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Knowledge of the chemical composition and absolute masses of Capella are keyto understanding the evolutionary state of this benchmark binary systemcomprising two giant stars. Previous efforts, including our own 2009 study,have largely failed to reach an acceptable agreement between the observationsand current stellar evolution models, preventing us from assessing the statusof the primary. Here we report a revision of the physical properties of thecomponents incorporating recently published high-precision radial velocitymeasurements, and a new detailed chemical analysis providing abundances formore than 20 elements in both stars. We obtain highly precise (to about 0.3%)masses of 2.5687 +/- 0.0074 and 2.4828 +/- 0.0067 solar masses, radii of 11.98+/- 0.57 and 8.83 +/- 0.33 solar radii, effective temperatures of 4970 +/- 50 Kand 5730 +/- 60 K, and independently measured luminosities based on the orbitalparallax (78.7 +/- 4.2 and 72.7 +/- 3.6 solar luminosities). We find anexcellent match to stellar evolution models at the measured composition of[Fe/H] = -0.04 +/- 0.06. Three different sets of models place the primary starfirmly at the end of the core helium-burning phase (clump), while the secondaryis known to be evolving rapidly across the Hertzprung gap. The measured lithiumabundance, the C/N ratio, and the 12C/13C isotopic carbon abundance ratio,which change rapidly in the giant phase, are broadly in agreement withexpectations from models. Predictions from tidal theory for the spin rates,spin-orbit alignment, and other properties do not fare as well, requiring a40-fold increase in the efficiency of the dissipation mechanisms in order tomatch the observations.

Capella (alpha Aurigae) revisited: New binary orbit, physical properties, and evolutionary state