A search for substellar companions to the two nearest brown dwarf systems

WISE J104915.57-531906.1 A+B and WISE J085510.83-071442.5 were recently discovered as the third and fourth closest known systems to the Sun, respectively (2.0 and 2.3 pc). The former consists of a L8+T0.5 binary and the latter is a probable Y dwarf and is the coldest known brown dwarf (∼250 K). We present a search for common proper motion companions to these brown dwarfs using multi-epoch mid-infrared images from the Spitzer Space Telescope. We have also obtained near-infrared adaptive optics (AO) images of WISE J104915.57-531906.1 A+B with the Very Large Telescope to search for companions at smaller separations than reached by Spitzer. No new companions are detected in either system. At projected separations of 25″-420″ (50-840 AU) for WISE J104915.57-531906.1 A+B and 4″-420″ (9-970 AU) for WISE J085510.83-071442.5, the Spitzer images are sensitive to companions with and 21.9, respectively, which correspond to masses of 1 for ages of Gyr and temperatures of 150 K. The detection limit in the AO images of WISE J104915.57-531906.1 A + B is at 3″-15″ (6-30 AU), or 7 for Gyr.