Stellar and planetary dynamics
Researchers: Rosemary Mardling, Daniel Dindas
In his Oppenheimer lecture entitled ``Gravity is cool, or, why our universe is as hospitable as it is'', Freeman Dyson discusses how time has two faces: the quick violent face and the slow gentle face; the face of the destroyer and the face of the preserver. These two faces he attributes entirely to gravity and the ease with which gravitational energy can change irreversibly into other forms of energy. The simplest system exhibiting these two faces is that of three gravitating bodies; for most configurations, the slow gentle face is the norm, while for a very important subset, violence is the order of the day. In fact it is this violence, resulting in the collision of two of the bodies or the ejection of one of the bodies from the system, which is responsible for much of the structure we see in the universe, from planets to giant elliptical galaxies.
Rosemary Mardling has developed new mathematical methods to study the general three-body problem, allowing for a much deeper understanding of the many astrophysical processes which rely on three-body instability to proceed. Together with members of the Dynamics group at Monash as well as collaborators at Cambridge, Oxford, UC Santa Cruz and CfA at Harvard, she studies problems which range from the dynamics of short-period extrasolar planets with multiple companions, to the ``gravitational chemistry'' of dense stellar systems. Current and recent members of the group are Gareth Kennedy (stellar dynamics at the Galactic Centre; globular cluster dynamics; resonant extrasolar planets), Keith Hsuan (planetary dynamics in binary star systems), Karen Lewis (transiting extrasolar moons; pulsar planet moons), Marie Gibbon (resonant extrasolar planets), and Jarrod Hurley (star cluster dynamics; stellar and binary evolution).
Fundamental to much of the work of the Monash Dynamics group is three-body stability. Except for the special case in which one body is massless (the so-called restricted three-body problem), the problem of determining which general triple configurations are stable has long been regarded as theoretically intractable. Researchers have had no choice but to determine stability by numerically integrating the equations of motion for individual systems, a CPU-intensive option which, for many applications, can be extremely inefficient. Using the formalism of Mardling (itself based on the chaos theory concept of resonance overlap), it is now possible to determine the stability of any triple configuration quickly and easily, with the associated algorithm being used by groups around the world for a wide range of applications. For example, without such an algorithm, star cluster simulations would grind to a halt whenever a stable triple was formed via the collision of two binaries, because a tiny permanent subsystem would be responsible for setting the integration time step indefinitely.