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We present a high-resolution (R ∼ 35 000), high signal-to-noise (S/N = 350) Magellan/MIKE spectrum of the bright extremely metal-poor star 2MASS J1808−5104. We find [Fe/H] = −4.01 (spectroscopic LTE stellar parameters), [Fe/H] = −3.8 (photometric stellar parameters), and [Fe/H] = −3.7 (spectroscopic NLTE stellar parameters). We measured a carbon-to-iron ratio of [C/Fe] = 0.38 from the CH G-band. J1808−5104 is thus not carbon-enhanced, contrary to many other stars with similarly low-iron abundances. We also determine, for the first time, a barium abundance ([Ba/Fe] = −0.78), and obtain a significantly reduced upper limit for the nitrogen abundance ([N/Fe] < −0.2). For its [Ba/Fe] abundance, J1808−5104 has a lower [Sr/Ba] ratio compared to other stars, consistent with behaviour of stars in ultra-faint dwarf galaxies. We also fit the abundance pattern of J1808−5104 with nucleosynthesis yields from a grid of Population III supernova models. There is a good fit to the abundance pattern that suggests J1808−5104 originated from gas enriched by a single massive supernova with a high explosion energy of E = 10 × 1051 erg and a progenitor stellar mass of M = 29.5 M⊙. Interestingly, J1808−5104 is a member of the Galactic thin disc, as confirmed by our detailed kinematic analysis and calculated stellar actions and velocities. Finally, we also established the orbital history of J1808−5104 using our time-dependent Galactic potential the ORIENT. J1808−5104 appears to have a stable quasi-circular orbit and been largely confined to the thin disc. This unique orbital history, the star’s very old age (∼13.5 Gyr), and the low [C/Fe] and [Sr/Ba] ratios suggest that J1808−5104 may have formed at the earliest epoch of the hierarchical assembly of the Milky Way, and it is most likely associated with the primordial thin disc.

 

Understanding the origin of the elements has been a decades-long pursuit, with many open questions remaining. Old stars found in the Milky Way and its dwarf satellite galaxies can provide answers because they preserve clean element abundance patterns of the nucleosynthesis processes that operated some 13 billion years ago, enabling reconstruction of the chemical evolution of the elements. This review focuses on the astrophysical signatures of heavy neutron-capture elements made in the s-, i-, and r-processes found in old stars. A highlight is the recently discovered r-process galaxy Reticulum II, which was enriched by a neutron star merger. These results show that old stars in dwarf galaxies provide a novel means to constrain the astrophysical site of the r-process, ushering in much-needed progress on this major outstanding question. This nuclear astrophysics research complements the many experimental and theoretical nuclear physics efforts into heavy-element formation, and also aligns with results on the gravitational-wave signature of neutron star mergers.