Abstract
Of the variations in the elemental abundance patterns of stars enhanced with r-process elements, the variation in the relative actinide-to-lanthanide ratio is among the most significant. We investigate the source of these actinide differences in order to determine whether these variations are due to natural differences in astrophysical sites, or due to the uncertain nuclear properties that are accessed in r-process sites. We find that variations between relative stellar actinide abundances is most likely astrophysical in nature, owing to how neutron-rich the ejecta from an r-process event may be. Furthermore, if an r-process site is capable of generating variations in the neutron-richness of its ejected material, then only one type of r-process site is needed to explain all levels of observed relative actinide enhancements.
Highlights
The rapid neutron-capture (“r ”) process is the physical mechanism by which about half the elements heavier than iron in the Solar System were created
After applying the AD distribution of Ye to each of the nuclear variations considered, we find that the actinide abundance and relative actinide-to-lanthanide ratio are lowered to levels that are roughly in agreement with abundances derived from observations of metal-poor stars
We have investigated the source of the stellar actinide-boost phenomenon to uncover whether this boost has an origin in key nuclear data or indicates some distinct r -process site
Summary
The rapid neutron-capture (“r ”) process is the physical mechanism by which about half the elements heavier than iron in the Solar System were created. Stars with low metallicity indicate few nucleosynthetic events by supernovae that would otherwise release an abundance of iron into the primordial gas of the interstellar medium In this way, the elemental abundances in low-metallicity stars record the chemical signatures of nucleosynthetic events preceding their formation. Stars over-enhanced with Eu relative to Fe compared to the Sun are called “r -process enhanced” and are divided into two categories: “r -I,” with 0.3 < [Eu/Fe] ≤ 1.0 (i.e., between a factor of 2 and 10 greater than the Solar value), and “r -II,” with [Eu/Fe] > 1.0 (i.e., over a factor of 10 greater than the Solar System) These r -I and r -II stars are relics of prolific r -process event(s) that occurred before the gas was enriched by supernovae, and are considered tracers of nearly pure r -process events. Stars with an over-abundance of [Th/Eu] relative to the Solar System are considered “actinide-boost” stars, occurring in about 30% of r -process enhanced stars
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