Overview of Research Activities:
From Protoplanetary Disks to Planets
Dr. Mario van den Ancker
One of the biggest astronomical discoveries made in the last decade is that of planets orbiting stars other than our sun. The realization that planets of several times the mass of Jupiter could orbit their parent stars at distance less than 1 AU was at odds with the planet-formation models developed at the time. Despite 10 years of frantic development of planet-formation scenarios, the present situation is still chaotic. Several competing theories exist for the formation of planets that try to explain the diversity of observed planetary systems. At present it is not clear which of these theories -- if any -- is correct.
Current observational efforts are mainly aimed at studying present-day fully-formed planetary systems, either through their effect on their host star (radial-velocity studies, transit searches), or more recently through AO-based direct imaging studies. These observations -- although undoubtedly successful in finding additional exo-planets -- may not be able to distinguish between different formation scenarios for exoplanetary systems.Another approach is therefore needed. Since planets are expected to form out of the material in disks around young stars, the formation of a planet will thoroughly alter the structure of the circumstellar disk of their host star, forming significant gaps in the radial distribution of matter around the star. Additionally, processing of solids (annealing/crystallization) and changes in the overall gas/dust ratio are thought to happen during the planet formation process and are believed to alter the observational signature of the disk.
For several of the problems outlined above, the infrared is a natural wavelength regime to use, since it allows us to study thermal emission from dust as well as many atomic and molecular emission lines, the major coolants of the warm gas in the star forming region. The launch of the Infrared Space Observatory (ISO) marked the first opportunity to exploit the full potential of the infrared in this respect. Recent developments in space-based (the increased sensitivity offered by Spitzer) and ground-based (the unprecedented spatial resolution offered by VISIR at the VLT and MIDI at the VLTI) instrumentation are greatly expanding our observational capacities in the thermal infrared. Breakthroughs in our understanding of the evolution of dust and its possible eventual condensation into planetary systems are expected to come from this emerging scientific discipline in the near future.
Observationally, Herbig Ae/Be stars, the intermediate-mass (2--10 M) equivalents of Classical T Tauri stars, are often the preferred systems to use for these studies since they tend to be brighter and appear more spatially extended, and are thus easier to study than their low-mass counterparts. Detailed studies of individual Herbig Ae/Be star disks have shown that there is a broad diversity in their composition and disk structure. In particular the presence of amorphous/crystalline dust, the visible fraction of carbon-rich dust and the amount of flaring in the disk widely differs from system to system. However, at present we do not know whether all these observational properties arise from an intrinsically different disk structure (which would give rise to the formation of planetary systems with widely varying properties), whether we are simply seeing different evolutionary stages of essentially the same initial disk (which would give rise to a more homogeneous ensemble of planets), or a mixture of both possibilities.
To remedy this situation, I have started a program to systematically investigate the properties of a statistically significant sample of Herbig Ae/Be stars through means of observations of gas and dust in their disks. Studies that are currently under way include: