1. Introduction: Brown dwarfs as companions
Despite extensive imaging surveys (e.g. Oppenheimer et al. 2000), only three brown dwarfs were confirmed so far by both spectroscopy and proper motion as companions to normal stars: Gl 229 B (Nakajima et al. 1995, Oppenheimer et al. 1995), G 196-3 B (Rebolo et al. 1998), and Gl 570 D (Burgasser et al. 2000). A few more candidates were presented, GG Tau Bb (White et al. 1999), CoD-33°7795 B (Lowrance et al. 1999, henceforth L99; Webb et al. 1999, W99), and HR 7329 B (Lowrance et al. 2000), but either spectroscopy or proper motions were not available. Brown dwarfs and L-dwarfs can also have companions (Basri & Martín 1999, Martín et al. 1999). Radial velocity surveys yielded a large number of planet candidates, but only few brown dwarfs are among them, e.g. HD 10697 (Zucker & Mazeh 2000).
Because young objects are still relatively luminous due to ongoing accretion and/or contraction (Burrows et al. 1997, Brandner et al. 1997, Malkov et al. 1998), imaging surveys for sub-stellar objects in star forming regions or as companions to isolated young nearby stars should be more fruitful. E.g., L99 and W99 found a faint object called CoD-33°7795 B just north of the isolated M1.5-type T Tauri star CoD-33°7795 A (Gregorio-Hetem et al. 1992), a kinematic member of the nearby TW Hya association (TWA, see Kastner et al. 1997). The Hipparcos satellite obtained the parallaxes of four out of 14 TWA members, so that we can assume the mean distance of those four stars for the other stars not observed by Hipparcos (including CoD-33°7795 A), namely pc.
The companion candidate CoD-33°7795 B is mag fainter than the primary star in the infrared, and its IHJK colors are consistent with spectral type M8 to M8.5 (L99, W99). Based on its colors, its small separation from CoD-33°7795 A, and its galactic latitude, it was concluded that this object could well be a brown dwarf companion, but neither a spectrum nor the proper motion were available for corroboration. Weintraub et al. (2000) presented additional HST NICMOS narrow-band filter photometry, also consistent with a young late M-type brown dwarf, but the epoch difference (0.2 yrs) between their and previous images were not sufficient for obtaining the proper motion of CoD-33°7795 B.
© European Southern Observatory (ESO) 2000
Online publication: August 23, 2000