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The Visual Orbit and Evolutionary State of 12 Bootes

We report on the determination of the visual orbit of the double-lined spectroscopic binary system 12 Bootes with data obtained by the Palomar Testbed Interferometer in 1998 and 1999. 12 Boo is a nearly equal-mass double-lined binary system whose spectroscopic orbit is well known. We have estimated the visual orbit of 12 Boo from our interferometric visibility data fit both separately and in conjunction with archival radial velocity data. Our 12 Boo orbit is in good agreement with the spectroscopic results, and the physical parameters implied by a combined fit to our visibility data and radial velocity data result in precise component masses. In particular, the orbital parallax of the system is determined to be 27.09 $\pm$ 0.41 mas, and masses of the two components are determined to be 1.435 $\pm$ 0.023 M$_{\sun}$ and 1.409 $\pm$ 0.020 M$_{\sun}$, respectively. Somewhat remarkably, even though the two components are nearly equal mass, the system exhibits a significant brightness difference between the components in the near infrared and visible. We attribute this brightness difference to evolutionary differences between the two components in their transition between main sequence and giant evolutionary phases, and based on theoretical isochrones we can estimate a system age. Further, because the atmospheres of the two components are becoming more convective, we project the system components are currently at or near synchronous rotation, and the system orbit is in the process of circularizing.