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

masses up to 10 solar masses for a number of clusters and found that the mass distribution over this range can
be described by a single lognormal functional form. Second, we showed that the cluster’s MF is invariant, i.e.,
the same for all the regions we studied, which suggest that the mass distribution of stars and brown dwarfs does
not depend much on environmental conditions at the epoch of formation. These results are now reasonably
accounted for by new models of star (and brown dwarf) formation which rely on the fragmentation of molecular
clouds resulting from MHD supersonic turbulence.
Star formation models also predict a minimum mass for fragmentation of order of 3-8 Jupiter masses. Hence,
the Initial Mass Function (IMF) is expected to have a lower limit at this scale. While the sensitivity of the
optical surveys we have performed so far reached about 30 Jupiter masses, the advent of infrared mosaic cameras
will now allow us to detect brown dwarfs in young clusters down to a few Jupiter masses. We have therefore
submitted a new large program at CFHT using WIRCAM, built in part at LAOG, in order to probe the low
mass end of the mass function of young clusters, in the range from a few to 30 Jupiter masses. If granted,
the surveys will start in 2006 and last for several semesters, thus completing our current results obtained from
optical surveys (CFHT 12K, MEGACAM) and providing new clues to the formation and dynamical evolution
of free-floating planetary mass objects in young clusters.
As most of the collaborators involved in the optical surveys over the last few years will also participate to
the new IR surveys (Cambridge, Arcetri, Potsdam, Cardiff, Kiel, Madrid, Canarias, Caltech, CFHT), we very
recently applied for another FP6 RTN which will focus, at least in part, on the investigation of the extremes
(upper and lower ends) of the IMF.
In parallel, we have embarked in the modelling of the dynamical evolution of the lowest mass populations of
star forming regions and young open clusters. Preliminary results obtained from numerical simulations using Nbody
codes (Moraux & Clarke 2005; Kroupa et al. 2003) suggest that the various competing scenarios of brown
dwarf formation (collapse, ejection, etc.) may yield distinct cinematical signatures at young ages. Additional
numerical simulations using the most recent N-body codes (in collaboration with Aarseth, Cambridge) will
refine the predicted dynamical evolution of young brown dwarfs in clusters, which we shall then be able to
confront to the results of our CFHT large program.
6.4.3 The low-mass population of the solar neighbourhood
The multiplicity of M dwarfs
With the goals of measuring very accurate dynamical masses and estimating the multiplicity fraction of the very
low mass stars, we carried large programmes on nearby M-dwarfs. These programmes are needed to constrain
star formation theories (multiplicity) and stellar physics (evolutionary tracks, mass-luminosity relations). By
doing so, we uncovered more than 20 new companions (Beuzit et al. 2003; Forveille et al. 2004; 2005).
Multiplicity properties are fairly well known for solar type stars (Duquennoy & Mayor 1991, A&A, 248,
485; Halbwachs et al. 2003, A&A, 397, 159), but it is not the case for the lower main sequence. To date no
determination of the orbital element distribution for M-dwarf binaries has been published. However, multiplicity
fractions ranging from 25% (Leinert et al. 1997, A&A, 325, 159) to 42% (Fischer & Marcy 1992, ApJ, 396, 178)
have been announced.
Our combination of radial velocity and adaptive optics imaging on M dwarfs is the most complete survey
to date for stellar companions in the solar neighbourhood. Our results (Marchal et al. 2003; Marchal et al.
2005 in prep) combined with those of Hinz et al. (2002, AJ, 123, 2027) for very wide binaries enabled us to
derive the multiplicity of M dwarfs. The main results are: (1) a stellar multiplicity rate of 26 ± 3%; (2) M- and
G-dwarfs have very similar separation distributions for separations under 10 a.u. (see Fig. 6.8), (3) at wider
separations M dwarfs have a large companion deficit relative to G dwarfs (see Fig. 6.8), (4) for both M- and
G-dwarfs, the mass ratio distribution is a function of period, (5) similarly to G dwarfs, brown dwarfs are very
rare within ∼100 a.u. of M dwarfs (the “so-called” brown dwarf desert).
These results favor a scenario where the G and M dwarfs form with identical multiplicity properties. The
deficit of M dwarfs at large separations being due to preferential breakup of the looser sytems via dynamical
interactions and orbital decay in compact cluster (Sterzik & Durisen 1998, A&A, 339, 95). The contrasting mass
ratio distributions at small and large orbital separations suggest that two distinct formation modes operate,
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