HE2159-0551: A very metal-poor peculiar low-Ba giant – a candidate from the LMS-1/Wukong

Goethe-Universität, Institute for Applied Physics, Max-von-Laue-Straße 1, 60438 Frankfurt am Main, Germany

^★ Corresponding author: schichtel@em.uni-frankfurt.de

Received: 2 July 2025
Accepted: 28 August 2025

Abstract

Aims. We present a comprehensive spectroscopic and kinematic analysis of the very metal-poor ([Fe/H] = −2.60 ± 0.20 dex) giant star HE2159-0551. By investigating the star’s chemodynamic characteristics, we seek to address its formation, evolution, and role in the chemical enrichment of the early cosmos and the possible connection to known merger events.

Methods. From high-resolution data, we performed a 1D, local thermodynamic equilibrium analysis using PyMoogi. We conducted a detailed abundance analysis of 23 elements (C, N, O, Na, Mg, Al, Si, K, Ca, Sc, Ti, V, Cr, Mn, Fe, Co, Ni, Zn, Sr, Y, Zr, Ba, and Eu) to derive abundances or place limits. Finally, we computed orbital parameters to investigate the kinematics and, thus, the nucleosynthetic origin of HE2159-0551.

Results. The analysis yields significant insights into the chemical composition of HE2159-0551, highlighting its peculiarities among very metal-poor stars. The star shows signs of internal mixing and a peculiar abundance pattern, particularly regarding the heavy elements. We find a low Ba abundance (s-process element) and a suppressed contamination of r-process elements, even though r-process enrichment may be expected in very metal-poor stars. The orbital parameters and kinematic properties indicate that HE2159-0551 is a thick disk star, connected to the old and metal-poor LMS-1/Wukong progenitor resulting from an early merger.

Conclusions. Our kinematic analysis suggests a potential connection of HE2159-0551 to the merger LMS-1/Wukong. We conclude that the nucleosynthesis processes responsible for the star’s enrichment in heavy elements are different from those observed in many other metal-poor stars, as neither the main r- nor s-process can explain the derived abundance pattern. A possible weak-r or vp-process forming Sr-Zr but not Ba might have mixed into a low level of underlying r-process material in the low-mass LMS-1/Wukong system.