Mass segregation

About: Mass segregation is a research topic. Over the lifetime, 1024 publications have been published within this topic receiving 57729 citations.

Mass Segregation in Young Magellanic Clouds Star Clusters: Four Clusters observed with HST

Abstract: We present the results of our investigation on the phenomenon of mass segregation in young star clusters in the Magellanic Clouds. HST/WFPC2 observations on NGC 1818, NGC 2004 & NGC 2100 in the Large Magellanic Cloud and NGC 330 in the Small Magellanic Cloud have been used for the application of diagnostic tools for mass segregation: i) the radial density profiles of the clusters for various mass groups and ii) their mass functions (MFs) at various radii around their centres. All four clusters are found to be mass segregated, but each one in a different manner. Specifically not all the clusters in the sample show the same dependence of their density profiles on the selected magnitude range, with NGC 1818 giving evidence of a strong such relation and NGC 330 showing only a hint of the phenomenon. NGC 2004 did not also show any significant signature of mass segregation in its density profiles. The MFs radial dependence provides clear proof of the phenomenon for NGC 1818, NGC 2100 and NGC 2004, while for NGC 330 it gives only indications. An investigation on the constraints introduced by the application of both diagnostic tools is presented. We also discuss the problems related to the construction of a reliable MF for a cluster and their impact on the investigation of the phenomenon of mass segregation. We find that the MFs of these clusters as they were constructed with two methods, are comparable to Salpeter's IMF. A discussion is given on the dynamical status of the clusters and a test is applied on the equipartition among several mass groups in them. Both showed that the observed mass segregation in the clusters is of primordial nature.

Mass functions and structure of the young open cluster NGC 6611

Abstract: We use J, H and K S 2MASS photometry to study colour-magnitude (CMDs) and colour-colour diagrams, structure and mass distribution in the ionizing open cluster NGC 661 1. Reddening variation throughout the cluster region is taken into account followed by field-star decontamination of the CMDs. Decontamination is also applied to derive the density profile and luminosity functions in the core, halo and overall (whole cluster) regions. The field-star deconlamination showed that the lower limit of the main sequence (MS) occurs at 5 M ○. . Based on the fraction of K S excess stars in the colour-colour diagram we estimate an age of 1.3 ± 0.3 Myr which is consislent with the presence of a large number of premain sequence (PMS) stars. The distance from the Sun was estimated from known O V stars in the cluster area and the turn-on stars connecting the PMS and MS, resulting in d ○. = 1.8 ± 0.5 kpc. The radial density distribution including MS and PMS stars is fitted by a King profile with a core radius R core = 0.70 ± 0.08 pc. The cluster density profile merges into the background at a limiting radius R lim = 6.5 ± 0.5 pc. From the field-star subtracted luminosity functions we derive the mass functions (MFs) in the form Φ(m) m -(1+x) . In the halo and through the whole cluster the MFs have slopes X = 1.52 ± 0.13 and X = 1.45 ± 0.12, respectively, thus slightly sleeper than Salpeter's IMF. In the core the MF is flat, X = 0.62 ± 0.16, indicating some degree of mass segregation since the cluster age is a factor ∼2 larger than the relaxation time. Because of the very young age of NGC 6611, part of this effect appears to be related to the molecular cloud-fragmentation process itself. We detect 362 ± 120 PMS stars. The total observed mass including detected MS (in the range 5-85 M ○. ) and PMS stars amounts to ∼1600 M ○. , thus more massive than the Trapezium cluster. Compared to older open clusters of different masses, the overall NGC 6611 fits in the relations involving structural and dynamical parameters. However, the cure is atypical in the sense that it looks like an old/dynamically evolved core. Again, part of this effect must he linked to formation processes.

Addendum: "Hubble Space Telescope Evidence for an Intermediate-Mass Black Hole in the Globular Cluster M15: II. Kinematical Analysis and Dynamical Modeling"

Abstract: It has been reported that there is an error in the figure in Dull et al. (1997, D97) that shows the radial M/L profile in Fokker-Planck models of M15. We discuss how this modifies the interpretation of our kinematical data. These imply the existence of a dark and compact mass component near the center of M15, either a single black hole (BH) or a collection of dark remnants that have sunk to the cluster center due to mass segregation. We previously showed that the latter interpretation is in conflict with the D97 M/L profile, which supported the BH interpretation. We repeat our analysis here with the corrected D97 profile. Models without a BH are now found to be statistically acceptable (within 1-sigma), although inclusion of a BH still provides a marginally better fit. It does not necessarily follow that dark remnants are now the preferred interpretation of the data. The D97 models, as well as N-body models by Baumgardt et al. (2002), assume that all neutron stars are retained during cluster evolution. This conflicts with predictions of the neutron-star retention rate (typically below 10 per cent) based on pulsar kick velocities. The presence of a single BH therefore continues to be a viable interpretation of the data. The best fit BH mass with the corrected D97 M/L profile is M_BH = 1.7^{+2.7}_{-1.7} x 10^3 solar masses, and with a constant M/L it is M_BH = 3.2^{+2.2}_{-2.2} x 10^3 solar masses. A model that includes both neutron star escape and mass segregation would probably yield a value between these numbers. This agrees with the correlation between velocity dispersion and BH mass inferred for galaxies. However, with the presently available models and data it is neither uniquely implied nor ruled out that M15 has an intermediate-mass BH.

Intermediate-mass black hole induced quenching of mass segregation in star clusters

Abstract: In many theoretical scenarios it is expected that intermediate-mass black holes (IMBHs, with masses M � 10 2 Y 10 4 M� ) reside at the centers of some globular clusters. However, observational evidence for their existence is limited. Several previous numerical investigations have focused on the impact of an IMBH on the cluster dynamics or brightness profile. Here we instead present results from a large set of direct N-body simulations including single and binarystars.TheseshowthatthereisapotentiallymoredetectableIMBHsignature,namelyonthevariationof theaverage stellar mass between the center and the half-light radius. We find that the existence of an IMBH quenches mass segregation and causes the average mass to exhibit only modest radial variation in collisionally relaxed star clusters. This differs from when there is no IMBH. To measure this observationally requires high-resolution imaging at the level of that already available from the Hubble Space Telescope (HST) for the cores of a large sample of Galactic globular clusters. With a modest additional investment of HST time to acquire fields around the half-light radius, it will be possible to identify the best candidate clusters to harbor an IMBH. This test can be applied only to globulars with a half-light relaxation time P1 Gyr, which is required to guarantee efficient energy equipartition due to twobody relaxation. Subject headingg globular clusters: general — methods: n-body simulations — stellar dynamics

Detection rate estimates of gravity waves emitted during parabolic encounters of stellar black holes in globular clusters

Abstract: The rapid advance of gravitational wave (GW) detector facilities makes it very important to estimate the event rates of possible detection candidates. We consider an additional possibility of GW bursts produced during parabolic encounters (PEs) of stellar-mass compact objects in globular clusters (GCs). We estimate the rate of successful detections for specific detectors: the initial Laser Interferometric Gravitational-Wave Observatory (InLIGO), the French-Italian gravitational wave antenna VIRGO, the near-future Advanced-LIGO (AdLIGO), the space-based Laser Interferometric Space Antenna (LISA), and the Next Generation LISA (NGLISA). Simple GC models are constructed to account for the compact object mass function, mass segregation, number density distribution, and velocity distribution. We both calculate encounters classically and account for general relativistic corrections by extrapolating the results for infinite mass ratios. We also include the cosmological redshift of waveforms and event rates. We find that typical PEs with masses m1 = m2 = 40 M? are detectable with matched filtering over a signal-to-noise ratio S/N = 5 within a distance dL ~ 200 Mpc for InLIGO and VIRGO, z = 1 for AdLIGO, 0.4 Mpc for LISA, and 1 Gpc for NGLISA. We estimate single data stream detection rates of 5.5 ? 10-5 yr-1 for InLIGO, 7.2 ? 10-5 yr-1 for VIRGO, 0.063 yr-1 for AdLIGO, 2.9 ? 10-6 yr-1 for LISA, and 1.0 yr-1 for NGLISA, for reasonably conservative assumptions. These estimates are subject to uncertainties in the GC parameters, most importantly the total number and mass distribution of BHs in the cluster core. In reasonably optimistic cases, we get 1 detection for AdLIGO per year. We expect that a coincident analysis using multiple detectors and accounting for GW recoil capture significantly increases the detection rates. The regular detection of GWs during PEs would provide a unique observational probe for constraining the stellar BH mass function of dense clusters.