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R. J. White, A. M. Ghez (UCLA)
We present a high spatial resolution UV, optical and NIR imaging survey of forty-five binary stars in Taurus-Auriga with separations of 10 - 1000 AU. These observations were carried out using the WFPC2 aboard the Hubble Space Telescope and speckle imaging at the IRTF. These measurements are used to determine the stellar and circumstellar properties of binary star components in order to understand their formation and the effects of a close companion on circumstellar material. The accretion signatures and mass accretion rates for primary stars are similar to singles stars and suggest that a companion as close as 10 AU has little effect on the mass accretion rate. Primary stars appear to have, on average, a higher accretion rate than secondary stars. The inner, truncated disks of the closest (< 100 AU) binaries are most likely being replenished from a low angular momentum, circumbinary reservoir. These circumstellar properties, in combination with coeval ages of binary stars and the distribution of mass ratios and secondary masses, support a scenario in which binary stars form via fragmentation.
A spectroscopic case study of the young quadruple GG Tau shows that the relative ages of binary stars offer a powerful test of evolutionary models and the temperature scale for young, low mass stars. The evolutionary models of Baraffe et al. (1998) are shown to yield the most consistent ages using a temperature scale intermediate between that of dwarf and giant stars. The lowest mass component of this system, GG Tau Bb, appears to be substellar with a mass of only forty-six times that of Jupiter (0.044 Msun). GG Tau Bb is currently the lowest mass, spectroscopically confirmed companion to a T Tauri star, and is one of the coldest, lowest mass T Tauri objects in the Taurus-Auriga star forming region.