The abundances of r-process elements of very metal-poor stars capture the history of the r-process enrichment in the early stage of star formation in a galaxy. Currently, various types of astrophysical sites including neutron star mergers (NSMs), magneto-rotational supernovae, and collapsars, are suggested as the origin of r-process elements. The time delay between the star formation and the production of r-process elements is the key to distinguish these scenarios, with the caveat that the diffusion of r-process elements in the interstellar medium may induce the delay in r-process enrichment because r-process events are rare. Here we study the observed Ba abundance data of very metal-poor stars as the tracer of the early enrichment history of r-process elements. We find that the gradual increase of [Ba/Mg] with [Fe/H], which is remarkably similar among the Milky Way and classical dwarfs, Requires a significant time delay (100 Myr–1 Gyr) of r-process events from star formation rather than the diffusion-induced delay. We stress that this conclusion is robust to the assumption regarding s-process contamination in the Ba abundances because the sources with no delay would overproduce Ba at very low metallicities, even without the contribution from the s-process. Therefore, we conclude that sources with a delay, possibly NSMs, are the origins of r-process elements.
Bibliographical noteFunding Information:
We thank the anonymous referee for useful comments. We also thank Yutaka Hirai, Tadafumi Matsuno, Naoki Yoshida, Brian Metzger, and Daniel Siegel for insightful comments. Y.T. is supported by JSPS KAKENHI grant No. 20J21795. K. H. is supported by JSPS Early-Career Scientists grant No. 20K14513. The research of P.B. was funded by the Gordon and Betty Moore Foundation through grant GBMF5076.
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