A near-infrared search for companions around very low luminosity young stellar objects in Taurus

We have carried out a near-infrared search for companions around 23 very low luminosity young stellar objects (YSOs) in the Taurus molecular cloud. After sophisticated photometry and analysis, five extremely low luminosity YSO (ELL-YSO) candidate companions were identified by both their near-infrared colors and proximity to the primary (separation less than 6"). They show infrared excess, as do ordinary YSOs. Their absolute J-band magnitudes range from 9 to 12 mag. The masses of these companions are estimated from their J-band luminosities, which use recent evolutionary tracks for very low mass objects. It is found that all are young brown dwarf candidates. The Ks-band magnitude difference between the ELL-YSO candidate companions and their primaries ranges from 2 to 6 mag, significantly larger than observed in T Tauri binaries. It is suggested that the companions are formed by the fragmentation of a disk around the primary. The binary frequency is 22(-10)(+9) % for systems with a period ranging between 10(7)-10(8) days. This frequency is consistent with that of T Tauri stars, but it is significantly higher than that of low-mass main-sequence stars.