Joint r-process Enrichment by Supernovae with a Metallicity Threshold and Neutron Star Mergers

Abstract The enrichment history of r -process elements has been imprinted on the stellar abundances that change in accordance with increasing metallicity in galaxies. Close examination of the Eu/Fe feature caused by stars in nearby galaxies, including the Large Magellanic Cloud (LMC), shows its perplexity. The decreasing trend of the Eu/Fe feature is followed by a nearly constant value; this trend is generally attributed to an onset of the delayed Fe release from Type Ia supernovae (SNe Ia), which is the same interpretation of the α /Fe feature. However, this feature appears in the LMC at Fe/H of approximately −0.7, which is significantly higher than that for the α /Fe case (≈−2). This result potentially indicates the presence of an overlooked property of the r -process site that remains unseen in the study of the Milky Way. Here, we propose that this Eu/Fe-knee feature is created by a fade-out of core-collapse SNe producing r -process elements; these elements along with neutron star mergers (NSMs) promote the r -process enrichment under the condition for this specific SNe such that their occurrence is limited to a low-metallicity environment. This metallicity threshold for the occurrence rate of r -process SNe at a subsolar is nearly identical to that for long gamma-ray bursts whose origin may be connected to fast-rotating massive stars. Moreover, we reason that the contribution of Eu from NSMs is crucial to maintain a high Eu/Fe at an early stage in dwarf galaxies by a balance with Fe from SNe Ia; both enrichments via NSMs and SNe Ia proceed with similar delay time distributions.