Neutron-capture elements in a sample of field metal-poor N-rich dwarfs

Abstract

Context. The aim of this work is to measure the abundances of n-capture elements in a sample of six metal-poor N-rich dwarfs that were formed in globular clusters, and subsequently became unbound from the cluster. These N-rich stars, HD 25329, HD 74000, HD 160617, G 24-3, G53-41, and G90-3, were previously studied in Paper I.
Aims: The abundances of the n-capture elements in these stars were compared to the abundances in normal metal-poor dwarfs and in globular cluster stars in the same metallicity range in order to find evidence of an enrichment of the material from which these N-rich stars were formed, by the ejecta of massive asymptotic giant branch stars (AGB) inside the cluster.
Methods: The abundances of 15 elements, from Sr to Yb, were derived line by line by comparing the observed profiles to synthetic spectra in a sample of six metal-poor N-rich dwarfs and nine classical metal-poor dwarfs.
Results: We show that, generally speaking, the behaviours of the intermediate metal-poor stars here studied and the extremely metal-poor stars are very different. In particular, the scatter of the [X/Fe] ratios is much smaller since many more stars contribute to the enrichment. Among our six metal-poor N-rich stars, three stars (G24-3 and HD 74000 and maybe also HD 160617) present an enrichment in elements formed by the s-process, typical of a contribution of AGB stars, unexpected at the metallicity of these stars. This suggests that the intracluster medium from which these stars were formed was enriched by a first generation of massive AGB stars. Another N-rich star, G53-41, is also rich in s-process elements, but since it is more metal-rich this could be due to the normal galactic enrichment by low-mass AGB stars before the formation of the cluster. In contrast, two stars (G 90-3 and HD 25329) have an abundance pattern compatible with a pure r-process such as that seen in metal-poor stars with [Fe/H] < −1.5.

Based on observations collected at the European Organisation for Astronomical Research in the Southern Hemisphere (Archives of programmes 090.B-0504(A) PI: Chaname; 095.D-0504(A) PI: Melendez; 076.B-0166(A) PI: Pasquini; 067.D-0086(A) PI: Gehren; 071.B-0529(A) PI: Silva; 065.L-0507(A) PI: Primas), and collected at the W. M. Keck Observatory, archive programme G401H, PI: Melendez. One star was also observed at ESO with the spectrograph ESPRESSO, programme 107.22RU.001 PI: Spite, and two stars were observed at the Observatoire de Haute Provence (Archives of programme 10A.PNPS.HALB, PI: Halbwachs) and at the Narval spectrograph of the Observatoire du Pic du Midi, programme L172N04, PI: Spite.