Paper: astro-ph/0509795
Date: Tue, 27 Sep 2005 09:01:39 GMT (269kb)

Title: A Hubble Space Telescope ACS Search for Brown Dwarf Binaries in the
Pleiades Open Cluster

Authors: H. Bouy, E. Moraux, J. Bouvier, W. Brandner, E. L. Martin, F. Allard,
I. Baraffe, M. Fernandez

Categories: astro-ph

Comments: 29 pages, 7 figures, 6 tables, accepted for publication in ApJ
\\
We present the results of a high-resolution imaging survey for brown dwarf
binaries in the Pleiades open cluster. The observations were carried out with
the Advance Camera for Surveys onboard the Hubble Space Telescope. Our sample
consists of 15 bona-fide brown dwarfs. We confirm 2 binaries and detect their
orbital motion, but we did not resolve any new binary candidates in the
separation range between 5.4AU and 1700AU and masses in the range
0.035--0.065~Msun. Together with the results of our previous study (Martin et
al., 2003), we can derive a visual binary frequency of 13.3$^{+13.7}_{-4.3}$\%
for separations greater than 7~AU masses between 0.055--0.065~M$_{\sun}$ and
mass ratios between 0.45--0.9$<q<$1.0. The other observed properties of
Pleiades brown dwarf binaries (distributions of separation and mass ratio)
appear to be similar to their older counterparts in the field.

\\ ( http://arXiv.org/abs/astro-ph/0509795 , 269kb)

Paper: astro-ph/0509798
Date: Tue, 27 Sep 2005 10:21:22 GMT (94kb)

Title: Review on low-mass stars and brown dwarfs

Authors: G. Chabrier (1), I. Baraffe (1), F. Allard (1) and P.H. Hauschildt
(2) ((1) Ecole Normale Superieure de Lyon, C.R.A.L. (2) Hamburger Sternwarte,
Hamburg)

Categories: astro-ph

Comments: Invited review, "Resolved Stellar Populations", Cancun, april 2005
\\
In this review, we examine the successes and weaknesses of modern low-mass
star and brown dwarf theory.(1) We first focus on the mechanical (equation of
state) and thermal (atmosphere) properties and on the evolution. We then
examine the current shortcomings of the theory and we discuss recent
observational analysis which have suggested discrepancies between models and
observations.(2) We then examine the stellar and brown dwarf IMF and suggest
that a power-law above the average thermal Jeans mass (about 1 Msol) rolling
over a lognormal form below this limit adequately reproduces the observations
of field and young cluster stellar and brown dwarf distributions. This yields a
reasonably accurate estimate of the stellar and brown dwarf Galactic census.
Finally (3) we argue that the combination of turbulence driven fragmentation at
large scale and gravity at small scales provides an appealing solution for the
general star and brown dwarf formation mechanism. It also provides a physical
ground for the aforementioned power-law + lognormal form for the IMF, whereas a
series of different power laws lacks such a physical motivation. At last, we
argue that the deuterium-burning limit as the distinction between stars and
planets has no physical foundation in this modern star formation scheme.
Opacity limited fragmentation extending down to a few (< 10) jupiter masses,
due to shocks, anisotropy or magnetic fields, provides a much more robust
limit, even though difficult to determine accurately. Therefore, the various
"direct" detections of exoplanets claimed recently in the literature are most
likely regular low-mass brown dwarfs and the direct detection of an extrasolar
planet remains for now elusive.

\\ ( http://arXiv.org/abs/astro-ph/0509798 , 94kb)