HD 222925: A New Opportunity to Explore the Astrophysical and Nuclear Conditions of r-process Sites

With the most trans-iron elements detected of any star outside the solar system, HD 222925 represents the most complete chemical inventory among metal-poor stars enhanced with elements made by the rapid neutron capture (“r”) process. As such, HD 222925 may be a new “template” for the observational r-process, where before the (much higher-metallicity) solar r-process residuals were used. In this work, we test under which conditions a single site accounts for the entire elemental r-process abundance pattern of HD 222925. We found that several of our tests—with the single exception of the black hole–neutron star merger case—challenge the single-site assumption by producing an ejecta distribution that is highly constrained, in disagreement with simulation predictions. However, we found that ejecta distributions that are more in line with simulations can be obtained under the condition that the nuclear data near the second r-process peak are changed. Therefore, for HD 222925 to be a canonical r-process template likely as a product of a single astrophysical source, the nuclear data need to be reevaluated. The new elemental abundance pattern of HD 222925—including the abundances obtained from space-based, ultraviolet (UV) data—call for a deeper understanding of both astrophysical r-process sites and nuclear data. Similar UV observations of additional r-process–enhanced stars will be required to determine whether the elemental abundance pattern of HD 222925 is indeed a canonical template (or an outlier) for the r-process at low metallicity.

在太阳系外的恒星中，HD 222925所探测到的超铁元素数量最多，它代表了贫金属恒星中最完整的化学组成清单，这些恒星因快速中子俘获（“r”）过程产生的元素而含量增加。因此，HD 222925可能成为观测r过程的一个新“模板”，在此之前使用的是（金属含量高得多的）太阳r过程残留物。在这项工作中，我们测试在哪些条件下单个场所能够解释HD 222925的整个元素r过程丰度模式。我们发现，我们的几项测试——除了黑洞 - 中子星合并的情况这一单一例外——通过产生受到高度限制的抛射物分布，对单一场所假设提出了挑战，这与模拟预测不一致。然而，我们发现，在改变第二个r过程峰值附近的核数据的条件下，可以获得更符合模拟的抛射物分布。因此，要使HD 222925成为可能作为单个天体物理源产物的典型r过程模板，就需要重新评估核数据。HD 222925新的元素丰度模式——包括从太空紫外线（UV）数据获得的丰度——需要对天体物理r过程场所和核数据有更深入的理解。要确定HD 222925的元素丰度模式在低金属含量下是否确实是r过程的典型模板（还是异常值），还需要对其他r过程增强的恒星进行类似的紫外线观测。