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Capella (Alpha Aurigae) Revisited: New Binary Orbit, Physical Accepted for publication in The Astrophysical Journal A Preprint typeset using LTEX style emulateapj v. 5/2/11 CAPELLA (α AURIGAE) REVISITED: NEW BINARY ORBIT, PHYSICAL PROPERTIES, AND EVOLUTIONARY STATE Guillermo Torres1, Antonio Claret2, Kreˇsimir Pavlovski3, and Aaron Dotter4 Accepted for publication in The Astrophysical Journal ABSTRACT 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 ± 50K 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. Subject headings: binaries: general — binaries: spectroscopic — stars: abundances — stars: evolution — stars: fundamental parameters — stars: individual (Capella) 1. INTRODUCTION firm that notion, as it has not been possible to achieve As one of the brightest binary stars in the sky, a satisfactory fit to the global properties of both stars Capella (α Aurigae, HD34029, HR1708, HIP24608, simultaneously at a single age when assuming the bulk chemical composition the system is believed to have. The G8 III+G0 III, Porb = 104 days, V = 0.07) has been studied for more than a century with a wide range of secondary, on the other hand, is clearly on its way across techniques and at all observable wavelengths.5 A per- the Hertzprung gap. sistent source of frustration for several decades has been The uncertainty in the masses was thought to have the difficulty in determining accurate absolute masses for been solved in the T09 analysis, which improved the for- the components, despite the wealth of astrometric and mal precision by about a factor of three compared to spectroscopic measurements available. The history of previous estimates, and documented efforts to control this problem has been related by several authors (e.g., systematic errors in the radial velocities that have likely Batten et al. 1991; Barlow et al. 1993), and most re- plagued the determination of the velocity amplitude of cently in our earlier paper (Torres et al. 2009, hereafter the rapidly-rotating secondary star for decades, as de- T09). The challenge associated with the masses has hin- scribed there. Despite this, it was still not possible to dered efforts to pin down the precise evolutionary state establish the state of the primary component unambigu- of the more massive primary star, which has widely been ously when enforcing a single age. arXiv:1505.07461v1 [astro-ph.SR] 27 May 2015 considered to be a core helium-burning object, based In the interim, Weber & Strassmeier (2011) have pre- mostly on timescale arguments. Disappointingly, current sented a new spectroscopic study of Capella based on stellar evolution models have so far largely failed to con- much higher-quality observational material that leads to significantly larger masses for both stars than in our 2009 1 Harvard-Smithsonian Center for Astrophysics, 60 Gar- study, by several times the stated uncertainties. In par- den St., Cambridge, MA 02138, USA; e-mail: gtor- ticular, the spread in mass between the stars increased [email protected] from 1% to about 3.5%, which is an enormous differ- 2 Instituto de Astrof´ısica de Andaluc´ıa, CSIC, Apartado 3004, ence for a pair of giants, and could drastically change 18080 Granada, Spain; e-mail: [email protected] 3 Department of Physics, Faculty of Science, University of the assessment of their relative state of evolution. In ad- Zagreb, Bijeni˘cka cesta 32, 10000 Zagreb, Croatia; e-mail: dition, we have now made a new determination of the [email protected] chemical composition of Capella that is appreciably dif- 4 Research School of Astronomy and Astrophysics, Australian ferent from the abundance assumed in the earlier pa- National University, Canberra, ACT, 2611, Australia; e-mail: [email protected] per, and is a key ingredient for the comparison with 5 Capella has a wide common proper motion companion that stellar evolution models. These two developments mo- is itself a visual binary composed of M dwarfs. The system is tivate us to take a fresh look at the system in order to therefore a hierarchical quadruple. Revised properties of the M investigate the impact of the new measurements. Fur- dwarf pair are reported in the Appendix. 2 Torres et al. thermore, Weber & Strassmeier (2011) have presented on the active secondary star, or template mismatch.6 An evidence that the orbit of Capella may be very slightly additional indication of possible biases was the fact that eccentric, unexpectedly, whereas all previous studies in- a small offset (0.267 ± 0.079 kms−1) was found between cluding our own assumed it is circular. It is of inter- the primary and secondary velocities in the global orbital est, therefore, to revisit our 2009 study of tidal evolution fit of T09 that could not be accounted for by differences in the system (orbit circularization and rotational syn- in the gravitational redshift between the stars, and was chronization) with more sophisticated models than used ascribed to similar reasons as the secondary residual pat- there, especially in light of the new masses. tern. We have organized the paper as follows. In Sect. 2 More recently Weber & Strassmeier (2011) have re- we describe the new spectroscopic observations of ported new RV measurements for Capella based on a Weber & Strassmeier (2011) and comment on the issue very large set of more than 400 spectra obtained with the of systematics in the radial velocities, in comparison with STELLA ´echelle spectrograph (Strassmeier et al. 2010) our previous results from 2009. A revised orbital fit for on a 1.2 m robotic telescope in Tenerife, Spain. These Capella is the presented in Sect. 3, using also astromet- spectra are of far superior quality than those of T09, both ric and other measurements from T09. Sect. 4 reports in terms of wavelength coverage (two orders of magnitude a new detailed chemical analysis of both stars from dis- larger) and signal-to-noise ratios. Weber & Strassmeier entangled spectra, along with a determination of the at- (2011) derived velocities using a similar two-dimensional mospheric parameters. The revised physical properties cross-correlation approach as T09, and also performed of the stars are collected in Sect. 5, and are compared numerical simulations to assess and correct for system- against three different sets of stellar evolution models in atic errors caused by the measuring technique. The ve- Sect. 6. Key chemical diagnostics available for Capella locity scatter from their orbital fit is 0.064 km s−1 for are also compared with model predictions in this sec- the primary and 0.297 kms−1 for the secondary, seven tion. Then in Sect. 7 we examine the evolution of orbital and three times smaller than in T09, respectively. The and stellar properties subjected to the influence of tidal key difference in this data set compared to T09 is in mechanisms, as a test of that theory. Sect. 8 presents a the resulting velocity semi-amplitude of the secondary −1 discussion of the results and concluding remarks. Finally, star (KB = 26.840 ± 0.024 kms ), which is more the Appendix provides an update on the orbital proper- than 6σ larger than reported by T09 (KB = 26.260 ± ties of the wide common proper motion companion of 0.087 kms−1). This difference alone leads to absolute Capella. masses for Capella that are 4% larger than in T09 for the primary, and 2% for the secondary, a very significant change that exceeds the formal mass errors by a factor of 2. RADIAL VELOCITIES many. The semi-amplitudes KA of the primary star, on the other hand, are in virtually perfect agreement (see The numerous historical radial-velocity (RV) measure- below). ments of Capella have been discussed at length in our Despite the much improved random errors of T09 study, which highlighted how challenging it has been Weber & Strassmeier (2011), the residuals of the sec- to determine accurate values for the rapidly rotating ondary velocities from their spectroscopic orbital model secondary star (v sin i ≈ 35 kms−1), whereas those of −1 (see their Figure 2) still display a phase-dependent pat- the sharp-lined primary (v sin i ≈ 4 kms ) have been tern reminiscent of the one in T09, also with a semi- quite consistent over the last century. T09 presented amplitude of roughly twice the errors, but at a much 162 new RV determinations for both components based lower level in absolute terms.
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