Metal-poor stars with available detailed information about their chemical inventory pose powerful empirical benchmarks for nuclear astrophysics .
Here we present our spectroscopic chemical abundance investigation of the metal-poor ( \mathrm { [ Fe / H ] } = -1.60 \pm 0.03 dex ) , r -process-enriched ( \mathrm { [ Eu / Fe ] } = 0.73 \pm 0.10 dex ) halo star HD 20 using novel and archival high-resolution data at outstanding signal-to-noise ratios ( up to \sim 1000 Å ^ { -1 } ) .
By combining one of the first asteroseismic gravity measurements in the metal-poor regime from a TESS light curve with the spectroscopic analysis of iron lines under non-local thermodynamic equilibrium conditions , we derive a set of highly accurate and precise stellar parameters .
These allow us to delineate a reliable chemical pattern that is comprised of solid detections of 48 elements , including 28 neutron-capture elements .
Hence , we establish HD 20 among the few benchmark stars that have almost complete patterns and possess low systematic dependencies on the stellar parameters .
Our light-element ( Z \leq 30 ) abundances are representative of other , similarly metal-poor stars in the Galactic halo with contributions from core-collapse supernovae of type II .
In the realm of the neutron-capture elements , our comparison to the scaled solar r -pattern shows that the lighter neutron-capture elements ( Z \lesssim 60 ) are poorly matched .
In particular , we find imprints of the weak r -process acting at low metallicities .
Nonetheless , by comparing our detailed abundances to the observed metal-poor star BD +17 3248 , we find a persistent residual pattern involving mainly the elements Sr , Y , Zr , Ba , and La .
These are indicative of enrichment contributions from the s -process and we show that mixing with material from predicted yields of massive , rotating AGB stars at low metallicity considerably improves the fit .
Based on a solar ratio of heavy- to light- s elements – at odds with model predictions for the i -process – and a missing clear residual pattern with respect to other stars with claimed contributions from this process , we refute ( strong ) contributions from such astrophysical sites providing intermediate neutron densities .
Finally , nuclear cosmochronology is used to tie our detection of the radioactive element Th to an age estimate for HD 20 of 11.0 \pm 3.8 Gyr .