2008ApJ...687.1264M

While the strong anticorrelation between chromospheric activity and age has led to the common use of the Ca II H and K emission index (R'_HK=L_HK/L_bol) as an empirical age estimator for solar-type dwarfs, existing activity-age relations produce implausible ages at both high and low activity levels. We have compiled R'_HK data from the literature for young stellar clusters, richly populating for the first time the young end of the activity-age relation. Combining the cluster activity data with modern cluster age estimates and analyzing the color dependence of the chromospheric activity age index, we derive an improved activity-age calibration for F7-K2 dwarfs (0.5 mag<B-V<0.9 mag). We also present a more fundamentally motivated activity-age calibration that relies on conversion of R'_HK values through the Rossby number to rotation periods and then makes use of improved gyrochronology relations. We demonstrate that our new activity-age calibration has typical age precision of ∼0.2 dex for normal solar-type dwarfs aged between the Hyades and the Sun (∼0.6-4.5 Gyr). Inferring ages through activity-rotation-age relations accounts for some color-dependent effects and systematically improves the age estimates (albeit only slightly). We demonstrate that coronal activity as measured through the fractional X-ray luminosity (R_X=L_X/L_bol) has nearly the same age- and rotation-inferring capability as chromospheric activity measured through R'_HK. As a first application of our calibrations, we provide new activity-derived age estimates for a volume-limited sample of the 108 solar-type field dwarfs within 16 pc.