Carbon and oxygen isotopic ratios in Arcturus and Aldebaran

Abstract

Context. We re-analyzed the carbon and oxygen isotopic ratios in the atmospheres of the two bright K giants Arcturus (α Boo) and Aldebaran (α Tau). Aims. These stars are in the evolutionary stage following the first dredge-up (FDU). Previous determinations (dating back more than 20 years) of their 16 O/ 18 O ratios showed a rough agreement with FDU expectations; however, the estimated 16 O/ 17 O and 12 C/ 13 C ratios were lower than in the canonical predictions for red giants. Today these anomalies are interpreted as signs of the occurrence of non-convective mixing episodes. We therefore re-investigated this problem to verify whether the observed data can be reproduced in this scenario and if the fairly well determined properties of the two stars can help us in fixing the uncertain parameters that characterize non-convective mixing and in constraining its physical nature. Methods. We used high-resolution infrared spectra from the literature to derive the 12 C/ 13 C and 16 O/ 17 O/ 18 O ratios from CO molecular lines near 5 μm, using the local termodynamic equilibrium (LTE) spectral synthesis method. We made use of the recently published ACE-FTS atlas of the infrared solar spectrum for constructing an updated atomic and molecular line lists in this spectral range. We also reconsidered the determination of the stellar parameters to build the proper atmospheric and evolutionary models. Results. We found that both the C and the O isotopic ratios for the two stars considered actually disagree with pure FDU predictions. This reinforces the idea that non-convective transport episodes occurred in these stars. By reproducing the observed elemental and isotopic abundances with the help of parametric models for the coupled occurrence of nucleosynthesis and mass circulation, we derived constraints on the properties of non-convective mixing, providing information on the so far elusive physics of these phenomena. We find that very slow mixing, like that associated to diffusive processes, is incapable of explaining the observed data, which require a fast transport. Circulation mechanisms with speeds intermediate between those typical of diffusive and convective mixing are necessary. We conclude with a word of caution on the conclusions possible at this stage however, as the parameters for the mass transport are fairly sensitive to the stellar mass and initial composition. At least for α Boo, reducing the uncertainty still remaining on these data would be highly desirable.

Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields

“…The only point of comparison for this quantity is provided by α Boo. Our value ( 12 C/ 13 C = 8 ± 1) closely agrees with those in the literature, which range from 6 to 10 (e.g., Pilachowski et al 1997;Pavlenko 2008;Abia et al 2012). …”

CHEMICAL EVOLUTION OF THE INNER 2 DEGREES OF THE MILKY WAY BULGE: [α/Fe] TRENDS AND METALLICITY GRADIENTS

“…The stellar parameters we use for α Boo are those from the detailed investigation of these authors: T eff = 4286 K, log g = 1.66 (cgs), [Fe/H] = −0.52, χ micro = 1.7 km s −1 , and χ macro = 6.3 km s −1 . Furthermore, we used 12 C/ 13 C = 9 as determined by Abia et al (2012) and C, N, and O abundances as derived in Ryde et al (2009) and Abia et al (2012). These are important in order to synthesize the molecular lines in the K band properly.…”

THE AGE AND MASS OF THE α HERCULIS TRIPLE-STAR SYSTEM FROM A MESA GRID OF ROTATING STARS WITH 1.3 ⩽M/M☉⩽ 8.0

“…The stellar mass has an additional critical role: The surface abundances of AGBs depend on the efficiency of the previous dredge-up episodes in addition to (extra) non-convective mixing mechanisms (e.g., Karakas et al 2010;Abia et al 2012). Different proposed extra mixing mechanisms are rotation and atomic diffusion (Maeder & Meynet 2012), internal gravity waves (Talon & Charbonnel 2005), magnetic dynamo (Busso et al 2007), and thermohaline mixing after the sub-giant phase (Charbonnel & Zahn 2007;Cantiello & Langer 2010).…”