Capella (α Aurigae) Revisited: New Binary Orbit, Physical Properties, and Evolutionary State

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 (∼0.3%) masses of 2.5687 ± 0.0074 {M}_⊙ and 2.4828 ± 0.0067 {M}_⊙ , radii of 11.98 ± 0.57 {R}_⊙ and 8.83 ± 0.33 {R}_⊙ , effective temperatures of 4970 ± 50 and 5730 ± 60 K, and independently measured luminosities based on the orbital parallax (78.7 ± 4.2 {L}_⊙ and 72.7 ± 3.6 {L}_⊙ ). We find an excellent match to stellar evolution models at the measured composition of [{Fe}/{{H}}]=-0.04+/- 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 ¹²C/¹³C 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.