Preliminary Orbit and Masses of the Nearby Binary L Dwarf GJ 1001BC

We present the preliminary results of a continuing VLT program to map the orbit of the nearby binary L4.5 dwarf GJ 1001BC (LHS 102BC). Since discovering its duplicity in 2002 and 2003 using HST's NICMOS and ACS, we have obtained high-resolution images of GJ 1001BC at three epochs between October 2004 and November 2005 using the NAOS/CONICA system at VLT-UT4 (Yepun). Our HST and VLT images cover ~75% of GJ 1001BC's 4 year orbit. A least-squares fit of a Keplerian orbit yields a combined binary mass of 0.100 ± 0.026 M_sun for a tentative parallactic distance measurement of 13.0 ± 0.7 pc to the M dwarf GJ 1001A. Hypothetically assuming a 3:2 mass ratio for the nearly equal-luminosity L dwarfs, we estimate masses of 0.060 ± 0.016 M_sun and 0.040 ± 0.010 M_sun for GJ 1001B and C, respectively. If these preliminary values are sustained by our continuing orbit and parallax measurements, then GJ 1001C will be the least massive L dwarf for which a dynamical mass has been measured.