Studies of multiple stellar systems - IV. The triple-lined spectroscopic system Gliese 644

We present a radial velocity study of the triple-lined system Gliese 644 an d derive spectroscopic elements for the inner and outer orbits with periods of 2.965 5 and 627 d. We also utilize old visual data, as well as modem sp eckle and adaptive optics observations, to derive a new astrometric solutio n for the outer orbit. These two orbits together allow us to derive masses for each of the three components in the system: M-A = 0.410 +/-0.028 (6.9 p er cent), M-Ba = 0.336 +/-0.016 (4.7 per cent), and M-Bb = 0.304 +/-0.014 ( 4.7 per cent) M-circle dot. We suggest that the relative inclination of the two orbits is very small. Our individual masses and spectroscopic light ra tios for the three M stars in the Gliese 644 system provide three points fo r the mass-luminosity relation near the bottom of the main sequence, where the relation is poorly determined. These three points agree well with theor etical models for solar metallicity and an age of 5 Gyr. Our radial velocit ies for Gliese 643 and vB 8, two common proper motion companions of Gliese 644, support the interpretation that all five M stars are moving together i n a physically bound group. We discuss possible scenarios for the formation and evolution of this configuration, such as the formation of all five sta rs in a sequence of fragmentation events leading directly to the hierarchic al configuration now observed, versus formation in a small N cluster with s ubsequent dynamical evolution into the present hierarchical configuration.