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While the strong anti-correlation between chromospheric activity and age hasled to the common use of the Ca II H & K emission index (R'_HK = L_HK/L_bol) asan empirical age estimator for solar type dwarfs, existing activity-agerelations produce implausible ages at both high and low activity levels. Wehave compiled R'_HK data from the literature for young stellar clusters, richlypopulating for the first time the young end of the activity-age relation.Combining the cluster activity data with modern cluster age estimates, andanalyzing the color-dependence of the chromospheric activity age index, wederive an improved activity-age calibration for F7-K2 dwarfs (0.5 < B-V < 0.9mag). We also present a more fundamentally motivated activity-age calibrationthat relies on conversion of R'_HK values through the Rossby number to rotationperiods, and then makes use of improved gyrochronology relations. Wedemonstrate 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 accountsfor some color-dependent effects, and systematically improves the age estimates(albeit only slightly). We demonstrate that coronal activity as measuredthrough the fractional X-ray luminosity (R_X = L_X/L_bol) has nearly the sameage- and rotation-inferring capability as chromospheric activity measuredthrough R'_HK. As a first application of our calibrations, we provide newactivity-derived age estimates for the nearest 100 solar-type field dwarfs (d <15 pc).

Improved Age Estimation for Solar-Type Dwarfs Using Activity-Rotation Diagnostics