47 research outputs found
Binaries and Globular Cluster Dynamics
We summarize the results of recent theoretical work on the dynamical
evolution of globular clusters containing primordial binaries. Even a very
small initial binary fraction (e.g., 10%) can play a key role in supporting a
cluster against gravothermal collapse for many relaxation times. Inelastic
encounters between binaries and single stars or other binaries provide a very
significant energy source for the cluster. These dynamical interactions also
lead to the production of large numbers of exotic systems such as ultracompact
X-ray binaries, recycled radio pulsars, double degenerate systems, and blue
stragglers. Our work is based on a new parallel supercomputer code implementing
Henon's Monte Carlo method for simulating the dynamical evolution of dense
stellar systems in the Fokker-Planck approximation. This new code allows us to
calculate very accurately the evolution of a cluster containing a realistic
number of stars (N ~ 10^5 - 10^6) in typically a few hours to a few days of
computing time. The discrete, star-by-star representation of the cluster in the
simulation makes it possible to treat naturally a number of important
processes, including single and binary star evolution, all dynamical
interactions of single stars and binaries, and tidal interactions with the
Galaxy.Comment: 15 pages, to appear in `The Influence of Binaries on Stellar
Population Studies', ed. D. Vanbeveren (Kluwer
Relativistic Dynamics and Extreme Mass Ratio Inspirals
It is now well-established that a dark, compact object (DCO), very likely a
massive black hole (MBH) of around four million solar masses is lurking at the
centre of the Milky Way. While a consensus is emerging about the origin and
growth of supermassive black holes (with masses larger than a billion solar
masses), MBHs with smaller masses, such as the one in our galactic centre,
remain understudied and enigmatic. The key to understanding these holes - how
some of them grow by orders of magnitude in mass - lies in understanding the
dynamics of the stars in the galactic neighbourhood. Stars interact with the
central MBH primarily through their gradual inspiral due to the emission of
gravitational radiation. Also stars produce gases which will subsequently be
accreted by the MBH through collisions and disruptions brought about by the
strong central tidal field. Such processes can contribute significantly to the
mass of the MBH and progress in understanding them requires theoretical work in
preparation for future gravitational radiation millihertz missions and X-ray
observatories. In particular, a unique probe of these regions is the
gravitational radiation that is emitted by some compact stars very close to the
black holes and which could be surveyed by a millihertz gravitational wave
interferometer scrutinizing the range of masses fundamental to understanding
the origin and growth of supermassive black holes. By extracting the
information carried by the gravitational radiation, we can determine the mass
and spin of the central MBH with unprecedented precision and we can determine
how the holes "eat" stars that happen to be near them.Comment: Update from the first version, 151 pages, accepted for publication @
Living Reviews in Relativit
Relativistic Binaries in Globular Clusters
Galactic globular clusters are old, dense star systems typically containing
10\super{4}--10\super{7} stars. As an old population of stars, globular
clusters contain many collapsed and degenerate objects. As a dense population
of stars, globular clusters are the scene of many interesting close dynamical
interactions between stars. These dynamical interactions can alter the
evolution of individual stars and can produce tight binary systems containing
one or two compact objects. In this review, we discuss theoretical models of
globular cluster evolution and binary evolution, techniques for simulating this
evolution that leads to relativistic binaries, and current and possible future
observational evidence for this population. Our discussion of globular cluster
evolution will focus on the processes that boost the production of hard binary
systems and the subsequent interaction of these binaries that can alter the
properties of both bodies and can lead to exotic objects. Direct {\it N}-body
integrations and Fokker--Planck simulations of the evolution of globular
clusters that incorporate tidal interactions and lead to predictions of
relativistic binary populations are also discussed. We discuss the current
observational evidence for cataclysmic variables, millisecond pulsars, and
low-mass X-ray binaries as well as possible future detection of relativistic
binaries with gravitational radiation.Comment: 88 pages, 13 figures. Submitted update of Living Reviews articl
Estimate of the transition value of librational invariant curves
We investigate the break-down threshold of librational invariant curves. As a model problem, we consider a variant of a mapping introduced by M. Henon, which well describes the dynamics of librational motions surrounding a stable invariant point. We verify in concrete examples the applicability of Greene's method, by computing the instability transition values of a sequence of periodic orbits approaching an invariant curve with fixed noble frequency. However, this method requires the knowledge of the location of the periodic orbits within a very good approximation. This task appears to be difficult to realize for a libration regime, due to the different topology of the phase space. To compute the break-down threshold, we tried an alternative method very easy to implement, based on the computation of the fast Lyapunov indicators and frequency analysis. Such technique does not require the knowledge of the periodic orbits, but again, it appears very difficult to have a precision better than Greene's method for the computation of the critical parameter