CNO and F abundances in the barium star HD 123396

Abstract

[Abridged] Barium stars are moderately rare, chemically peculiar objects, which are believed to be the result of the pollution of an otherwise normal star by material from an evolved companion on the asymptotic giant branch (AGB). We aim to derive carbon, nitrogen, oxygen, and fluorine abundances for the first time from the infrared spectra of the barium red giant star HD 123396 to quantitatively test AGB nucleosynthesis models for producing barium stars via mass accretion. High-resolution and high S/N infrared spectra were obtained using the Phoenix spectrograph mounted at the Gemini South telescope. The abundances were obtained through spectrum synthesis of individual atomic and molecular lines, using the MOOG stellar line analysis program, together with Kurucz's stellar atmosphere models. The analysis was classical, using 1D stellar models and spectral synthesis under the assumption of local thermodynamic equilibrium. We confirm that HD 123396 is a metal-deficient barium star ([Fe/H] = -1.05), with A(C) = 7.88, A(N) = 6.65, A(O) = 7.93, and A(Na) = 5.28 on a logarithmic scale where A(H) = 12, leading to [(C+N)/Fe] ~ 0.5. The A(CNO) group, as well as the A(Na) abundances, is in excellent agreement with those previously derived for this star using high resolution optical data. We also found A(F) = 4.16, which implies [F/O] = 0.39, a value that is substantially higher than the F abundances measured in globular clusters of a similar metallicity, noting that there are no F measurements in field stars of comparable metallicity. The observed abundance pattern of the light elements (CNO, F and Na) recovered here as well as the heavy elements (s-process) studied elsewhere suggest that the surface composition of HD 123396 is well fitted by the predicted abundance pattern of a 1.5Msun AGB model star with Z = 0.001.

8 pages, 4 figures, accepted for publication in A&A (language corrected)

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• The s process in AGB stars as constrained by a large sample of Barium stars
• When binaries keep track of recent nucleosynthesis: The Zr - Nb pair in extrinsic stars as an s-process diagnostic
• Atmospheric parameters and chemical properties of red giants in the CoRoT asteroseismology fields
• CNO and F abundances in the globular cluster M22 (NGC 6656)
• Fluorine in the Solar neighborhood: No evidence for the neutrino process
• Fluorine in the solar neighborhood - is it all produced in AGB-stars?
• Fluorine in the solar neighbourhood: the need for several cosmic sources
• Fluorine Abundances of Galactic Low-Metallicity Giants