Abstract
Metal-poor stars were formed during the early epochs when only massive stars had time to evolve and contribute to the chemical enrichment. Low-mass metal-poor stars survive until the present and provide fossil records of the nucleosynthesis of early massive stars. On the other hand, short-lived radionuclides (SLRs) in the early solar system (ESS) reflect the nucleosynthesis of sources that occurred close to the proto-solar cloud in both space and time. Both the ubiquity of Sr and Ba and the diversity of heavy-element abundance patterns observed in single metal-poor stars suggest that some neutron-capture mechanisms other than the r-process might have operated in early massive stars. Three such mechanisms are discussed: the weak s-process in non-rotating models with initial carbon enhancement, a new s-process induced by rapid rotation in models with normal initial composition, and neutron-capture processes induced by proton ingestion in non-rotating models. In addition, meteoritic data are discussed to constrain the core-collapse supernova (CCSN) that might have triggered the formation of the solar system and provided some of the SLRs in the ESS. If there was a CCSN trigger, the data point to a low-mass CCSN as the most likely candidate. An 11.8 M⊙ CCSN trigger is discussed. Its nucleosynthesis, the evolution of its remnant, and the interaction of the remnant with the proto-solar cloud appear to satisfy the meteoritic constraints and can account for the abundances of the SLRs 41Ca, 53Mn, and 60Fe in the ESS.
Highlights
Metal-poor stars were formed during the early epochs when only massive stars had time to evolve and contribute to the chemical enrichment
Massive stars evolve quickly to end in core-collapse supernovae (CCSNe) and the neutrino-driven winds from the proto-neutron stars created in these events can produce Sr
These so-called carbon-enhanced metal-poor (CEMP)-no stars constitute ∼ 20%, 40%, and 80% of all the metal-poor stars with [Fe/H] ≤ −2, −3, and −4, respectively. Such stars were most likely formed out of the general interstellar medium (ISM) that had been enriched by very early CCSNe with elevated abundances of C relative to Fe. While such stars have low masses in order to survive until the present, they must have been formed along with massive CEMP stars that already exploded as CCSNe
Summary
Observations show that heavy elements such as Sr (with mass numbers A ∼ 86–88) and Ba (A ∼ 134–138) are prevalent in metal-poor stars [1]. This diversity calls for massive stellar sources for neutron-capture processes other than the r-process. All of the above observations concern single metal-poor stars Both the ubiquity of Ba and the diversity of La/Eu in these stars reflect enrichment of the general interstellar medium (ISM) by massive stellar sources, not some localized surface contamination through mass transfer of e.g., the s-process products from binary companions
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