Untitled - Laboratoire d'Astrophysique de l'Observatoire de Grenoble

Figure 6.8: Distribution of separations for multiple M-dwarfs of the Solar neighbourhood, compared with the
distribution of solar-type multiples.
one that produces a flat mass ratio distribution over a broad period range, and a second that only forms short
period binaries with nearly equal masses.
The multiplicity of L dwarfs
Where do free-floating brown dwarfs come from? Are they ejected stellar embryos (Reipurth & Clarke 2001, AJ,
122, 432), or do they form like “isolated” stars, by fragmentation of molecular cores? Searching for companions
around L dwarfs is the logical next step to get a complete view of multiplicity accross the mass spectrum and
answer these questions.
To search for companions to L dwarfs, a part of the DENIS sample of ultracool dwarfs (Delfosse et al. 2003)
was observed with HST/WFPC2 (Bouy et al. 2003). 15 new ultracool binaries are reported and statistical
elements determined. The observed frequency of ultracool binaries is lower than that of binaries with G-type
primaries in the separation range from 0.42 AU to 80 AU. There is also a clear deficit of ultracool binaries
with separations wider than 15 AU and a tendency for the ultracool binaries to have mass ratios near unity.
Noteworthy, this astrometric work lead to the first determination of the orbit and dynamical masses of a binary
brown dwarfs: the L dwarf 2MASSW J0746425+2000321 (Bouy et al. 2004), mentioned before in this report.
The deficit of wide ultracool dwarfs binaries reported above by Bouy et al. (2003), but also in Close et al.
(2003, ApJ, 598, 1265) and Gizis et al. (2003, AJ, 125, 3302), is frequently presented as an argument in favour
of the embryo-ejection scenario for the formation of brown dwarfs. To test this scenario, we obtained infrared
images of a very large sample of ultracool dwarfs (250 DENIS late-M and L dwarfs) to detect companion at
separation larger than 1arcsec. We detected the first wide binary for this range of mass (Billères et al. 2005,
see Fig. 6.9), which is clearly not formed by the embryo-ejection scenario. One consequence of this discovery is
that, although rare, low-mass pairs exist and brown dwarfs do not form solely by embryo ejection.
Surveys for field brown dwarfs
For the last 10 years we have used the DENIS survey to find very-late M- and L-dwarfs in the solar neighbourhood
(see for example Crifo et al. 2005; Kendall et al. 2004; Phan-Bao et al. 2003; and Delfosse et al. 2003 for
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