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A. M. Ghez (UCLA), R. J. White (U. Texas), D. J. Sand (Caltech)
We present the results of a speckle imaging K[2.2 micron]-band survey conducted with the W. M. Keck 10-m telescope of 44 of the lowest mass T Tauri stars in Taurus-Auriga. Twenty-three companion stars are identified with separations ranging from 0."027 to 7."2 and magnitude differences of up to 5.5; 17 of these are new detections. All companions should have been detected over the angular separation range of 0."05 to 2."6 (7 - 360 AU) and the magnitude difference range of 0.0 to 2.75. Within this region, the companion star fraction is 0.27 +- 0.08, which is somebut lower but statistically comparable to that found in surveys that focused on the somewhat more massive (approx. solar mass) stars 0.45 +- 07. An analysis of the magnitude difference distribution, which is a proxy of mass ratio, as a function of separation suggests that there is a separation dependence. Splitting the sample of binary stars in half (at approx. 20 AU), we find that the closer pairs have a much higher fraction of equal mass systems. These results are consistent with models of binary star formation based on fragmentation followed by accretion.
Follow-up observations have measured the orbital motion of the closest binary star pairs. Continued monitoring over the next few years should provide exact total masses for these systems, some of which appear to be sub-stellar, and will thereby add important additional constraints for the wide array of existing evolutionary models, which are particularly uncertain towards lower masses.
This work was supported a grant number NAG-6975 from NASA's Origins of Solar Systems program.
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