Origins of Brown Dwarfs

Abstract

The formation of objects below or close to the hydrogen burning limit is currently vividly discussed and is one of the main open issues in the field of the origins of stars and planets. Applying various observational techniques, we explored a sample of brown dwarfs and very low-mass stars in the ChaI star forming cloud at an age of only a few million years and determined fundamental parameters for their formation and early evolution. Tracking the question of how frequent are brown dwarf binaries and if brown dwarfs have planets, one of the first radial velocity (RV) surveys of brown dwarfs sensitive down to planetary masses is carried out based on high-resolution spectra taken with UVES at the VLT. The results hint at a low multiplicity fraction, which is in contrast to the situation for young low-mass stars. Testing recent formation scenarios, which propose an ejection out of the birth place in the early accretion phase, we carried out a precise kinematic analysis of the brown dwarfs in our sample in comparison with T Tauri stars in the same field. This yielded the first empirical upper limit for possible ejection velocities of a homogeneous group of brown dwarfs. Rotation is a fundamental parameter for objects in this early evolutionary phase. By means of studying the line broadening of spectral features in the UVES spectra as well as by tracing rotational modulation of their lightcurves due to surface spots in photometric monitoring data, one of the first rotation rates of very young brown dwarfs have been determined. In the light of the presented observational results, the current scenarios for the formation of brown dwarfs are discussed.