Testing Stellar Models with an Improved Physical Orbit for 12 Bootis

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Abstract
In a previous paper we reported on the binary system 12 Bootis and its evolutionary state. In particular, the 12 Boo primary component is in a rapid phase of evolution; hence accurate measurement of its physical parameters makes it an interesting test case for stellar evolution models. Here we report on a significantly improved determination of the physical orbit of the double-lined spectroscopic binary system 12 Boo. We have a 12 Boo interferometry data set spanning 6 yr with the Palomar Testbed Interferometer, a smaller amount of data from the Navy Prototype Optical Interferometer, and a radial velocity data set spanning 14 yr from the Harvard-Smithsonian Center for Astrophysics. We have updated the 12 Boo physical orbit model with our expanded interferometric and radial velocity data sets. The revised orbit is in good agreement with previous results, and the physical parameters implied by a combined fit to our visibility and radial velocity data result in precise component masses and luminosities. In particular, the orbital parallax of the system is determined to be 27.72 ± 0.15 mas, and masses of the two components are determined to be 1.4160 ± 0.0049 and 1.3740 ± 0.0045 M☉, respectively. These mass determinations are more precise than those in the previous report by a factor of 4-5. As indicated in the previous publication, even though the two components are nearly equal in 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 models, we can estimate a system age of approximately 3.2 Gyr. Comparisons with stellar models suggest that the 12 Boo primary may be just entering the Hertzsprung gap, but that conclusion is highly dependent on details of the models. Such a dynamic evolutionary state makes the 12 Boo system a unique and important test for stellar models.

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