Steve McMillan

Missing Link Found? The "Runaway" Path to Supermassive Black Holes

Observations of stellar kinematics, gasdynamics, and masers around galactic nuclei have now firmly established that many galaxies host central supermassive black holes (SMBHs) with masses in the range of ~106-109 M☉. However, how these SMBHs formed is not well understood. One reason for this situation is the lack of observations of intermediate-mass BHs (IMBHs), which could bridge the gap between stellar mass BHs and SMBHs. Recently, this missing link (i.e., an IMBH) has been found in observations made by ASCA and Chandra of the central region of the starburst galaxy M82. Subsequent observations by Subaru have revealed that this IMBH apparently coincides with a young compact star cluster. Based on these findings, we suggest a new formation scenario for SMBHs. In this scenario, IMBHs first form in young compact star clusters through runaway merging of massive stars. While these IMBHs are forming, the host star clusters sink toward the galactic nucleus through dynamical friction and upon evaporation deposit their IMBHs near the galactic center. The IMBHs then form binaries and eventually merge via gravitational radiation, forming an SMBH.

BINARIES IN GLOBULAR-CLUSTERS

Binary stars in a globular cluster (hereafter, GC) may be primordial (i.e. formed along with the cluster), or the result of cluster dynamics. “Dynamical” binaries can result from conservative three-body encounters (e.g. Spitzer, 1987) if a third star can carry away enough kinetic energy to leave two others bound, or from dissipative two-body encounters, if two stars happen to pass within a few stellar radii of one other (Fabian, Pringle, & Rees, 1975). Such non-primordial systems are likely to be found primarily in evolved GC cores, both because conditions are more favorable for making them there, and because of mass segregation. Knowledge of the formation process allows reasonable estimates to be made of their mass and energy distributions. The initial spatial, mass, and energy distributions of primordial binaries, on the other hand, are largely unknown.

Dynamical Formation of Close Binary Systems in Globular Clusters

We know from observations that globular clusters are very efficient catalysts in forming unusual short-period binary systems or their offspring, such as low-mass X-ray binaries (LMXBs; neutron stars accreting matter from low-mass stellar companions), cataclysmic variables (white dwarfs accreting matter from stellar companions), and millisecond pulsars (rotating neutron stars with spin periods of a few milliseconds). Although there has been little direct evidence, the overabundance of these objects in globular clusters has been attributed by numerous authors to the high densities in the cores, which leads to an increase in the formation rate of exotic binary systems through close stellar encounters. Many such close binary systems emit X-radiation at low luminosities (LX 1034 ergs s-1) and are being found in large numbers through observations with the Chandra X-Ray Observatory. Here we present conclusive observational evidence of a link between the number of close binaries observed in X-rays in a globular cluster and the stellar encounter rate of the cluster. We also make an estimate of the total number of LMXBs in globular clusters in our Galaxy.

On the Central Structure of M15

We present a detailed comparison between the latest observational data on the kinematical structure of the core of M15, obtained with the Hubble Space Telescope Space Telescope Imaging Spectrograph and Wide Field Planetary Camera 2 instruments, and the results of dynamical simulations carried out using the special purpose GRAPE-6 computer. The observations imply the presence of a significant amount of dark matter in the cluster core. In our dynamical simulations, neutron stars and/or massive white dwarfs concentrate to the center through mass segregation, resulting in a sharp increase in toward the center. While consistent with the presence of M/L a central black hole, the Hubble Space Telescope data can also be explained by this central concentration of stellar mass compact objects. The latter interpretation is more conservative, since such remnants result naturally from stellar evolution, although runaway merging leading to the formation of a black hole may also occur for some range of initial conditions. We conclude that no central massive object is required to explain the observational data, although we cannot conclusively exclude such an object at the level of. Our findings are similar to500-1000 M-circle dot. Our findings are unchanged when we reduce the assumed neutron star retention fraction in our simulations from 100% to 0%.

A Dynamical Model for the Globular Cluster G1

We present a comparison between the observational data on the kinematical structure of G1 in M31, obtained with the Hubble Space Telescope Wide Field Planetary Camera 2 and Space Telescope Imaging Spectrograph instruments, and the results of dynamical simulations carried out using the special purpose computer GRAPE-6. We have obtained good fits for models starting from single-cluster King model initial conditions and even better fits when starting our simulations with a dynamically constructed merger product of two star clusters. In the latter case, the results from our simulations are in excellent agreement with the observed profiles of luminosity, velocity dispersion, rotation, and ellipticity. We obtain a mass-to-light ratio of M/L = 4.0 ± 0.4 and a total cluster mass of M = (8 ± 1) × 106 M☉. Given that our dynamical model can fit all available observational data very well, there seems to be no need to invoke the presence of an intermediate-mass black hole in the center of G1.

The Astrophysical Multipurpose Software Environment

We present the open source Astrophysical Multi-purpose Software Environment (AMUSE), a component library for performing astrophysical simulations involving different physical domains and scales. It couples existing codes within a Python framework based on a communication layer using MPI. The interfaces are standardized for each domain and their implementation based on MPI guarantees that the whole framework is well-suited for distributed computation. It includes facilities for unit handling and data storage. Currently it includes codes for gravitational dynamics, stellar evolution, hydrodynamics and radiative transfer. Within each domain the interfaces to the codes are as similar as possible. We describe the design and implementation of AMUSE, as well as the main components and community codes currently supported and we discuss the code interactions facilitated by the framework. Additionally, we demonstrate how AMUSE can be used to resolve complex astrophysical problems by presenting example applications.

Star cluster evolution with primordial binaries. I - A comparative study

The evolution of equal-mass star clusters containing a mass fraction of about 20 percent binaries has been followed using direct integration, making one run each for a total number of stars of N = 282 and N = 563, and four runs for N = 1126. For comparison the evolution of an equivalent star system where the binaries were replaced by stars twice as heavy as the other stars was followed. The pre-core-collapse evolution is driven by mass segregation between the equal-mass single stars and the binaries, which are twice as heavy. After core collapse, the cluster shows, on average, a smooth reexpansion driven by a steady rate of burning (hardening) of primordial binaries. With so much primordial fuel present, the postcollapse cluster core is significantly larger than is the case in comparison runs without primordial binaries. 64 refs.

How Many Young Star Clusters Exist in the Galactic Center

We study the evolution and observability of young compact star clusters within ~200 pc of the Galactic center. Calculations are performed using direct N-body integration on the GRAPE-4, including the effects of both stellar and binary evolution and the external influence of the Galaxy. The results of these detailed calculations are used to calibrate a simplified model applicable over a wider range of cluster initial conditions. We find that clusters within 200 pc of the Galactic center dissolve within ~70 Myr. However, their projected densities drop below the background density in the direction of the Galactic center within ~20 Myr, effectively making these clusters undetectable after that time. Clusters farther from the Galactic center but at the same projected distance are more strongly affected by this selection effect and may go undetected for their entire lifetimes. Based on these findings, we conclude that the region within 200 pc of the Galactic center could easily harbor some 50 clusters with properties similar to those of the Arches or the Quintuplet systems.

The Evolution of a primordial binary population in a globular cluster

A simple model is presented for the evolution of a primordial binary population in a globular cluster. Monte Carlo simulations are given for an initial population of 50,000 binaries against a fixed background population of 500,000 single stars in a tidally truncated cluster model. Individual histories of all binaries are followed through mass segregation, scattering recoil, escape from the cluster, or coalescence. It is found that most binaries are destroyed by binary-binary interactions, with the rest escaping in the point-mass approximation. In a more realistic model, the majority of the rest merge. At any instant, most of the remaining binaries are drifting in toward the center before their first strong encounter. A typical binary spends most of its active life in or near the cluster core. The few binaries which receive a recoil sufficient to place them in the halo past the half-mass radius remain there long enough to make a significant contribution to the radial binary distribution.

MODEST-1: Integrating stellar evolution and stellar dynamics

Abstract We summarize the main results from MODEST-1, the first workshop on MOdeling DEnse STellar systems. Our goal is to go beyond traditional population synthesis models, by introducing dynamical interactions between single stars, binaries, and multiple systems. The challenge is to define and develop a software framework to enable us to combine in one simulation existing computer codes in stellar evolution, stellar dynamics, and stellar hydrodynamics. With this objective, the workshop brought together experts in these three fields, as well as other interested astrophysicists and computer scientists. We report here our main conclusions, questions and suggestions for further steps toward integrating stellar evolution and stellar (hydro)dynamics.