5 research outputs found
Boyle's law and gravitational instability
We have re-examined the classical problem of the macroscopic equation of
state for a hydrostatic isothermal self-gravitating gas cloud bounded by an
external medium at constant pressure. We have obtained analytical conditions
for its equilibrium and stability without imposing any specific shape and
symmetry to the cloud density distribution. The equilibrium condition can be
stated in the form of an upper limit to the cloud mass; this is found to be
inversely proportional to the power 3/2 of a form factor \mu characterizing the
shape of the cloud. In this respect, the spherical solution, associated with
the maximum value of the form factor, \mu = 1, turns out to correspond to the
shape that is most difficult to realize. Surprisingly, the condition that
defines the onset of the Bonnor instability (or gravothermal catastrophe) can
be cast in the form of an upper limit to the density contrast within the cloud
that is independent of the cloud shape. We have then carried out a similar
analysis in the two-dimensional case of infinite cylinders, without assuming
axisymmetry. The results obtained in this paper generalize well-known results
available for spherical or axisymmetric cylindrical isothermal clouds that have
had wide astrophysical applications, especially in the study of the
interstellar medium.Comment: 9 pages, 2 figures, to appear in A&
Monte Carlo Simulations of Star Clusters - VI. The globular cluster NGC 6397
We describe Monte Carlo models for the dynamical evolution of the nearby
globular cluster NGC 6397. The code includes treatments of two-body relaxation,
most kinds of three- and four-body interactions involving primordial binaries
and those formed dynamically, the Galactic tide, and the internal evolution of
both single and binary stars. We arrive at a set of initial parameters for the
cluster which, after 12Gyr of evolution, gives a model with a fairly
satisfactory match to the surface brightness profile, the velocity dispersion
profile, and the luminosity function in two fields. We describe in particular
those aspects of the evolution which distinguish this cluster from M4, which
has a roughly similar mass and Galactocentric distance, but a qualitatively
different surface brightness profile. Within the limitations of our modelling,
we conclude that the most plausible explanation for the difference is
fluctuations: both clusters are post-collapse objects, but sometimes have
resolvable cores and sometimes not.Comment: 11 pages, 12 figures, MNRAS, in press; revised in response to referee
repor
Intermediate and extreme mass-ratio inspirals — astrophysics, science applications and detection using LISA
Black hole binaries with extreme (gtrsim104:1) or intermediate (~102–104:1) mass ratios are among the most interesting gravitational wave sources that are expected to be detected by the proposed laser interferometer space antenna (LISA). These sources have the potential to tell us much about astrophysics, but are also of unique importance for testing aspects of the general theory of relativity in the strong field regime. Here we discuss these sources from the perspectives of astrophysics, data analysis and applications to testing general relativity, providing both a description of the current state of knowledge and an outline of some of the outstanding questions that still need to be addressed. This review grew out of discussions at a workshop in September 2006 hosted by the Albert Einstein Institute in Golm, Germany
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