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

Abstract

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 solartype 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 -2as 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).

An extreme planetary system around HD 219828

“…These values are even lower than those found in Pace (2013) (log R HK between −4.896 and −4.880), attesting to the low activity level of this star. This low value for the activity level would correspond to a rotational period of 28.7 ± 0.6 days and 31.7 ± 0.8 days, according to the calibrations of Noyes et al (1984) and Mamajek & Hillenbrand (2008), respectively. A projected rotational velocity v sin i = 2.9 km s −1 is derived from the FWHM of the HARPS crosscorrelation function (see e.g.…”

“…((a)) Using the activity level and the calibrations of Noyes et al (1984) and Mamajek & Hillenbrand (2008). ((b)) Based on HARPS spectra using a calibration similar to the one presented by Santos et al (2002).…”

A High-Precision Near-Infrared Survey for Radial Velocity Variable Low-Mass Stars Using Cshell and a Methane Gas Cell

“…The young age of all stars in this sample is strongly constrained by kinematic membership in these young associations, typically in addition to lithium abundance measurements or comparison to isochrones (e.g., Torres et al 2008), except for the starεEridani, which is not a member of any known young moving group. In this case, the age is constrained to 400-800 Myr by Mamajek & Hillenbrand (2008). No selection cut was applied on projected rotational velocities; all stars in this sample have measured v i sin values in the literature ranging from 2 to 70 km s −1 .…”

The HARPS search for southern extra-solar planets

“…Yet this technic is less sensitive for stars with slow rotations as BD −08 • 2823. Indeed, according to Noyes et al (1984) and Mamajek & Hillenbrand (2008), the activity level for BD −08 • 2823 implies a stellar rotation period of ∼37 days. The Hipparcos data of BD −08 • 2823 include From top to bottom, the five panels show the periodograms of the radial velocities, of the FWHMs, contrasts and bisectors of the CCFs, and finally of the log R HK activity indexes.…”