Dynamical Mass Estimates for Incomplete Orbits: Young Multiple Stars in Taurus and Ophiuchus

We present recent measurements of the orbital motion in the binaries DF Tau and ZZ Tau and in the triples Elias 12, T Tau, and V853 Oph. We observed these systems with the Fine Guidance Sensors on the Hubble Space Telescope and with adaptive optics imaging at the W. M. Keck and Gemini North Observatories. Based on our measurements and those presented in the literature we perform preliminary orbital analyses for DF Tau, ZZ Tau, Elias 12 Na-Nb, and T Tau Sa-Sb. Because the orbital coverage in most of these systems does not yet span a sufficient portion of the orbit, we are not able to find definitive orbit solutions. By using a Monte Carlo search technique we explored the orbital parameter space allowed by the current set of data available for each binary. We constructed weighted distributions for the total mass of the binaries derived from a large sample of possible orbits that fit the data. These mass distributions show that the total mass is already well defined. We compute total mass estimates of 0.7,0.66, 1.13, and 4.13 M for DF Tau, ZZ Tau, Elias 12 Na-Nb, and T Tau Sa-Sb, respectively, using a distance of 140 pc. For Elias 12 Na-Nb, where the orbital coverage spans ~164°, we compute a preliminary orbit solution with a period of ~9-12 yr. By including an earlier lunar occultation measurement, we also find a likely orbit solution for ZZ Tau with a period of ~32 yr. With additional measurements to continue mapping the orbits the derived dynamical masses will be useful in constraining the theoretical tracks of pre-main-sequence evolution.