Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO)
Our research field is the behaviour of gases in the interstellar space, i.e. astrophysical hydrodynamics. Most matter in the universe is in the form of gas, and the dynamics of gases determines the formation of planets, stars, and galaxies. Motions in cosmic gases are often supersonic, leading to the formation of shocks. The gas can couple to radiation, either by absorbing or emitting it, and the presence of small dust particles also influence the dynamics and thermal state of the gas. All of these processes have different time and length scales, which requires the use of sophisticated numerical approaches, such as adaptive mesh refinement, especially when studying them in three dimensions. This program proposal combines several projects within our research group at the Sterrewacht Leiden. The projects are: 1. Extragalactic jets and the intergalactic medium. 2. Structure and evolution of dwarf galaxies. 3. Dynamics of dust-driven stellar winds. 4. Stellar wind bubbles. 5. Young planets interacting with disks. - Ad 1. Extragalactic jets and the intergalactic medium. A fraction of observed galaxies posess active nuclei, in which an accretion disk around the central black hole produces a powerful outflow or jet. This jet makes its way through the galaxy and beyond it, pushing out of the way the material inside and around the galaxy. This interaction of the jet with the intergalactic medium plays an important role in determining the state of the gas in galaxy clusters. The gas inside the galaxy may be severely disturbed by the jet. One suspected consequence of jet activity is the triggering of the formation of stars, by compressing material in the shock waves surrounding the jet. We study this process both in detail, and on larger scales. We use both 2 and 3-dimensional models, adding the effects of radiative losses, and background heating. - Ad 2. Structure and evolution of galaxies. One of the major problems in astrophysics is the formation and subsequent evolution of galaxies. In this project we study the processes responsible for shaping current day galaxies. Essential for this is the realistic modelling of the interstellar medium contained in the galaxy, as well as star formation and stellar feedback. We model the dynamics of the gas and the stars in model galaxies in 3 dimensions, and follow the changes induced by the processes of star formation and feedback from stellar winds and supernovae. - Ad 3. Dynamics of dust-driven stellar winds. The massive winds of Asymptotic Giant Branch (AGB) stars are the most important contributor to the replenishment of the interstellar medium, which is a prerequisite for star formation. These winds often strongly influence the optical appearance of these stars, eventually turning them into pure infrared objects. Most probably, AGB star winds are driven by a combination of stellar pulsation and dust formation (e.g. Winters et al. 2000, A&A 361, 641). Newly formed dust grains are accelerated by radiation pressure and drag the gas outwards. Previous models have assumed that all the relevant chemical, radiative and dynamical processes occur in a stable, spherically symmetry way. However, recent observations and theoretical investigations (Woitke 2000, A&A 358, 665) have indicated that these winds can be highly irregular and unstable, with chaotic, event-like dust formation epochs in only limited regions above the stellar surface. Therefore, we are building multi-dimensional models for AGB star winds, which requires to couple hydrodynamics to chemistry, dust formation and radiative transfer. Our aim is to understand the various instabilities and structure formation processes in these outflows and to make qualified interpretations of high angular-resolution observations of AGB stars. - Ad4. Stellar wind bubbles. Both low and high mass stars produce during parts of their life stellar winds. These winds interact with the surrounding environment leading to the formation of so-called stellar winds bubbles. Surprisingly, the nebulae resulting from this expulsion phase are rarely spherical. More often they show a pronounced bipolar or multipolar shape. These seemingly enigmatic forms can be reproduced by a model in which the wind is interacting with a warped disk surrounding the star. We study the interaction of the stellar wind with these warped disks in two and three dimensions. - Ad 5. Young planets interacting with discs. The formation of a planetary system around a young star takes places in the accretion disk surrounding the star. This disk consists of gas and dust. The disk not only supplies the material for building the planets, but the after forming the planets also gravitationally interact with the disk, which changes their orbits, and may also grow by accreting material form the disk. All of these processes are directly relevant for understanding our own solar system, as well as the planetary systems around other stars, of which some 100 are now known. We concentrate on the coupling of the between the young planet and the gas/dust disk. The gravitational forces from the newly formed planet opens up a gap in the disk. The opening of the gap changes the accretion onto the planet, and by breaking the symmetry, make produce a torque on the planet, changing its orbit. The gap itself is interesting in that it is easier to detect than the planet itself, especially in the radiation produced by the dust particles.