A holistic approach to carbon-enhanced metal-poor stars

By considering the various CEMP subclasses separately, we try to derive, from the specific signatures imprinted on the abundances, parameters (such as metallicity, mass, temperature, and neutron source) characterizing AGB nucleosynthesis from the specific signatures imprinted on the abundances, and separate them from the impact of thermohaline mixing, first dredge-up, and dilution associated with the mass transfer from the companion.To put CEMP stars in a broad context, we collect abundances for about 180 stars of various metallicities, luminosity classes, and abundance patterns, from our own sample and from literature. First, we show that there are CEMP stars which share the properties of CEMP-s stars and CEMP-no stars (which we call CEMP-low-s stars). We also show that there is a strong correlation between Ba and C abundances in the s-only CEMP stars. This strongly points at the operation of the 13C neutron source in low-mass AGB stars. For the CEMP-rs stars (seemingly enriched with elements from both the s- and r-processes), the correlation of the N abundances with abundances of heavy elements from the 2nd and 3rd s-process peaks bears instead the signature of the 22Ne neutron source. Adding the fact that CEMP-rs stars exhibit O and Mg enhancements, we conclude that extremely hot conditions prevailed during the thermal pulses of the contaminating AGB stars. Finally, we argue that most CEMP-no stars (with no overabundances for the neutron-capture elements) are likely the extremely metal-poor counterparts of CEMP neutron-capture-rich stars. We also show that the C enhancement in CEMP-no stars declines with metallicity at extremely low metallicity ([Fe/H]~< -3.2). This trend is not predicted by any of the current AGB models.

💡 Research Summary

This paper presents a comprehensive, subclass‑by‑subclass investigation of carbon‑enhanced metal‑poor (CEMP) stars with the aim of disentangling the nucleosynthetic imprint of the donor asymptotic‑giant‑branch (AGB) companion from the subsequent surface mixing and dilution that occur after mass transfer. The authors assembled a homogeneous abundance database for roughly 180 CEMP objects spanning a wide range of metallicities, luminosity classes, and chemical patterns, drawing from their own high‑resolution spectroscopic observations as well as from the literature.

The first major result is the identification of a transitional group, dubbed CEMP‑low‑s, which shares characteristics of both the classic CEMP‑s (s‑process‑rich) and CEMP‑no (neutron‑capture‑poor) populations. In these stars a tight positive correlation between barium and carbon abundances is observed, strongly pointing to the operation of the ¹³C(α,n)¹⁶O neutron source in low‑mass AGB companions. The ¹³C source operates at relatively modest temperatures (~9 × 10⁷ K) and provides a long‑duration, low‑flux neutron exposure that builds up the first and second s‑process peaks in a characteristic way.

In contrast, the CEMP‑rs stars exhibit simultaneous enrichment in both s‑ and r‑process elements. Their nitrogen abundances correlate with the heavy‑element abundances belonging to the second and third s‑process peaks (e.g., Ba, La, Ce, Nd, Sm). This pattern is best explained by a dominant ²²Ne(α,n)²⁵Mg neutron source, which requires much higher temperatures (~3 × 10⁸ K) during thermal pulses. The high‑temperature environment also accounts for the observed oxygen and magnesium enhancements, as the same α‑capture reactions that liberate neutrons from ²²Ne also synthesize O and Mg. Consequently, the authors argue that the contaminating AGB stars of CEMP‑rs objects experienced extremely hot thermal pulses, likely associated with intermediate‑mass (≈ 3–5 M☉) AGB stars.

The paper then turns to the CEMP‑no population, traditionally defined by the lack of neutron‑capture element overabundances. The authors find that, at the lowest metallicities (