Parallaxes of ultra cool brown dwarf calibrators.

Brown dwarfs are extremely important objects for our understanding of stellar and planetary formation and evolution. Lying at the limit of the coolest stars, they are poorly understood and the atmospheric models of are not yet reliable. Beyond this population, very cool brown dwarfs (Teff˜300 K) were recently discovered constituting new spectral classes. A precise distance is the fundamental parameter to be able to correctly derive their masses, ages and thus calibrate the atmospheric models of this type of object. In 2012 we started an observational program to measure the trigonometric parallax of 5 such extreme objects recently detected prototypes of cool brown dwarfs. Observations started at the 4.1m SOAR/Spartan telescope and will continue until the end of 2014. Here we present an overview on the development of this project