The nearby late M star Gliese 569B was recently found by adaptive optics im
aging to be a double with separation similar to1 AU. To explore the orbital
motion and masses, we have undertaken a high-resolution (similar to0"05) a
strometric study. Images were obtained over 1.5 yr with bispectrum speckle
interferometry at the 6.5 m Multiple Mirror Telescope (MMT) and 6 m Special
Astrophysical Observatory telescope. Our data show motion corresponding to
more than half the orbital period and constrain the total mass to be great
er than 0.115 M-circle dot, with a most probable value of 0.145 M-circle do
t. Higher masses cannot be excluded without more extended observations, but
from statistical analysis we find an 80% probability that the total mass i
s less than 0.21 M-circle dot.
An infrared spectrum of the blended B double obtained with the MMT has been
modeled as a blend of two different spectral types, chosen to be consisten
t with the measured J- and K-band brightness difference of a factor of simi
lar to2. The blended fit is not nearly as good as that to a pure M8.5+ temp
late. Therefore, we hypothesize that the brighter component likely has two
unresolved components with near equal masses, each the same as the fainter
component.
If G1 569B is a triple, our dynamical limits suggest each component has a m
ass of 50(-4)(+23) M-jup. We infer an age for the system of 300 Myr from it
s kinematic motion, which places it as a member of the Ursa Major moving gr
oup. All the above parameters are consistent with the latest DUSTY evolutio
n models for brown dwarfs.