- Stellar dynamics, galactic and extragalactic astronomy
- In this research area most of the staff members are active. Stars form
conglomerates that are called stellar systems. These contain from several
hundred thousand (globular clusters) to several thousands of millions stars
(galaxies). The study of stellar systems is a very vigorous research area
in astronomy. Currently it is still not known how exactly stellar systems
were born. Astronomers all over the world are probing the deep universe
to find the progenitors of the galaxies as we know them today, using the
most powerful telescopes.
At the Astronomical Observatory, the inner dynamical structure of stellar
systems is studied. The main goal is to understand how galaxies can be
in equilibrium, and how they evolve in time. This is done mainly by studying
their light, both photometrically and spectroscopically, and by producing
models that fit the observations. From these models, the essential characteristics
of galaxies can be inferred.
More specifically, topics under study include
- globular clusters
- round and flattened elliptical galaxies
- triaxial configurations embedded in integrable potentials
- stellar populations in disk-like configurations, such as star samples
in our galaxy
- barlike and spiral instabilities in disks
- orbital structure in axisymmetric potentials
Much of this work is based on a Quadratic Programming algorithm, developed
by H. Dejonghe, which has the advantage of great analytical flexibility,
and fits distribution functions to a variety of data, including star counts,
photometry, projected velocity distributions and/or their moments, proper
motions and numerical data obtained by simulations. Another component focuses
on solutions of the collisionless Boltzmann equation coupled to the Poisson
equation.
The research tools are diverse. Sometimes the staff is in the position
to acquire data themselves at the large telescopes of ESO, but mostly the
data are obtained through collaborations with other astronomers abroad.
There is also considerable attention for analytical work, but the importance
of numerical techniques is continuously increasing. Image
processing is a routine tool. A 1-person technical staff takes care of
the instruments.
- Space Plasma Waves
- Plasmas in space (outside the Earth) are characterized by the presence
of magnetic fields and more than one species of ions, often generated in
different conditions and streaming with respect to each other. Some of
the typical and most studied examples are the interaction of the solar
wind (the expanding outer corona of the Sun) with cometary environments,
or of the heliosphere (defined as the realm where the solar wind plasma
dominates) with the surrounding interstellar matter. Highly successful
missions to comets P/Giacobini-Zinner, P/Halley and P/Grigg-Skjellerup
have shown how assimilation of cometary ions into the solar wind occur
through collective effects which generate low-frequency turbulence. Why
low-frequency electromagnetic fluctuations dominate is not quite clear.
Theoretical descriptions of such plasma modes and instabilities involve
at least two distinct ion populations, rendering the treatment more complicated.
Wave theory traditionally starts from linear modes, but the quasilinear
and nonlinear aspects determine the ultimate levels of low-frequency turbulence.
Good agreement has been reached in this stimulating interplay between theoretical
concepts and observational evidence. There are similar applications to
certain types of solar flares, magnetospheric and auroral plasmas and dusty
plasmas in the solar system. The framework can even be used to model nonlinear
mode coupling in stellar pulsations.
In particular, the study of waves in dusty plasmas has become a subject
in its own right. Dust occurs in many astrophysical circumstances, like
interstellar and circumstellar media, planetary rings and cometary comae
and tails. These dust grains are immersed in ambient plasmas and become
electrically charged by various processes, after which they interact with
electromagnetic fields. Intriguing phenomena observed in the 1980s by Voyager
cameras and attributed to micron-sized charged dust are radial spokes in
the B-ring and braids in the F-ring
of Saturn, not explainable by classical mechanical pictures. As the
dust grains can have very high negative charges and in proportion even
higher masses than ordinary ions, characteristic frequencies are considerably
smaller than corresponding electron or ion quantities, giving rise to new
low-frequency eigenmodes. Further complications arise because the dust
charges fluctuate, leading to new electrostatic and electromagnetic instabilities.
- We are interested in general treatments for low-frequency modes in
plasmas with different ion species, and have studied appropriate generalizations
of canonical nonlinear evolution equations, applicable to space plasmas.
The modelling of variable dust charges and its influence on plasma waves
is a recent theoretical challenge, for which much remains to be done.
