2009/06/04 by Guillermo Torres, A. Claret, Antonio Claret +2 · 5 citations
Physics and Astronomy · #Astro and Planetary Science #Astronomy #Astrophysics #Astrophysics and Star Formation Studies #Binary number #Binary star #Giant star #Mass ratio #Metallicity #Orbit (dynamics) #Orbital elements #Orbital inclination #Orbital period #Parallax #Physics #Radial velocity #Rotation period #Stars #Stellar evolution #Stellar, planetary, and galactic studies #astro-ph.SR
paper · pdf · doi:10.1088/0004-637x/700/2/1349
published as Astrophys.J.700:1349-1381,2009 · 42 pages in emulateapj format, including figures and tables. To appear in the 2009 August 10 issue of The Astrophysical Journal
arxiv created 2009/06/04 · openalex publication_date 2009/07/13 · arxiv updated 2009/12/01 · openalex created_date 2016/06/24 · openalex updated_date 2026/08/05
We report extensive radial velocity measurements of the two giant components of the detached, 104 day period binary system of Capella. Our highly accurate three-dimensional orbital solution based on all existing spectroscopic and astrometric observations including our own yields much improved masses for the primary and secondary of 2.466 ± 0.018 M ☉ and 2.443 ± 0.013 M ☉ , with relative errors of only 0.7% and 0.5%, respectively. The mass ratio is considerably closer to unity than previously believed, which has an impact on assessing the evolutionary state of the system. Improved values are presented also for the radii (11.87 ± 0.56 R ☉ and 8.75 ± 0.32 R ☉ ), effective temperatures (4920 ± 70 K and 5680 ± 70 K), and luminosities (79.5 ± 4.8 L ☉ and 72.1 ± 3.6 L ☉ ). The distance is determined to be 13.042 ± 0.028 pc, based on the accurate orbital parallax. The projected rotational velocities and individual rotation periods are also known. Capella is unique among evolved stars in that, in addition to all of the above, the chemical composition is known as well. This includes the overall metallicity [m/H], the carbon isotope ratio 12 C/ 13 C for the primary, and the lithium abundance and C-to-N ratios for both components. We present new or revised values for some of these. The latter three quantities are sensitive diagnostics of evolution, and change drastically for giants as a result of the deepening of the convective envelope during the first dredge-up. The secondary is crossing the Hertzprung gap, while the primary is believed to be in the longer-lived core helium-burning phase. Previous studies using only the masses, temperatures, and luminosities have found good agreement with stellar evolution models placing the primary in the clump. Here, we compare all of the constraints simultaneously against three sets of current models. We find that they are unable to match all of the observations for both components at the same time, and at a single age, for any evolutionary state of the primary. This shows the great importance of chemical information for assessing the evolutionary state of giant stars. A comparison with models of tidal evolution yields similarly disappointing results, when tested against the fact that the orbit is circular, the primary is rotating synchronously, the secondary ∼12 times faster than synchronous, and the spin axes are apparently aligned with the axis of the orbit. When confronted in detail, our understanding of the advanced stages of stellar evolution is thus still very incomplete.