Context. While M dwarfs are the most abundant stars in the Milky Way, there is still large uncertainty about their basic physical properties (mass, luminosity, radius, etc.) as well as their formation environment. Precise knowledge of multiplicity characteristics and how they change in this transitional mass region, between Sun-like stars on the one side and very low mass stars and brown dwarfs on the other, provide constraints on low mass star and brown dwarf formation. Aims. In the largest M dwarf binary survey to date, we search for companions to active, and thus preferentially young, M dwarfs in the solar neighbourhood. We study their binary/multiple properties, such as the multiplicity frequency and distributions of mass-ratio and separation, and identify short period visual binaries, for which orbital parameters and hence dynamical mass estimates can be derived in the near future. Methods. The observations are carried out in the SDSS i and z band using the Lucky Imaging camera AstraLux Sur at the ESO 3.5 m New Technology Telescope. Lucky Imaging is a very efficient way of observing a large sample of stars at an angular resolution close to the diffraction limit. Results. In the first part of the survey, we observed 124 M dwarfs of integrated spectral types M 0−M 6 and identified 34 new and 17 previously known companions to 44 stars. We derived relative astrometry and component photometry for these binary and multiple systems. More than half of the binaries have separations smaller than 1 and would have been missed in a simply seeing-limited survey. Correcting our sample for selection effects yields a multiplicity fraction of 32 ± 6% for 108 M dwarfs within 52 pc and with angular separations of 0.1 −6.0 , corresponding to projected separations of 3−180 AU at median distance 30 pc. Compared to earlytype M dwarfs (M > ∼ 0.3 M ), later-type (and hence lower mass) M dwarf binaries appear to have closer separations, and more similar masses.

Abstract. Based on high-resolution UVES spectra we found that the radial velocity (RV ) dispersion of nine of twelve known young bona fide and candidate brown dwarfs in the Cha I dark cloud is 2.0 km s −1 , i.e. significantly smaller than the RV dispersion of T Tauri stars in Cha I (3.6 km s −1 ) and only slightly larger than the dispersion of the surrounding molecular gas (1.2 km s −1 ) (Mizuno et al. 1999). This result indicates that the majority of these brown dwarfs are not ejected with high velocity out of a dense region as proposed by some formation scenarios for brown dwarfs. The mean RV values are consistent with the objects being kinematic members of Cha I. The RV dispersion of the T Tauri stars confined to the Cha I region is based on a compilation of T Tauri stars with known RV s from the literature plus three T Tauri stars observed with UVES and unpublished RV s for nine T Tauri stars. Time-resolved spectroscopy revealed RV variations for five out of nine of the bona fide and candidate brown dwarfs in Cha I, which could be due to orbiting planets or surface features. Furthermore we derived rotational velocities v sin i and the Lithium 6708Å equivalent width.

We present results of a radial velocity (RV) survey for planets and brown dwarf (BD) companions to very young BDs and (very) low-mass stars in the Cha I star-forming cloud. Time-resolved high-resolution echelle spectra of Cha Hα 1-8 and Cha Hα 12 (M6-M8), B34 (M5), CHXR 74 (M4.5), and Sz 23 (M2.5) were taken with UVES at the VLT between 2000 and 2004. The precision achieved for the relative RVs range between 40 and 670 m s −1 and is sufficient to detect Jupiter mass planets around the targets. This is the first RV survey of very young BDs. It probes multiplicity, which is a key parameter for formation in an as yet unexplored domain, in terms of age, mass, and orbital separation. We find that the subsample of ten BDs and very low-mass stars (VLMSs, M < ∼ 0.12 M , spectral types M5−M8) has constant RVs on time scales of 40 days and less. For this group, estimates of upper limits for masses of hypothetical companions range between 0.1 M Jup and 1.5 M Jup for assumed orbital separations of 0.1 AU. This hints at a rather small multiplicity fraction for very young BDs/VLMSs, for orbital separations of < ∼ 0.1 AU and orbital periods of < ∼ 40 days. Furthermore, the non-variable objects demonstrate the lack of any significant RV noise due to stellar activity down to the precision necessary to detect giant planets. Thus, very young BDs/VLMSs are suitable targets for RV surveys for planets. Three objects of the sample exhibit significant RV variations with peak-to-peak RV differences of 2−3 km s −1 . For the highest mass object observed with UVES (Sz 23, ∼0.3 M ), the variations are on time scales of days, which might be explained by rotational modulation. On the other hand, the BD candidate Cha Hα 8 (M6.5) and the low-mass star CHXR 74 (M4.5) both display significant RV variations on times scales of > ∼ 150 days, while they are both RV constant or show only much smaller amplitude variations on time scales of days to weeks, i.e. of the rotation periods. A suggested explanation for the detected RV variations of CHXR 74 and Cha Hα 8 is that they are caused by giant planets or very low-mass BDs of at least a few Jupiter masses orbiting with periods of several months or longer. Thus, the presented RV data indicate that orbital periods of companions to very young BDs and (very) low-mass stars are possibly several months or longer, and that orbital separations are > ∼ 0.2 AU. This parameter range has not been covered for all targets yet, but will be probed by follow-up observations. Furthermore, we show that the scaled down equivalent to the BD desert found around solar-like stars would be a giant planet desert around BD and VLMS primaries, if formed by the same mechanism. The present data test its existence for the targets in the limited separation range of the survey. So far, no hints of companions in a "giant planet desert" have been found.

Searches for companions of brown dwarfs by direct imaging mainly probe orbital separations greater than 3-10 AU. On the other hand, previous radial velocity surveys of brown dwarfs are mainly sensitive to separations smaller than 0.6 AU. It has been speculated that the peak of the separation distribution of brown dwarf binaries lies right in the unprobed range. This work extends high-precision radial velocity surveys of brown dwarfs for the first time out to 3 AU. Based on more than six years UVES/VLT spectroscopy the binary frequency of brown dwarfs and (very) low-mass stars (M4.25-M8) in Chamaeleon I was determined: 18 +20 −12 % for the whole sample and 10 +18 −8 % for the subsample of ten brown dwarfs and very low-mass stars (M < ∼ 0.1 M ). Two spectroscopic binaries were confirmed, the brown dwarf candidate Cha Hα 8 (previously discovered by Joergens & Müller) and the low-mass star CHXR 74. Since their orbital separations appear to be 1 AU or greater, the binary frequency at <1 AU might be less than 10%. Now for the first time companion searches of (young) brown dwarfs cover the whole orbital separation range, and the following observational constraints for models of brown dwarf formation can be derived: (i) the frequency of brown dwarf and very low-mass stellar binaries at <3 AU does not significantly exceed that at >3 AU; i.e. direct imaging surveys do not miss a significant fraction of brown dwarf binaries; (ii) the overall binary frequency of brown dwarfs and very low-mass stars is 10-30%; (iii) the decline in the separation distribution of brown dwarfs towards smaller separations seems to occur between 1 and 3 AU; (iv) the observed continuous decrease in the binary frequency from the stellar to the substellar regime is confirmed at <3 AU providing further evidence of a continuous formation mechanism from low-mass stars to brown dwarfs.

We have studied the photometric variability of very young brown dwarfs and very low mass stars (masses well below 0.2 M ) in the Cha I star-forming region. We have determined photometric periods in the Gunn i and R bands for the three M6.5-M7 type brown dwarf candidates Cha H 2, Cha H 3, and Cha H 6 of 2.2-3.4 days. These are the longest photometric periods found for any brown dwarf so far. If interpreted as rotationally induced, they correspond to moderately fast rotational velocities, which is fully consistent with their v sin i values and their relatively large radii. We have also determined periods for the two M5-M5.5 type very low mass stars B34 and CHXR 78C. In addition to the Gunn i-and R-band data, we have analyzed JHK S monitoring data of the targets, which have been taken a few weeks earlier and confirm the periods found in the optical data. Upper limits for the errors in the period determination are between 2 and 9 hr. The observed periodic variations of the brown dwarf candidates as well as of the T Tauri stars are interpreted as modulation of the flux at the rotation period by magnetically driven surface features, on the basis of a consistency with v sin i values as well as RÀi color variations typical for spots. Furthermore, the temperatures even for the brown dwarfs in the sample are relatively high (>2800 K) because the objects are very young. Therefore, the atmospheric gas should be sufficiently ionized for the formation of spots on one hand, and the temperatures are too high for significant dust condensation and hence variabilities due to clouds on the other hand. A comparison with rotational properties of older brown dwarfs shows that most of the acceleration of brown dwarfs takes place within the first 30 Myr or less. If magnetic braking plays a role, this suggests that the disk dissipation for brown dwarfs occurs between a few and 36 Myr.