Origins of Brown Dwarfs

Astrophysics, abstract
astro-ph/0501220

From: Viki Joergens [view email]

Date: Wed, 12 Jan 2005 16:42:29 GMT   (192kb)

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.

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