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

• DOI: 10.1086/591785
• Publication Date: 2008-10-31T22:48:58Z
• Authors: Eric E. Mamajek; Lynne A. Hillenbrand
• Keywords: Astrophysics (astro-ph); 0103 physical sciences; FOS: Physical sciences; Astrophysics; 01 natural sciences; 520; stars: activity — stars: chromospheres — stars: coronae — stars: fundamental parameters — stars: rotation — X-rays: stars

While the strong anti-correlation between chromospheric activity and age has led to the common use of the Ca II H & 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 < 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 the nearest 100 solar-type field dwarfs (d < 15 pc).<br/>78 pages, 15 figures, ApJ, in press. Files can also be downloaded from http://www.cfa.harvard.edu/~emamajek/cahk/<br/>