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HR 8799 is an hF0 mA5 gamma Doradus, lambda Bootis, Vega-type star best knownfor hosting four directly imaged candidate planetary companions. Using theCHARA Array interferometer, we measure HR 8799's limb-darkened angular diameterto be 0.342 +/- 0.008 mas; this is the smallest interferometrically measuredstellar diameter to date, with an error of only 2%. By combining ourmeasurement with the star's parallax and photometry from the literature, wegreatly improve upon previous estimates of its fundamental parameters,including stellar radius (1.44 +/- 0.06 R_Sun), effective temperature (7193 +/-87 K, consistent with F0), luminosity (5.05 +/- 0.29 L_Sun), and the extent ofthe habitable zone (1.62 AU to 3.32 AU). These improved stellar propertiespermit much more precise comparisons with stellar evolutionary models, fromwhich a mass and age can be determined, once the metallicity of the star isknown. Considering the observational properties of other lambda Bootis starsand the indirect evidence for youth of HR 8799, we argue that the internalabundance, and what we refer to as the effective abundance, is most likelynear-solar. Finally, using the Yonsei-Yale evolutionary models with uniformlyscaled solar-like abundances, we estimate HR 8799's mass and age consideringtwo possibilities: 1.516 +0.038/-0.024 M_Sun and 33 +7/-13 Myr if the star iscontracting toward the zero age main-sequence or 1.513 +0.023/-0.024 M_Sun and90 +381/-50 Myr if it is expanding from it. This improved estimate of HR 8799'sage with realistic uncertainties provides the best constraints to date on themasses of its orbiting companions, and strongly suggests they are indeedplanets. They nevertheless all appear to orbit well outside the habitable zoneof this young star.

The CHARA Array Angular Diameter of HR 8799 Favors Planetary Masses for Its Imaged Companions