Mass segregation

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

Strong mass segregation around a massive black hole

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.

M82-F: a doomed super star cluster?

Abstract: We present high dispersion echelle spectroscopy of the very luminous, young super star cluster (SSC) ‘F’ in M82, obtained with the 4.2-m William Herschel Telescope (WHT), for the purpose of deriving its dynamical mass and assessing whether it will survive to become an old globular cluster. In addition to the stellar lines, the spectrum contains complex Na I absorption and broad emission lines from the ionized gas. We measure a stellar velocity dispersion of 13.4±0.7 kms −1 , a projected half-light radius of 2.8 ± 0.3 pc from archival HST/WFPC2 images, and derive a dynamical mass of 1.2±0.1×10 6 M⊙, demonstrating that M82-F is a very massive, compact cluster. We determine that the current luminosity-to-mass ratio (LV /M)⊙ for M82-F is 45 ± 13. Comparison with spectral synthesis models shows that (LV /M)⊙ is a factor of � 5 higher than that predicted for a standard Kroupa (2001) initial mass function (IMF) at the well-determined age for M82-F of 60 ± 20 Myr. This high value of (LV /M)⊙ indicates a deficit of low mass stars in M82-F; the current mass function (MF) evidently is ‘top-heavy’. We find that a lower mass cutoff of 2–3M⊙ is required to match the observations for a MF with a slope � = 2.3. Since the cluster apparently lacks longlived low mass stars, it will not become an old globular cluster, but probably will dissolve at an age of 62 Gyr. We also derive up-dated luminosity-to-mass ratios for the younger SSCs NGC 1569A and NGC 1705-1. For the first object, the observations are consistent with a slightly steeper MF (� = 2.5) whereas for NGC 1705-1, the observed ratio requires the MF to be truncated near 2 M⊙ for a slope of � = 2.3. We discuss the implications of our findings in the context of large scale IMF variations; with the present data the top-heavy MF could reflect a local mass segregation effect during the birth of the cluster. M82-F likely formed in a dense molecular cloud; however, its high radial velocity with respect to the centre of M82 (� 175 km s −1 ) suggests it is on an eccentric orbit and now far from its birthplace, so the environment of its formation is unknown.

Using the minimum spanning tree to trace mass segregation

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.

A Dynamical Origin for Early Mass Segregation in Young Star Clusters

Abstract: Some young star clusters show a degree of mass segregation that is inconsistent with the effects of standard two-body relaxation from an initially unsegregated system without substructure, in virial equilibrium, and it is unclear whether current cluster formation models can account for this degree of initial segregation in clusters of significant mass. In this Letter we demonstrate that mergers of small clumps that are initially mass segregated, or in which mass segregation can be produced by two-body relaxation before they merge, generically lead to larger systems that inherit the progenitor clumps' segregation. We conclude that clusters formed in this way are naturally mass segregated, accounting for the anomalous observations and suggesting that this process of prompt mass segregation due to initial clumping should be taken into account in models of cluster formation and dynamics.

N-body modelling of globular clusters: masses, mass-to-light ratios and intermediate-mass black holes

Abstract: We have determined the masses and mass-to-light ratios of 50 Galactic globular clusters by comparing their velocity dispersion and surface brightness profiles against a large grid of 900 N-body simulations of star clusters of varying initial concentration, size and central black hole mass fraction. Our models follow the evolution of the clusters under the combined effects of stellar evolution and two-body relaxation allowing us to take the effects of mass segregation and energy equipartition between stars self-consistently into account. For a subset of 16 wellobserved clusters, we also derive their kinematic distances. We find an average mass-to-light ratio of Galactic globular clusters of =1.98 ± 0.03, which agrees very well with the expected M/L ratio if the initial mass function (IMF) of the clusters was a standard Kroupa or Chabrier mass function. We do not find evidence for a decrease in the average mass-to-light ratio with metallicity. The surface brightness and velocity dispersion profiles of most globular clusters are incompatible with the presence of intermediate-mass black holes (IMBHs) with more than a few thousand M in them. The only clear exception is ω Cen, where the velocity dispersion profile provides strong evidence for the presence of a ~40 000 M IMBH in the centre of the cluster.