Brown Dwarfs in the solar neighborhood : a high resolution study of their physical properties and the search for planetary mass companions
This thesis presents the results of an ongoing near-infrared high-resolution spectroscopic and photometric monitoring program with the VLT Natural Guide Star and Laser Guide Star Adaptive Optics systems to observe binary brown dwarfs (BD) with relatively short orbital periods. The chosen objects cover the entire spectral range from early L dwarfs, across the particularly interesting L/T transition to the late T dwarfs. For the L dwarf binary system Kelu-1 AB the results of the orbital fits are discussed, including the first dynamical mass estimate of the system. In addition, the first resolved spectra provide evidence for a third system component, making the system an excellent candidate to be the first pure BD triple system discovered so far. Photometric investigations on the L/T transition binary 2MASS0310+1648 AB revealed a very striking nature of the system, most likely resulting in the detection of the only fifth resolved L/T transition binary with a flux reversal between the components. Furthermore, extended resolved spectra of the T dwarf binary Indi Bab allowed, together with the Kelu-1 AB results, to test atmospheric and evolutionary models. The analysis seems to confirm the already previously reported trend that current models, at least partly, underestimate the masses of very low-mass stars and BDs. A spectral differential imaging program on a sample of twelve relatively young field L dwarfs in the solar neighborhood was carried out, using the HST, to search for planetary mass companions around BDs. Two BD binaries were resolved and the derived binary parameters like separations and mass ratios, as well as the overall binary fraction of the survey are consistent with the results of previous multiplicity surveys of BDs. Finally, a companion candidate to 2MASS0337-1758 is identified. First mass estimates place it at the BD/planetary mass boundary and make it the coolest and least massive resolved companion candidate to any L or T dwarf.