GHOST commissioning science results - II: A very metal-poor star witnessing the early galactic assembly

This study focuses on Pristine 180956.78−294759.8 (hereafter P180956, [Fe/H] = −1.95 ± 0.02), a star selected from the Pristine Inner Galaxy Survey (PIGS), and followed-up with the recently commissioned Gemini High-resolution Optical SpecTrograph (GHOST) at the Gemini South telescope. The GHOST spectrograph’s high efficiency in the blue spectral region (3700−4800 Å) enables the detection of elemental tracers of early supernovae (e.g. Al, Mn, Sr, and Eu). The star exhibits chemical signatures resembling those found in ultrafaint dwarf (UFD) systems, characterized by very low abundances of neutron-capture elements (Sr, Ba, and Eu), which are uncommon among stars in the Milky Way halo. Our analysis suggests that P180956 bears the chemical imprints of a small number (2 or 4) of low-mass hypernovae (∼ 10−15 M_⃝), which are needed to mostly reproduce the abundance pattern of the light-elements (e.g. [Si, Ti/Mg, Ca] ∼0.6), and one fast-rotating intermediate-mass supernova (∼ 300 km s⁻¹, ∼ 80−120 M_⃝), which is the main channel contributing to the high [Sr/Ba] (∼+1.2). The small pericentric (∼ 0.7 kpc) and apocentric (∼ 13 kpc) distances and its orbit confined to the plane (2 kpc) indicate that this star was likely accreted during the early Galactic assembly phase. Its chemo-dynamical properties suggest that P180956 formed in a system similar to a UFD galaxy accreted either alone, as one of the low-mass building blocks of the proto-Galaxy, or as a satellite of Gaia–Sausage–Enceladus. The combination of Gemini’s large aperture with GHOST’s high efficiency and broad spectral coverage makes this new spectrograph one of the leading instruments for near-field cosmology investigations.