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Title: Capella (alpha Aurigae) revisited: New binary orbit, physical properties, and evolutionary state
Author: Guillermo Torres (1), Antonio Claret (2), Kresimir Pavlovski (3), Aaron Dotter (4) ((1) Harvard-Smithsonian Center for Astrophysics, USA, (2) Instituto de Astrofisica de Andalucia, Spain, (3) University of Zagreb, Croatia, (4) Research School of Astronomy and Astrophysics, ANU, Australia)

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 (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 K and 5730 60 K, and independently measured luminosities based on the orbital parallax (78.7 4.2 and 72.7 3.6 solar luminosities). 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.

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Title: Binary orbit, physical properties, and evolutionary state of Capella (alpha Aurigae)
Authors: Guillermo Torres (CfA), Antonio Claret (IAA, Spain), Patrick A. Young (AZ State Univ.)

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 of 2.466 0.018 M_Sun and 2.443 0.013 M_Sun for the primary and secondary (relative errors of 0.7% and 0.5%). Improved values are derived also for the radii (11.87 0.56 R_Sun and 8.75 0.32 R_Sun), effective temperatures (4920 70 K and 5680 70 K), and luminosities (79.5 4.8 L_Sun and 72.1 3.6 L_Sun). The distance is determined to be 13.042 0.028 pc. Capella is unique among evolved stars in that, in addition to all of the above, the chemical composition is known, including the overall metallicity [m/H], the carbon isotope ratio 12C/13C for the primary, and the lithium abundance and C/N ratios for both components. 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. However, we find that current stellar evolution models 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. Similar problems are found when testing the rotational synchronization, spin axis alignment, and orbital circularisation of the system against tidal theory. We conclude that our understanding of the advanced stages of stellar evolution is still very incomplete.

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Title: Short Timescale Coronal Variability in Capella
Authors: Vinay L. Kashyap, Jennifer Posson-Brown

We analyse 205 ks of imaging data of the active binary, Capella, obtained with the Chandra High Resolution Camera Imager (HRC-I) to determine whether Capella shows any variability at timescales < 50 ks. We find that a clear signal for variability is present for timescales < 20 ks, and that the light curves show evidence for excess fluctuation over that expected from a purely Poisson process. This overdispersion is consistent with variability at the 2-7% level, and suggests that the coronae on the binary components of Capella are composed of low-density plasma and low-lying loops.

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Title: Chandra/HETGS Observations of the Capella System: the Primary as a Dominating X-ray Source
Authors: Kazunori Ishibashi, Daniel Dewey, David P. Huenemoerder, Paola Testa

Using the Chandra/High Energy Transmission Grating Spectrometer (hereafter HETGS) researchers have detected Doppler motion of Capella's X-ray emission lines in the 6 -- 25AA wave-band.
The observed motion follows the expected orbital motion of Capella's primary. This finding implies that the primary G8 III star, not the secondary G1 III star in the Hertzsprung gap, has been the dominant source of hot 10^6.8 -- 10^7K plasma at least in the last six years.
In addition, the results demonstrate the long-term stability of the HETGS and demonstrate small uncertainties of 25 and 33 km/s in the velocity determination with the HEG and MEG, respectively.

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Capella (Alpha Aurigae)is a yellow-white G-class star in the constellation Auriga, the `Charioteer'.
It is the sixth brightest star in the sky, third in the Northern hemisphere. It is a well-known spectroscopic binary of yellow giants, appears to form a physical system with Capella HL, an outlying pair of Red Dwarfs that orbit about a light year away from the star , and is a member of the extended Hyades moving group (coeval with the nuclear region of the nearby young cluster).

The two giant G-class stars have luminosities of around 50 and 80 times that of the Sun and lie less than 100 million km apart with an orbital period of 104.02 days. The two stars are in the process of becoming red giants in a few million more years as they continue to expand, cool, and brighten.

Capella lies at a distance of 42 light years
Right ascension 05h 16m 41.4s Declination +45 59' 53"

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