Mario van den Ancker
European Southern Observatory
Karl-Schwarzschild-Strasse 2
D-85748 Garching bei München
Phone: +49 89 3200 6602
Fax: +49 89 3200 6358

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_sun) 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:

  • A Spitzer spectroscopic survey of disks around intermediate-mass stars of various ages. Goal of this study is to extend the time-line of the evolution/processing of solid-state material started by ISO to more evolved -- and hence fainter -- systems. Spitzer spectroscopic data has been obtained for ~60% of the stars in our sample; data-processing is on-going. Ground-based follow-up with VISIR on the VLT has been obtained. Collaborators: Jeroen Bouwman (MPIA), Thomas Henning (MPIA), Roy van Boekel (MPIA), Rens Waters (Univ. of Amsterdam).

  • An investigation of the December 2003 FU Orionis outburst in Orion using FORS2 and NACO at the VLT and TIMMI2 at the ESO 3.6m telescope. FU Orionis events are short-lived events in which a protoplanetary disk is seen to increase its accretion rate by up to eight orders of magnitude. During these times, the material in the disk is heated up to temperatures that are otherwise unattanaible, and may experience significant thermal processing. It is believed that common chondritic material in our own solar system may have been formed during such a ``hot phase'' in the sun's disk. Collaborators: Davide Fedele (ESO), Monika Petr-Gotzens (ESO), Nancy Ageorges (ESO).

  • Studies of the gas content of protoplanetary disks in Herbig Ae/Be systems. Although clues to the evolution of disks have so far mainly come from its dust contents, the bulk of the disk mass will be gaseous -- mainly in the form of H2. It is this bulk disk mass which may be directly relevant for distinguishing between different planet formation scenarios. However, compared to the dust, the gas will be much harder to study. All of the low-lying transitions of the dominant constituent, molecular hydrogen, are located at mid-infrared wavelengths which makes them excruciatingly hard to observe. VISIR at the VLT holds the promise of being the first instrument that offers the combination of spectral resolution, spatial resolution and sensitivity that will allow us to study the gas content of protoplanetary disks in detail. As a starting point to this study, we are currently studying the contents of other gaseous components of protoplanetary disks through near-infrared spectroscopy (sensitive to CO), and optical spectroscopy of background stars seen through a pole-on disk surrounding a young star (sensitive to a host of low-ionization species). Collaborators: Andrés Carmona (MPIA), Thomas Henning (MPIA), Wing-Fai Thi (Univ. of Amsterdam), Miwa Goto (MPIA).

  • An ultra-high spectral resolution survey of the 6300 Å [O I] line in Herbig Ae/Be stars. In Herbig Ae/Be stars this non-thermal line is formed as a by-product of the photo-dissociation of OH in the surface layer of the disk. Spatially resolved ultra-high spectral resolution spectroscopy may reveal variations in the disk's surface density -- possibly providing direct evidence for the existence of disk gaps and hence on-going planet formation. Collaborators: Gerrit van der Plas (ESO), Bram Acke (Catholic University Leuven), Kees Dullemond (MPIA Heidelberg).