Abundances of neutron-capture elements in G 24-25

A halo-population CH subgiant^⋆,⋆⋆

¹ Key Laboratory of Optical Astronomy, National Astronomical Observatories, CAS, 20A Datun Road, Chaoyang District, 100012 Beijing PR China
e-mail: gzhao@nao.cas.cn
² Graduate University of the Chinese Academy of Sciences, 19A Yuquan Road, Shijingshan District, 100049 Beijing, PR China
³ Department of Physics and Astronomy, University of Aarhus, 8000 Aarhus C, Denmark
e-mail: pen@phys.au.dk
⁴ Observatorio Astronómico Nacional, Universidad Nacional Autónoma de México, Apartado Postal 877, CP 22800 Ensenada, BC, Mexico
e-mail: schuster@astrosen.unam.mx

Received: 18 August 2011
Accepted: 5 March 2012

Abstract

Aims. The differences between the neutron-capture element abundances of halo stars are important to our understanding of the nucleosynthesis of elements heavier than the iron group. We present a detailed abundance analysis of carbon and twelve neutron-capture elements from Sr up to Pb for a peculiar halo star G 24-25 with [Fe/H] = −1.4 in order to probe its origin.

Methods. The equivalent widths of unblended lines are measured from high resolution NOT/FIES spectra and used to derive abundances based on Kurucz model atmospheres. In the case of CH, Pr, Eu, Gd, and Pb lines, the abundances are derived by fitting synthetic profiles to the observed spectra. Abundance analyses are performed both relative to the Sun and to a normal halo star G 16-20 that has similar stellar parameters as G 24-25.

Results. We find that G 24-25 is a halo subgiant star with an unseen component. It has large overabundances of carbon and heavy s-process elements and mild overabundances of Eu and light s-process elements. This abundance distribution is consistent with that of a typical CH giant. The abundance pattern can be explained by mass transfer from a former asymptotic giant branch component, which is now a white dwarf.