Parallaxes of Southern Extremely Cool Objects I: Targets, Proper Motions and First Results

We present results from the PARallaxes of Southern Extremely Cool objects (PARSEC) program, an observational program begun in April 2007 to determine parallaxes for 122 L and 28 T southern hemisphere dwarfs using the Wide Field Imager on the ESO 2.2m telescope. The results presented here include parallaxes of 10 targets from observations over 18 months and a first version proper motion catalog. The proper motions were obtained by combining PARSEC observations astrometrically reduced with respect to the Second US Naval Observatory CCD Astrograph Catalog, and the Two Micron All Sky Survey Point Source Catalogue. The resulting median proper motion precision is 5mas/yr for 195,700 sources. The 140 0.3deg fields sample the southern hemisphere in an unbiased fashion with the exception of the galactic plane due to the small number of targets in that region. The proper motion distributions are shown to be statistically well behaved. External comparisons are also fully consistent. We will continue to update this catalog until the end of the program and we plan to improve it including also observations from the GSC2.3 database. We present preliminary parallaxes with a 4.2 mas median precision for 10 brown dwarfs, 2 of which are within 10pc. These increase by 20% the present number of L dwarfs with published parallaxes. Of the 10 targets, 7 have been previously discussed in the literature: two were thought to be binary but the PARSEC observations show them to be single, one has been confirmed as a binary companion and another has been found to be part of a binary system, both of which will make good benchmark systems. These results confirm that the foreseen precision of PARSEC can be achieved and that the large field of view will allow us to identify wide binary systems. Observations for the PARSEC program will end in early 2011 providing 3-4 years of coverage for all targets. The main expected outputs are: more than a 100% increase of the number of L dwarfs with parallaxes; to increment in conjuction with published results to at least 10 the number of objects per spectral subclass up to L9, and; to put sensible limits on the general binary fraction of brown dwarfs. We aim to contribute significantly to the understanding of the faint end of the H-R diagram and of the L/T transition region. Subject headings: Astrometry – Stars: low-mass, fundamental parameters, distances, proper motions Observatório Nacional/MCT, R. Gal. José Cristino 77, CEP20921-400, RJ, Brasil. 2 INAF/Osservatório Astronomico di Torino, Strada Osservatório 20, 10025 Pino Torinese, Italy 3 Observatório do Valongo/UFRJ, Ladeira Pedro Antônio 43, CEP20080-090, RJ, Brasil 4 UERJ, Faculdade de Oceanografia, R. São Francisco Xavier 524, CEP20550-900, RJ, Brasil Max Planck Institute for Astronomy, Koenigstuhl 17, D–69117 Heidelberg, Germany 6 Centre for Astrophysics Research, Science and Technology Research Institute, University of Hertfordshire, Hatfield AL10 9AB INAF/Istituto di Astrofisica Spaziale e Fisica Cosmica, Bologna, Via Gobetti 101, 40129 Bologna, Italy 8 Centro Universitário Estadual da Zona Oeste, Av. Manuel Caldeira de Alvarenga 1203, CEP23070-200, RJ,