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Measured Diameters of 2 F-stars in the Beta Pic Moving Group

We report angular diameters of HIP 560 and 21547, two F spectral type pre-main sequence members of the beta Pic Moving Group. We used the East-West 314-m long baseline of the CHARA Array. The measured limb-darkened angular diameters of HIP 560 and 21547 are 0.492+-0.032 and 0.518+-0.009 mas, respectively. The corresponding stellar radii are 2.1 and 1.6 Rsun for HIP 560 and HIP 21547 respectively. These values indicate that the stars are truly young. Analyses using the evolutionary tracks calculated by Siess, Dufour, and Forestini and the tracks of the Yonsei-Yale group yield consistent results. Analyzing the measurements on an angular diameter vs color diagram we find that the ages of the two stars are indistinguishable; their average value is 13+-2 MY. The masses of HIP 560 and 21547 are 1.65+-0.02 and 1.75+-0.05 Msun, respectively. However, analysis of the stellar parameters on a Hertzsprung-Russell Diagram yields ages at least 5 MY older. Both stars are rapid rotators. The discrepancy between the two types of analyses has a natural explanation in gravitational darkening.