The Formation of Brown Dwarfs

In the presented work, a population of twelve very young bona fide
and candidate brown dwarfs in the ChaI star forming cloud
(ChaHa1-12) was studied observationally in terms of their kinematic
properties, the occurrence of multiple systems among them as well
as their rotational characteristics. Based on high-resolution
spectra taken for nine out of the twelve objects with UVES at the
VLT, radial and rotational velocities have been measured with high
accuracy. A kinematic study of the sample showed that their radial
velocity dispersion is relatively small (2.2 kms). It is
significantly smaller than the radial velocity dispersion of the
T~Tauri stars in the field (3.6 kms) and slightly larger than that
one of the surrounding molecular gas (1.2 kms). This result
indicates that the studied brown dwarfs are not ejected during
their formation with velocities large than ~2 kms as proposed in
recent formation scenarios. The brown dwarfs may have larger 3D
velocities. However, brown dwarfs ejected during the early
accretion phase in directions with a significant fraction
perpendicular to the line-of-sight, would have flown out of the
field a long time ago. By means of time-resolved UVES spectra, a
radial velocity survey for close companions to the targets was
conducted. In addition, a direct imaging survey for wide companions
was carried out with the WFPC camera on board the HST, with FORS at
the VLT as well as with SofI at the NTT. With these two
complementary search methods, a wide range of possible companion
separations has been covered. For brown dwarf companions (> 13
M_Jup) to the targets, separations < 3 AU and between 50 and
1000 AU were covered. With more restricted separations (< 0.1 AU
and 300-1000 AU) the surveys were sensitive also to companion
masses down to 1 M_Jup. HST images of ChaHa2 hint at a binary
system comprised of two approximately equal-mass companions with a
separation of ~30 AU. No further indications for companions have
been found in the images. Furthermore, the radial velocities of the
targets turned out to be rather constant setting upper limits for
the mass Msini of possible companions to 0.1 M_Jup to 2 M_Jup These
findings hint at a rather low (smaller or equal 10%) multiplicity
fraction of the studied brown dwarfs. Furthermore, a photometric
monitoring campaign of the targets yielded the determination of
rotational periods for ChaHa2, 3 and 6 in the range of 2.2 to 3.4
days. These are the first rotational periods for very young brown
dwarfs and among the first for brown dwarfs at all. They are
complemented by measurements of rotational velocities vsini from
UVES spectra. The observations show that brown dwarfs at an age of
1-5 Myr display surface spots like T Tauri stars and are moderately
fast rotators in contrast to rapidly rotating old brown dwarfs
consistent with them being in an early contracting stage. A
comparison with rotational periods from the literature indicates
that most of the acceleration of brown dwarfs takes place in the
first 30 million years or less of their lifetime.