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.

非常贫金属恒星的r 过程元素的丰度捕捉了星系中恒星形成早期阶段r 过程富集的历史。目前，各种类型的天体物理站点，包括中子星并合（NSM）、磁旋转超新星和坍缩星，都被认为是r 过程元素的起源。恒星形成和r 过程元素产生之间的时间延迟是区分这些情景的关键，需要注意的是星际介质中r 过程元素的扩散可能会导致r 过程富集的延迟，因为r-进程事件很少见。在这里，我们研究了观察到的非常贫金属恒星的 Ba 丰度数据，作为r 过程元素早期富集历史的示踪剂。我们发现 [Ba/Mg] 与 [Fe/H] 的逐渐增加，这在银河系和经典矮星之间非常相似，需要来自恒星的r 过程事件的显着时间延迟（100 Myr–1 Gyr）形成而不是扩散引起的延迟。我们强调，这个结论对于关于Ba 丰度中s 过程污染的假设是稳健的，因为即使没有s-过程。因此，我们得出结论，具有延迟的源，可能是 NSM，是r进程元素的起源。