Star Formation

We study the evolution and structure of circumstellar discs, which are (theoretically) a natural by-product of the star-formation process and in which planets are expected to form. Besides the classical techniques of imaging and spectroscopy (Meeus et al., 2003) from the infrared to the millimeter regime, our studies make use of optical long-baseline interferometry. Instruments like AMBER (near-infrared) and MIDI (mid-infrared) allow a detailed investigation of the circumstellar material with an up to now unprecedented spatial resolution (van Boekel et al., 2006). Of special interest in this context are the so-called "infrared companions", i.e. stars within binary systems that are probably surrounded by a massive disk that extincts the light at visual and sometimes even near-infrared wavelengths (Correia et al., 2006). Also our studies of the properties and composition of dust in circumstellar disks around different types of young objects (Schuetz et al., 2005) benefit from interferometric observations, because the dust can now be investigated with MIDI not only for the whole disks, but even for different regions therein.

The mass is probably the most important parameter for the structure and evolution of a star. Therefore, empirical mass determinations are crucial for our understanding of stellar astrophysics. In particular, this is the case for low-mass pre-main-sequence stars. Here the results of the evolutionary models predicting the mass differ significantly from each other. So a comparison of their predictions with empirical results is very desirable to find out which model describes the reality best (Steffen et al., 2001). From orbit determinations of visual binaries the combined mass of the components can be derived if the distance to the object is known. We are conducting a program to detect and follow orbital motion in T Tauri, e.g. T Tau itself (Köhler & Ratzka, 2006), and nearby main-sequence M dwarf binary and multiple systems. Despite the fact that visual orbit determinations require a very long effort, there is no short-time substitute for this method for a dynamical mass determination.

The frequency of binary stars and the distribution of their periods and mass ratios is - along with the Initial Mass Function - one of the key observable outcomes of the star formation process. The main reason for this is the fact that the initial binary statistics is practically frozen after the formation phase and provides us with a fossil record of star formation. Studies of binaries among nearby solar-type main-sequence stars show that about 50% of the stars are binary or multiple systems. Our speckle-survey of low-mass pre-main-sequence stars in the Ophiuchus molecular clouds (Ratzka et al., 2005) found a similar result and surveys of Taurus-Auriga revealed a frequency as high as 80-100%. This shows that binaries are the rule and not the exception in the outcome of star formation. Another important probe for star formation scenarios are the frequency and properties of high-order multiple systems (triples, quadruples, etc.) which was the objective of an investigation by means of an adaptive optics survey with the VLT/NACO system (Correia et al., 2006; a nice example of a triple system is SR24 at the Ophiuchus region; for a resolved quadruple system see J4872 in Taurus).

We also study the low-mass initial mass function in massive star-forming regions, such as 30 Doradus in the Large Magellanic Cloud (LMC). Our methods include optical and near-IR imaging of the related fields from which photometrical data for each source can be derived. These data are subsequently compared with theoretical isochrones to obtain the initial mass function (IMF).

Brown dwarfs and low mass stars close to the hydrogen burning limit. This study involves the determination of the lower-mass end of the stellar mass function in star-forming regions, such as the Trapezium Cluster in Orion. As in the case of the study in 30 Dor, our methods include optical and near-IR imaging of the related fields from which photometrical data for each source can be derived. These data are subsequently compared with theoretical isochrones to obtain the initial mass function (IMF). IR molecular bands of water are used for the determination of spectral types of the brown dwarf candidates (Meeus et al., 2005). IR photometry and astrometry has been combined with deep X-ray studies in the Orion Ultradeep Project, to study the X-ray behaviour of Brown dwarfs in the Orion Nebular Cluster (ONC).

The stellar and substellar content of the Solar neighbourhood. A new high proper motion survey and its cross-correlation with near-infrared sky surveys is used as an effective tool for improving our knowledge on nearby stars. Within 10 pc from the Sun, there are still more than 25% of the stars and probably 95% of the brown dwarfs missing! Newly identified low-mass stellar neighbours (e.g. LHS 2090 at 6pc), ultracool subdwarfs (e.g. this visitor from the Galactic halo), and brown dwarfs (the nearest discovery: Epsilon Indi B, later resolved as a binary brown dwarf) serve as templates for more distant members of their class and are good candidates for different planet search methods.

Activity phenomena of young protostars, such as jets and outflows which are indicative of on-going mass loss. A range of observational techniques and theoretical modelling are applied for this study.