Search for stellar systems with large separation

Most stars of our Galaxy have a stellar companion. The characteristics of those multiple systems in terms of separation and relative mass, provide important information about the stellar formation. It is observed that the binarity frequency depends on the spectral type of the main star (or equivalently on its mass), with less massive stars being more often single stars, but this is not well understood, especially for the least massive stars and the so-called brown dwarfs: objects not massive enough to burn hydrogen. More observational constraints are required.

Here we propose to search for new wide companions in the Solar neighbourhood: most stellar pairs are barely resolved from the ground, but some are widely separated, by dozens of arcseconds  (the full Moon is 1800 arcseconds in diameter) and hundreds of astronomical units (the companion is much more distant from the main star than Pluto from the Sun). 

Wide binaries can then be used to constrain the stellar formation models; to study the evolution of stars and brown dwarfs with time (because they have the same age and composition); to constrain the density for massive black holes (which would destroy wide binaries). 

All stars close to the Sun move on the celestial sphere. The bounds between two wide companions are best recognised by checking that they both move at the angular velocity (called proper motion) and in the same direction. The student would search existing catalogues of proper motions (especially from the Sloan Digital Sky Survey) for pairs; select the most interesting pairs (low mass stars and brown dwarfs, very wide binaries) and reject chance or spurious associations. The student would also search for companions of known brown dwarfs using the new UKIDSS near infrared catalogue.

Most of the programs required for this search are already available and the search for new pairs could start very quickly, any time (except in August). Some experience in astronomy, computer  programming and the UNIX system would be useful, but not required. The student would be mentored by Dr. Bertrand Goldman (preferably in English) and Dr. Viki Joergens (also in German). For more information contact B. Goldman: goldman_at_mpia.de; tel. 06221-528-260.