CONCIERTO PARA EUFONIO

in Cha I is relatively small. The radial velocity dispersion of the brown dwarfs was determined to 2.2 km s−1 _{in comparison with that one of the surrounding}

molecular gas of 1.2 km s−1 _{and that one of T Tauri stars in the field of 3.6 km s}−1_.

This gives suggestive evidence that there is no run-away brown dwarf among the studied sample. It cannot be ruled out that some of them have a larger three- dimensional velocity dispersion. However, brown dwarfs born within the observed field and 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. This is true even for the smallest ejection velocities calculated by theorists. Therefore, the measurement of radial velocities is the very method to test if objects born in the field have significantly high velocities due to dynamical interactions during their formation process.

Very recent dynamical calculations (Bate et al. 2002, Sterzik & Durisen 2002) hint at rather small ejection velocities suggesting the possibility that the imprint of the ejection in the kinematics might not be an observable effect. They predict a velocity dispersion of ejected brown dwarfs of 2 km s−1 _{in three-dimensions and}

1.2 km s−1 _{in one-dimension. On the other hand, they predict that 10% of the}

brown dwarfs have a larger velocity than 5 km s−1 _{due to dynamical interactions.}

The velocities of the bona fide and candidate brown dwarfs in Cha I cover a total range of only 2.6 km/s. Therefore, it is concluded that the ejection-model for the formation of brown dwarfs is not a likely formation mechanism for the brown dwarfs in Cha I.

6.2

Multiplicity of brown dwarfs and

very low-mass stars in Cha I

The study of brown dwarf multiple systems and their properties, like binary frequency, mass ratio and orbital separation, provide important clues to their formation history. In addition, binaries are our only means to determine accurate masses and therefore provide a valuable test of evolutionary models. About four confirmed brown dwarf binaries (brown dwarf–brown dwarf pairs) are known to date, one of them, PPl 15 is a spectroscopic binary (Basri & Mart´ın 1999), the others are wide binaries with separations in the range of 1 to 10 AU. About the same number of candidates for brown dwarf binaries are awaiting confirmation. There is no planet known orbiting a brown dwarf. The lowest mass star with a radial velocity planet candidate is the M4-dwarf Gl 876, which has a mass of 0.3–0.4 M (Delfosse et al. 1998b).

In order to study the multiplicity of the brown dwarf population in Cha I, a ra- dial velocity survey for close spectroscopic companions as well as a deep direct

imaging survey for wide visual companions have been carried out. With these two complementary search methods, a wide range of possible companion separa- tions has been covered. The exact separations depend on the companion masses. For example, for brown dwarf companions (>13 MJup) to the targets, separations

<3 AU and between 50 and 1000 AU were covered. With more restricted separa- tions (<0.1 AU and 300–1000 AU) the surveys were sensitive also to companion masses down to 1 MJup.

The radial velocity survey was carried out with the high-resolution echelle spec- trograph UVES at the VLT. Time-resolved spectra have been taken of nine bona fide and candidate brown dwarfs, Cha Hα1–8 and 12, as well as of three very low-mass T Tauri stars, B 34, CHXR 74 and Sz 23. The radial velocities for eight out of nine observed bona fide and candidate brown dwarfs were found to be constant within the measurements uncertainties. Upper limits for the companion mass M2sinihave been derived and range for them between a tenth of a Jupiter

mass and 1–2 Jupiter masses. The very low-mass T Tauri star B 34 displays also a constant radial velocity (_≤ 70 m s−1_{) setting the upper limit for a companion}

mass M2sini to 0.3 MJup. The radial velocities for Cha Hα4 and the very low-

mas T Tauri stars CHXR 74 and Sz 23 show small (≤ 600 m s−1_{) but significant}

variations of their radial velocity, which correspond to companion masses M2sini

of 0.8 MJup, 2.6 MJup and 7 MJup, respectively. The nature of these variations has

still to be clarified. Such small amplitude variations can also be mimicked by sur- face spots as has been shown for T Tauri stars (Guenther et al. 2000). From the results of the radial velocity survey, it is concluded that spectroscopic binaries among the studied brown dwarfs in Cha I as well as close-in giant planets around them are rather rare.

The direct imaging survey for wide visual companions around Cha Hα1–12 was carried out with deep HST as well as NTT and VLT images. A companion candi- date has been found around Cha Hα5 at a separation of 1.500₍

∼240 AU). Its color is consistent with an early- to mid-L type object at the distance of the primary. However, recent follow-up spectroscopy of this companion candidate (Neuh¨auser et al. 2002b) showed that it is a highly reddened K-type background star. There are hints of binarity of Cha Hα2 from an elongated PSF in HST images. Follow- up images are planned in order to confirm or reject the binary hypothesis for Cha Hα2.

To summarize, despite the hints at Cha Hα2 being a wide binary from HST im- ages no indications for brown dwarf binaries among the twelve bona fide and candidate brown dwarfs in Cha I have been found. Furthermore, despite the vari- ations of the radial velocity of Cha Hα4, which could be due to a companion of less than 1 MJup, no evidence for low-mass companions around the twelve targets

has been found. There is the possibility that there are planetary mass compan- ions with masses below one Jupiter mass since the presented observations were

6.3. Rotation rates, surface spots and substellar angular momentum evolution93