The Hamburg/ESO R-process Enhanced Star survey (HERES) VIII. The r+s star HE 1405 0822

Abstract

Aims. The aim of this study is a detailed abundance analysis of the newly discovered r-rich star HE 1405−0822, which has [Fe/H] = −2.40. This star shows enhancements of both r- and s-elements, [Ba/Fe] = + 1.95 and [Eu/Fe] = 1.54, for which reason it is called r+s star. Methods. Stellar parameters and element abundances were determined by analyzing high-quality VLT/UVES spectra. We used Fe I line excitation equilibria to derive the effective temperature. The surface gravity was calculated from the Fe i/Fe ii and Ti i/Ti ii equilibria. Results. We determined accurate abundances for 39 elements, including 19 neutron-capture elements. HE 1405−0822 is a red giant. Its strong enhancements of C, N, and s-elements are the consequence of enrichment by a former AGB companion with an initial mass of less than 3 M⊙. The heavy n-capture element abundances (including Eu, Yb, and Hf) seen in HE 1405−0822 do not agree with the r-process pattern seen in strongly r-process-enhanced stars. We discuss possible enrichment scenarios for this star. The enhanced α elements can be explained as the result of enrichment by supernovae of type II. Na and Mg may have partly been synthesized in a former AGB companion, when the primary 22Ne acted as a neutron poison in the 13C-pocket