Multimessenger observations of the neutron star merger GW170817 and its kilonova proved that neutron star mergers can synthesize large quantities of r -process elements .
If neutron star mergers in fact dominate all r -process element production , then the distribution of kilonova ejecta compositions should match the distribution of r -process abundance patterns observed in stars .
The lanthanide fraction ( X _ { \text { La } } ) is a measurable quantity in both kilonovae and metal-poor stars , but it has not previously been explicitly calculated for stars .
Here , we compute the lanthanide fraction distribution of metal-poor stars ( [ Fe/H ] < -2.5 ) to enable comparison to current and future kilonovae .
The full distribution peaks at \log X _ { \text { La } } \sim - 1.8 , but r -process-enhanced stars ( [ Eu/Fe ] > 0.7 ) have distinctly higher lanthanide fractions ; \log X _ { \text { La } } \gtrsim - 1.5 .
We review observations of GW170817 and find general consensus that the total \log X _ { \text { La } } = -2.2 \pm 0.5 , somewhat lower than the typical metal-poor star and inconsistent with the most highly r -enhanced stars .
For neutron star mergers to remain viable as the dominant r -process site , future kilonova observations should be preferentially lanthanide-rich ( including a population of \sim 10 \% with \log X _ { \text { La } } > -1.5 ) .
These high- X _ { \text { La } } kilonovae may be fainter and more rapidly evolving than GW170817 , posing a challenge for discovery and follow-up observations .
Both optical and ( mid- ) infrared observations will be required to robustly constrain kilonova lanthanide fractions .
If such high- X _ { \text { La } } kilonovae are not found in the next few years , that likely implies that the stars with the highest r -process enhancements have a different origin for their r -process elements .