Abundance of heavy r-process elements in CEMP-rs stars: The role of the i-process

TL;DR

This study obtained high-resolution UVES spectra for 17 CEMP stars to derive abundances for twelve heavy r-process elements, many measured in the UV where lines are most sensitive to nucleosynthetic origin. By applying both model-independent abundance-distance diagnostics and model-dependent comparisons to s-process and i-process AGB yields, the authors securely classify stars as CEMP-rs or CEMP-s and demonstrate that the i-process can reproduce the abundance patterns of CEMP-rs stars. The work shows that Ta and other heavy elements provide crucial diagnostics that help differentiate nucleosynthesis channels, and it highlights the need for larger, more homogeneous samples and refined i-process models to map metallicity trends and the rs-s distinction across the CEMP population. Overall, the results support an important role for the i-process in shaping the heavy-element content of CEMP-rs stars and set a foundation for future multi-element, NLTE-aware abundance studies.

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, 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 twelve heavy r-elements, including, in some cases, tantalum, using high-resolution UVES spectra of seventeen 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.

Figures (15)

• Figure 1: Spectral fits for the determination of $^{12}$C/$^{13}$C ratio using the CH G band at 4310 Å in CS 29512$-$073. The upper (resp., middle and bottom) panel shows spectral synthesis with $^{12}$C/$^{13}$C$=30$ (resp., $19$ and $1.5$). The red curve depicts the synthetic spectrum for an abundance of $\log \epsilon(\mathrm{C}) = 7.45$, with the blue and green curves illustrating $\pm 0.3$ dex variations. The black line represents the observed spectrum, and the magenta line corresponds to the spectral synthesis without carbon.
• Figure 2: Spectral fits for determining the $^{12}$C/$^{13}$C ratio using the CN band at 3883 Å in CS 29512$-$073. The upper (resp., middle and bottom) panel shows spectral synthesis with $^{12}$C/$^{13}$C$= 30$ (resp., 11.5 and 1.5). The red curve depicts the synthetic spectrum for an abundance of $\log \epsilon(\mathrm{N}) = 6.10$, with the blue and green curves indicating the synthesis with $\pm 0.3$ dex variations. The black line represents the observed spectrum, and the magenta line corresponds to the spectral synthesis without nitrogen.
• Figure 3: The upper and lower left panels display the spectral fitting of the Ho2 lines for CS 22947$-$187 and HD 196944, while the upper and lower right panels display the Tb2 lines for HD 224959 and CS 30322$-$023. Red lines correspond to spectral syntheses with the adopted Ho2 abundances of $-$1.6 dex, $-$2.0 dex for CS 22947$-$187 and HD 196944, and Tb2 abundances of $-$0.75 dex, $-$2.0 dex for HD 224959 and CS 30322$-$023 respectively. Blue and green lines correspond to syntheses with abundances deviating by $\pm$0.3 dex from the adopted abundance. The black dashed line represents the observed spectrum. The magenta line corresponds to the synthesis with a null abundance for the corresponding element.
• Figure 4: Spectral fitting of the Yb2 and Tm2 lines is shown for two CEMP stars, CS 29512$-$073 and HD 196944 in the upper and lower panels, respectively. Red lines correspond to spectral syntheses with the adopted Yb2, Tm2 abundances of $-$0.55 dex, $-$1.20 dex for CS 29512$-$073, and $-$1.05 dex, $-$1.60 dex for HD 196944 respectively. The blue, green, magenta, and black curves have the same meaning as in Figure \ref{['Fig:TbHo']}.
• Figure 5: The spectral fits for the Ta2 lines at 3414.13 Å and 3446.85 Å are presented for HD 196944 in the left and for HD 224959 in the right panels, respectively. The red curve represents the synthesis with the adopted abundances ($-$0.6 dex for the left panel and $-$0.25 dex for the right panel). The blue, green, magenta, and black curves have the same meaning as in Figure \ref{['Fig:TbHo']}.