High-resolution spectroscopic observations of two s-process-enriched and carbon-poor post-AGB stars: GLMP 334 and IRAS 15482−5741^⋆,⋆⋆

¹ Observatório Nacional, Rua José Cristino, 77, CEP 20921-400, São Cristóvão, Rio de Janeiro-RJ, Brazil
e-mail: claudio@on.br
² Dipartimento di Fisica Generale, Università di Torino, via Pietro Giuria 1, 10125 Torino, Italy
e-mail: gallino@to.infn.it; bisterzo@to.infn.it

Received: 2 June 2011
Accepted: 22 September 2011

Abstract

Aims. The study of post-AGB stars provides important information on the s-process inside asymptotic giant branch (AGB) stars. We determined the atmospheric parameters and abundance patterns of two post-AGB stars: GLMP 334 and IRAS 15482−5741 to better understand their evolutionary state and the nature of the s-element enhancement of these stars.

Methods. We used high-resolution optical spectroscopy. Atmospheric parameters and abundances were determined in the local-thermodynamic-equilibrium model atmospheres of Kurucz (1993, CD-ROM 13, SAO) using the spectral analysis code MOOG. Spectroscopic observations are compared with nucleosynthesis AGB models of different initial masses and s-process efficiencies.

Results. Theoretical interpretations are presented and discussed. The observed distribution of neutron capture elements can be fitted with particular choices of AGB initial masses and of the ¹³C neutron source (¹³C-pocket strengths) obtained at the adopted [Fe/H] for each star, but the low [C/Fe] observed is incompatible with model predictions. The problem can be solved by increasing the metallicity by 0.2 dex, which is inside the spectroscopic uncertainty. The abundances of carbon and nitrogen observed in these stars may support the occurrence of an extra-mixing episode, the “cool bottom process”, even if its exact effect cannot be theoretically quantified yet.