-year data. In particular, we study how the measured properties of subhaloes vary as a function of the parent halo mass, the physical properties of the parent halo and redshift. The fraction of halo mass in substructures increases with increasing mass: it is of the order of 5 per cent for haloes with M200∼ 1013 M⊙ and of the order of 10 per cent for the most massive haloes in our sample, with M200∼ 1015 M⊙. There is, however, a very large halo-to-halo scatter that can be explained only in part by a range of halo physical properties, e.g. concentration. At a given halo mass, less concentrated haloes contain significantly larger fractions of mass in substructures because of the reduced strength of tidal disruption. Most of the substructure mass is located at the outskirts of the parent haloes, in relatively few massive subhaloes. This mass segregation appears to become stronger at increasing redshift, and should reflect into a more significant mass segregation of the galaxy population at different cosmic epochs. When haloes are accreted on to larger structures, their mass is significantly reduced by tidal stripping. Haloes that are more massive at the time of accretion (these should host more luminous galaxies) are brought closer to the centre on shorter time-scales by dynamical friction, and therefore suffer a more significant stripping. The halo merger rate depends strongly on the environment with substructure in more massive haloes suffering more important mergers than their counterparts residing in less massive systems. This should translate into a different morphological mix for haloes of different mass.

Formation of massive black holes in dense star clusters. ii. initial mass function and primordial mass segregation

Abstract: A promising mechanism to form intermediate-mass black holes is the runaway merger in dense star clusters, where main-sequence stars collide and form a very massive star (VMS), which then collapses to a black hole (BH) In this paper, we study the effects of primordial mass segregation and the importance of the stellar initial mass function (IMF) on the runaway growth of VMSs using a dynamical Monte Carlo code for N-body systems with N as high as 106 stars Our code now includes an explicit treatment of all stellar collisions We place special emphasis on the possibility of top-heavy IMFs, as observed in some very young massive clusters We find that both primordial mass segregation and the shape of the IMF affect the rate of core collapse of star clusters and thus the time of the runaway When we include primordial mass segregation, we generally see a decrease in core-collapse time (t cc) Although for smaller degrees of primordial mass segregation this decrease in t cc is mostly due to the change in the density profile of the cluster, for highly mass-segregated (primordial) clusters, it is the increase in the average mass in the core which reduces the central relaxation time decreasing t cc The final mass of the VMS formed is always close to ~10–3 of the total cluster mass, in agreement with previous studies and is reminiscent of the observed correlation between the central BH mass and the bulge mass of the galaxies As the degree of primordial mass segregation is increased, the mass of the VMS increases at most by a factor of three Flatter IMFs generally increase the average mass in the whole cluster, which increases t cc For the range of IMFs investigated in this paper, this increase in t cc is to some degree balanced by stellar collisions, which accelerate core collapse Thus, there is no significant change in t cc for the somewhat flatter global IMFs observed in very young massive clusters

N-body models of globular clusters: metallicity, half-light radii and mass-to-light ratios

Abstract: Size differences of approx. 20% between red (metal-rich) and blue (metal-poor) sub-populations of globular clusters have been observed, generating an ongoing debate as to weather these originate from projection effects or the difference in metallicity. We present direct N-body simulations of metal-rich and metal-poor stellar populations evolved to study the effects of metallicity on cluster evolution. The models start with N = 100000 stars and include primordial binaries. We also take metallicity dependent stellar evolution and an external tidal field into account. We find no significant difference for the half-mass radii of those models, indicating that the clusters are structurally similar. However, utilizing observational tools to fit half-light (or effective) radii confirms that metallicity effects related to stellar evolution combined with dynamical effects such as mass segregation produce an apparent size difference of 17% on average. The metallicity effect on the overall cluster luminosity also leads to higher mass-to-light ratios for metal-rich clusters.

The Harlow-Shapley Symposium on Globular Cluster Systems in Galaxies : Proceedings of the 126th Symposium of the International Astronomical Union, Held in Cambridge, Massachusetts, U.S.A., August 25-29, 1986

Abstract: I Review Papers on Harlow Shapley.- Shapley's Debate.- Shapley's Era.- Shapley's Impact.- II Review Papers on Globular Clusters in the Milky Way.- An Overview of the Globular Cluster System of the Galaxy.- Kinematics of the Galactic Globular Cluster System.- Globular Cluster Color-Magnitude Diagrams.- The Overall Abundances of Globular Clusters.- The Chemical Inhomogeneity within Globular Clusters.- Ages of the Galactic Globular Clusters.- Globular Cluster Luminosity Functions.- Globular Clusters and Field Halo Stars.- III: Review Papers on Globular Clusters in Nearby Galaxies.- Old Globular Clusters in the Magellanic Clouds.- Intermediate-Age Magellanic Cloud Globular Clusters.- M 31 Cluster System.- The Clusters of M 33.- The NGC 5128 Cluster System.- Dwarf Spheroidal Galaxies and Globular Clusters.- An Overview of Globular Systems in Distant Galaxies.- The M 87 Globular Cluster System.- IV Review Papers on Evolution of Globular Clusters.- The Evolution of the System of Globular Clusters.- Tidal Heating of Globular Clusters.- Cluster Swapping.- Galaxy Formation and Cluster Formation.- The Origin of Globular Clusters.- Surface Photometry of Globular Clusters.- X-Ray Binaries and Cluster Evolution.- Precollapse Evolution of Globular Clusters.- After Core Collapse, What?.- Disolution of Star Clusters in Galaxies.- V Review Papers on Globular Clusters as Tracers and HST.- Interstellar Matter in Globular Clusters.- Globular Clusters as Tracers of the Galaxy Mass Distribution.- Globular Clusters and Primordial Composition.- Stellar Evolution in Globular Clusters and HST.- Simulations of HST Observations of Globular Clusters.- VI Review Papers Summary.- Globular Cluster Systems in Galaxies: Main Trends and Future Directions.- VII Poster Papers on Harlow Shapley and Globular Clusters in the Milky Way.- Harlow Shapley: A View from the Harvard Archives.- Harlow Shapley and the University of Missouri.- Harlow Shapley and Red Giant Stars.- The Development of a Red-Giant Branch in Low to Intermediate Mass Stars.- New Main-Sequence Luminosity Functions for Globular Clusters.- Global Versus Local Mass Functions.- A New Survey of Globular Cluster Structural and Luminosity Parameters.- Axial Ratios and Orientations for 100 Galactic Globular Star Clusters.- Abundances in Stars in Globular Clusters from Palomar Ccd Spectra.- The Metal Abundance of Metal-Rich Globular Clusters.- The Composition of Warm Giants in M 71 and M 5.- The Integrated Spectra of Metal-Rich Galactic Globular Clusters: A Two-Parameter Family.- IUE Investigations at the Core of M 79.- One-Micron Photometry of Omega Centauri Giants.- On the Bimodal Distributions of Horizontal Branches.- Bimodal Distributions on the Horizontal Branch.- Horizontal-Branch Stars with Strong He Lines.- Spectra of BHB Stars in M 3, M 13 and M 92.- Four-Color Measures of BHB Stars in M 4, M 13 and M 55.- Globular Clusters in the Vilnius Photometric System.- The Metallicity Distribution Function of Halo Dwarfs and Globular Clusters.- The Similarity of the Halo Field K Giant Population with the Globular Cluster System of Our Galaxy.- Apparent Rotation of the Galactic Globular Cluster System.- Astrometric Distances of Globular Clusters.- Absolute Proper Motions and Space Motions of Globular Clusters.- A Search for Obscured Globular Clusters.- On the Color Excesses of Globular Clusters.- First Poster Paper Discussion.- VIII Poster Papers on Cluster Systems in Nearby Galaxies.- A Search for Globular Cluster Candidates in NGC 2403.- Search for Globular Clusters in the Nearby Galaxies II. NGC 3109.- A Complete Sample of Globular Clusters in NGC 5128.- Astronomical Catalogues in the M 31 Region.- The Blue Star Clusters of M 31.- Spatial Distribution of Globular Clusters in M 31.- Formation of Populous Clusters from Metal-Poor Gas in the Magellanic Clouds.- The Development of the Red Giant Branch in Magellanic Cloud Clusters: Progress Report.- The Age Distribution and Age-Metallicity Relation of Star Clusters in a Northern Region of the LMC.- Photometric Models for Globular Clusters From Population Synthesis.- Bvri Photometry of Star Clusters in the Bok Region of the Large Magellanic Cloud.- Ages and Metal Abundances of Star Clusters in the Magellanic Clouds.- Internal Dynamics of Magellanic Cloud Clusters.- Do Binary Clusters Exist in the Large Magellanic Cloud?.- Ellipticities of Globular Clusters in the Andromeda Galaxy.- Observed Variations in the Density Profiles of Star Clusters in the LMC.- Ratio of Early to Late Type Stars in SMC Clusters.- The SMC Cluster Lindsay 11.- Abundances of Young LMC Clusters.- The Abundance of the LMC Globular Cluster NGC 2213.- Deep Photometry of the Draco Dwarf Spheroidal Galaxy.- CCD Photometry in the Core of the Fornax Dwarf Galaxy.- A Candidate for the Recovered Nova of 1938 in the Globular Cluster M 14.- The Absolute Luminosity of RR Lyrae Variables.- The Distances to RR Lyrae Variables.- Double-Mode RR Lyrae Stars in IC 4499.- Short-Period Variables in Globular Clusters of Moderate Metallicity.- Second Poster Paper Discussion.- IX: Poster Papers on Cluster Systems in Distant Galaxies. Deep Photometry and CM Diagrams.- The Nuclei of Nucleated Dwarf Elliptical Galaxies - Are they Globular Clusters?.- The Globular Cluster System of M 87.- The Core of the M 87 Globular Cluster System.- U Photometry of Globular Clusters in the Central Region of M 87.- Globular Clusters Detected uin the Coma Cluster'S Central Giant Galaxy NGC 4874.- Globular Clusters in Different Types of Spiral Galaxies.- Globular Clusters in Lenticular Galaxies: NCG 3115.- Globular Clusters as Extragalactic Distance Indicators: Maximum Likelihood Methods.- Photometry of Faint Stars in Globular Clusters Using the Six Meter Telescope.- High Precision Photometry of 10,000 Stars in M 3.- Deep CCD Photometry in M 5.- Photographic Photometry of 4500 Stars in M 30.- An Automated HR Diagram for NGC 6809 (M 55).- Deep CCD Photometry in Omega Centauri and NGC 3201.- Deep CCD Photometry of OMEGA Centauri.- The Ages of Globular Clusters Derived from BVRI CCD Photometry.- Turnoffs and Ages of Globular Clusters.- The Dynamics of Globular Clusters in High Eccentricity Orbits.- Mass Distributions of Galaxies with Globular Cluster Systems.- The Dynamics of Globular Cluster Systems.- Third Poster Paper Discussion.- X Poster Papers on Formation and Evolution of Globular Clusters.- A Multicolor CCD Survey of Southern Globular Clusters.- The Structure of Collapsed Cluster Cores.- Radial Velocity Study of NGC 6712.- A Survey of Globular Cluster Velocity Dispersions.- Anistropy in OMEGA Centauri and 47 Tucanae.- Evolution of Globular Clusters Including a Degenerate Component.- Evolution of Globular Clusters with Tidally-Captured Binaries Through Core Collapse.- Binary Interactions in Star Clusters.- Tidal Effects on Stellar Evolution in Close Binaries Formed in Globular Clusters.- The Effects of Stellar Evolution and Galactic Tides on Globular Cluster Evolution.- The Spatial Distribution of Spectroscopic Binaries and Blue Stragglers in M 67.- Evidence for Mass Segregation in NGC 5466.- Origin and Radial Distribution of Faint Blue Horizontal-Branch Stars.- Variability of Omega Centauri Blue Stragglers: Clues to their Origin.- A Search for Optical Counterparts of Globular Cluster X-Ray Sources.- Low Luminosity Globular Cluster X-Ray Sources.- Exosat Observations of Omega Centauri.- New Methods for the Search for Hot Gas in Globular Clusters.- Radial Velocity Profiles for Anisotropic Spherically Symmetric Clusters: An Example.- Linear Density Waves in Globular Clusters.- On Gravothermal Oscillations.- Cooling and Fragmentation of Proto-Globular Cluster Clouds.- Formation of Globular Clusters and the First Stellar Generation.- Formation of Population III Objects Due to Cosmic Strings.- Fourth Poster Paper Discussion.- Random Quotes.- Name Index.- Object Index.- Addresses Of Participants.

The luminosity function and stellar mass-to-light ratio of the massive globular cluster NGC 2419

Abstract: We used archival Hubble Space Telescope Wide Field Camera 3 (WFC3) images to obtain the luminosity function of the remote globular cluster NGC 2419 from 2 mag above the horizontal branch level down to ≃ 3.0 mag below the turn-off point (to MI≃ 6.4), approximately covering the range of initial stellar masses . The completeness-corrected luminosity function does not display any change of shape over the radial range covered by the WFC3 data, out to ≃ 6 core radii (rc), or, equivalently, to ≃ 2 half-light radii. The luminosity function in this radial range is also identical to that obtained from ground-based data at much larger distances from the cluster centre (), in the magnitude range in which the two distributions overlap (MI≤ 4.0). These results support the conclusion by Dalessandro et al. that there is no significant mass segregation among cluster stars; hence, the stellar mass-to-light ratio remains constant with distance from the cluster centre. We fitted the observed luminosity function with theoretical counterparts with the proper age and metallicity from different sets of stellar evolution models, and we consistently derive a total V-band mass-to-light ratio by extrapolating to the hydrogen-burning limit, with a best-fitting value M/LV= 1.5 ± 0.1. On the other hand, assuming that there are no cluster stars with m≤ 0.3 M⊙, we establish a robust lower limit M/LV > 0.8. These estimates provide useful constraints for dynamical models of the cluster that were forced to consider the stellar mass-to-light ratio as a (nearly) free parameter.

The influence of stellar dynamical ejections and collisions on the relation between the maximum stellar and star cluster mass

Abstract: We perform the largest currently available set of direct N-body calculations of young star cluster models to study the dynamical influence, especially through the ejections of the most massive star in the cluster, on the current relation between the maximum stellar mass and the star cluster mass. We vary several initial parameters such as the initial half-mass radius of the cluster, the initial binary fraction and the degree of initial mass segregation. Two different pairing methods are used to construct massive binaries for more realistic initial conditions of massive binaries. We find that lower mass clusters () do not shoot out their heaviest star. In the case of massive clusters (), no most massive star escapes the cluster within 3 Myr regardless of the initial conditions if clusters have initial half-mass radii, r0.5, ≥0.8 pc. However, a few of the initially smaller sized clusters (r0.5= 0.3 pc), which have a higher density, eject their most massive star within 3 Myr. If clusters form with a compact size and their massive stars are born in a binary system with a mass ratio biased towards unity, the probability that the mass of the most massive star in the cluster changes due to the ejection of the initially most massive star can be as large as 20 per cent. Stellar collisions increase the maximum stellar mass in a large number of clusters when clusters are relatively dense ( and r0.5= 0.3 pc) and binary rich. Overall, we conclude that dynamical effects hardly influence the observational maximum stellar mass–cluster mass relation.

The influence of stellar-dynamical ejections and collisions on the relation between the maximum-star and star-cluster-mass

Abstract: We perform the largest currently available set of direct N-body calculations of young star cluster models to study the dynamical influence, especially through the ejections of the most massive star in the cluster, on the current relation between the maximum-stellar-mass and the star-cluster-mass. We vary several initial parameters such as the initial half-mass radius of the cluster, the initial binary fraction, and the degree of initial mass segregation. Two different pairing methods are used to construct massive binaries for more realistic initial conditions of massive binaries. We find that lower mass clusters ( = 1000 Msun), no most-massive star escapes the cluster within 3 Myr regardless of the initial conditions if clusters have initial half-mass radii, r_0.5, >= 0.8 pc. However, a few of the initially smaller sized clusters (r_0.5 = 0.3 pc), which have a higher density, eject their most massive star within 3 Myr. If clusters form with a compact size and their massive stars are born in a binary system with a mass-ratio biased towards unity, the probability that the mass of the most massive star in the cluster changes due to the ejection of the initially most massive star can be as large as 20 per cent. Stellar collisions increase the maximum-stellar-mass in a large number of clusters when clusters are relatively dense (M_ecl >= 10^3 Msun and r_0.5 = 0.3 pc) and binary-rich. Overall, we conclude that dynamical effects hardly influence the observational maximum-stellar-mass -- cluster mass relation.

A search for mass segregation of stars and brown dwarfs in ρ Ophiuchi

Abstract: We apply two different algorithms to search for mass segregation to a recent observational census of the ρ Ophiuchi star-forming region. First, we apply the ΛMSR method, which compares the minimum spanning tree (MST) of a chosen subset of stars to MSTs of random subsets of stars in the cluster, and determine the mass segregation ratio, ΛMSR. Secondly, we apply the m–Σ method, which calculates the local stellar surface density around each star and determines the statistical significance of the average surface density for a chosen mass bin compared to the average surface density in the whole cluster. Using both methods, we find no indication of mass segregation (normal or inverse) in the spatial distribution of stars and brown dwarfs in ρ Ophiuchi. Although ρ Ophiuchi suffers from high visual extinction, we show that a significant mass segregation signature would be detectable, albeit slightly diluted, despite dust obscuration of centrally located massive stars.

A search for mass segregation of stars and brown dwarfs in \rho\ Ophiuchi

Abstract: We apply two different algorithms to search for mass segregation to a recent observational census of the rho Ophiuchi star forming region. Firstly, we apply the Lambda_MSR method, which compares the minimum spanning tree (MST) of a chosen subset of stars to MSTs of random subsets of stars in the cluster, and determine the mass segregation ratio, Lambda_MSR. Secondly, we apply the m-Sigma method, which calculates the local stellar surface density around each star and determines the statistical significance of the average surface density for a chosen mass bin, compared to the average surface density in the whole cluster. Using both methods, we find no indication of mass segregation (normal or inverse) in the spatial distribution of stars and brown dwarfs in rho Ophiuchi. Although rho Ophiuchi suffers from high visual extinction, we show that a significant mass segregation signature would be detectable, albeit slightly diluted, despite dust obscuration of centrally located massive stars.

A structural analysis of star-forming region afgl 490

Abstract: We present Spitzer IRAC and MIPS observations of the star-forming region containing intermediate-mass young stellar object (YSO) AFGL 490. We supplement these data with near-IR Two Micron All Sky Survey photometry and with deep Simultaneous Quad Infrared Imaging Device observations off the central high-extinction region. We have more than doubled the known membership of this region to 57 Class I and 303 Class II YSOs via the combined 1-24 {mu}m photometric catalog derived from these data. We construct and analyze the minimum spanning tree of their projected positions, isolating one locally overdense cluster core containing 219 YSOs (60.8% of the region's members). We find this cluster core to be larger yet less dense than similarly analyzed clusters. Although the structure of this cluster core appears irregular, we demonstrate that the parsec-scale surface densities of both YSOs and gas are correlated with a power-law slope of 2.8, as found for other similarly analyzed nearby molecular clouds. We also explore the mass segregation implications of AFGL 490's offset from the center of its core, finding that it has no apparent preferential central position relative to the low-mass members.