CNO and F abundances in the barium star HD 123396

¹ Departamento de Astronomía y AstrofísicaPontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
e-mail: abrito@astro.puc.cl
² Centre for Astrophysics and Supercomputing, Swinburne University of Technology, Hawthorn, Victoria 3122, Australia
³ Research School of Astronomy and Astrophysics, The Australian National University, Cotter Road, Weston, ACT 2611, Australia
⁴ Universidade de São Paulo, IAG, Rua do Matão 1226, Cidade Universitária, São Paulo 05508-900, Brazil

Received: 28 January 2001
Accepted: 15 July 2011

Abstract

Context. 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).

Aims. 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.

Methods. 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.

Results. 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.

Conclusions. 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 1.5 M_⊙ AGB model star with Z = 0.001. Thus, the AGB mass transfer hypothesis offers a quantitatively viable framework.