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

Abstract

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, Eu). The star exhibits chemical signatures resembling those found in ultra-faint dwarf systems, characterised by very low abundances of neutron-capture elements (Sr, Ba, 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, ∼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 an ultra-faint dwarf 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.