Knowledge of the chemical composition and absolute masses of Capella are key to understanding the evolutionary state of this benchmark binary system comprising two giant stars .
Previous efforts , including our own 2009 study , have largely failed to reach an acceptable agreement between the observations and current stellar evolution models , preventing us from assessing the status of the primary .
Here we report a revision of the physical properties of the components incorporating recently published high-precision radial velocity measurements , and a new detailed chemical analysis providing abundances for more than 20 elements in both stars .
We obtain highly precise ( \sim 0.3 % ) masses of 2.5687 \pm 0.0074 M _ { \sun } and 2.4828 \pm 0.0067 M _ { \sun } , radii of 11.98 \pm 0.57 R _ { \sun } and 8.83 \pm 0.33 R _ { \sun } , effective temperatures of 4970 \pm 50 K and 5730 \pm 60 K , and independently measured luminosities based on the orbital parallax ( 78.7 \pm 4.2 L _ { \sun } and 72.7 \pm 3.6 L _ { \sun } ) .
We find an excellent match to stellar evolution models at the measured composition of { [ Fe / H ] } = -0.04 \pm 0.06 .
Three different sets of models place the primary star firmly at the end of the core helium-burning phase ( clump ) , while the secondary is known to be evolving rapidly across the Hertzprung gap .
The measured lithium abundance , the C/N ratio , and the ^ { 12 } C/ ^ { 13 } C isotopic carbon abundance ratio , which change rapidly in the giant phase , are broadly in agreement with expectations from models .
Predictions from tidal theory for the spin rates , spin-orbit alignment , and other properties do not fare as well , requiring a 40-fold increase in the efficiency of the dissipation mechanisms in order to match the observations .