Abundance of Heavy r -process Elements in CEMP-rs Stars: The Role of the i -process

Abstract Carbon-enhanced metal-poor (CEMP) stars are ancient stars enriched in carbon and heavy elements. Some of these stars exhibit enhanced s -process and/or r -process elements, and hence are classified as CEMP-s, CEMP-rs, or CEMP-r. This classification is challenging due to the limited availability of heavy element abundances, particularly among r -process elements. Heavy r -process elements such as terbium, holmium, thulium, ytterbium, lutetium, tantalum, and iridium have rarely been measured because their sensitive lines are located in the ultraviolet. However, they provide sensitive diagnostics of the s -, r -, and i - nucleosynthetic processes. In this work, we aim to obtain a secure classification of CEMP-s and -rs stars and investigate whether the i -process can account for the measured abundance patterns in CEMP-rs stars. We derive the abundance profiles, notably for 12 heavy r-elements, including, in some cases, tantalum, using high-resolution UVES spectra of 17 CEMP-s and -rs stars. Based on indicators such as the [s/r] abundance ratio or the model-independent “abundance distance”, nine stars are confirmed as CEMP-rs and six as CEMP-s. The classification of two objects remains uncertain. The i -process satisfactorily reproduces the abundance patterns of CEMP-rs stars. However, larger samples are needed to confirm trends with metallicity and clarify how CEMP-rs stars differ from CEMP-s stars.