Infrared spectrum and proper motion of the brown dwarf companion of HR 7329 in Tucanae

Abstract

Up to now only four brown dwarf companions to normal stars have been found and confirmed by both spectroscopy and proper motion (namely Gl 229 B, G 196-3 B, Gl 570 D, and CoD$-33 ^{\circ} 7795$ B). On the basis of an optical spectrum taken with HST/STIS Lowrance et al. (2000) recently pointed out another possible candidate companion. The companion candidate is located at a distance of $4^{\prime \prime}$ from the A0-star HR 7329, which is considered as a member of a moving group of young stars in Tucanae located at a distance of only ∼ 48 pc. In order to confirm or disregard the companion nature of the candidate, we have determined the proper motion of the brown dwarf candidate with an epoch difference of 1.8 years, and found that it is consistent with a co-moving companion of HR 7329. Additional to the proper motion measurement, we have also taken an H -band spectrum using ISAAC on the ESO-VLT. From this spectrum, we conclude that the companion candidate has spectral type M 7 to M 8, which is in agreement with the optical spectrum. We thus conclude that HR 7329 B is most likely a brown dwarf companion. The mass ratio of this pair (A0 to M 7-8, i.e. ∼ 100:1) is the largest known among brown dwarf companions, which is relevant for studying the formation of brown dwarfs as companions.