Multiplicity of tight ultra low mass systems

A number of scenarios for the formation of brown dwarfs are currently being discussed. Binary properties constrain these scenarios. While many imaging surveys were carried out to find visuell binaries, few surveys were conducted to find short-period binaries. By deriving the frequency of short-period binary brown dwarfs, the importance of momentum transfer and tidal interactions for the formation of binary brown dwarfs can be studied. A radial velocity survey is the most efficient way to find short-period brown dwarf binaries. In this work, high-resolution spectra of 27 very-low mass stars and brown dwarfs were analyzed. The data was taken with UVES at the VLT (Paranal, Chile) in the years 2001 to 2004. Parts of this data were already analyzed by Guenther and Wuchterl (2003). For the work presented here, all data from the years 2001 to 2004 was used. Additionally, the accuracy of the radial velocity measurement was improved. Two spectroscopic binaries were confirmed. It was shown that there is no additional binary with a period of ! 40 days in this sample. By improving the accuracy of the measurements and increasing the number of spectra, the binary candidate, LHS 292, turned out not to be a binary. Thus, the binary frequency is 7.4 ± 1.4%. This frequency supports the solar-like formation scenario of very low-mass stars and brown dwarfs and it shows that momentum transfer during the formation is not of great importance. To study the effects of magnetic fields on the atmospheres of brown dwarfs, I observed the brown dwarf LP 944-20 with EFOSC2 on the 3.6 m telescope in La Silla, Chile. LP 944-20 is an ideal object for this purpose. The magnetic field strength at the surface of LP 944-20 may be nearly 1 kG, even in quiescence. There are no significant variations, neither in the equivalent widths of the H! and Na D emission lines nor the temperature. It can be concluded that LP 944-20 has a very homogeneous atmosphere. It is highly unlikely that spots or other prominent surface features exist on this object.