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M. Simon (SUNY-SB), S.T. Holfeltz, E.P. Nelan, G. Schaefer (STScI)
We are mapping the orbits of the pre-main sequence multiple systems DF Tau, ZZ Tau, HV Tau, Elias 12, and V853 Oph using one of HST's Fine Guidance Sensors. These observations will enable us to measure the masses of these PMS stars and thus to calibrate theories of their evolution to the main sequence. We have observed the companions of DF Tau and ZZ Tau as they have moved through more than 90 degrees in position angle, so it is now possible to fit preliminary orbits. The FGS detects the IR-discovered close tertiary in V853 Oph and finds it to be strongly variable at optical wavelengths. Our program also yields the parallaxes of these systems, so we will be able to confirm or refute the puzzling Hipparcos distances of DF Tau and BP Tau.