2022MNRAS.517.3993M

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.