GHOST Commissioning Science Results II: a very metal-poor star witnessing the early Galactic assembly

TL;DR

This paper reports on the detailed chemical analysis of a very metal-poor star, P180956, using high-resolution spectroscopy, revealing its origins and the early Galactic assembly processes through its unique elemental signatures.

Contribution

It presents the first detailed chemical characterization of P180956 with GHOST, linking its properties to early Galactic assembly and accretion of ultra-faint dwarf systems.

Findings

Star shows chemical signatures similar to ultra-faint dwarf galaxies.

Abundance pattern suggests enrichment by a few low-mass hypernovae.

Orbit indicates the star was likely accreted during early Galaxy formation.

Abstract

This study focuses on Pristine$\_180956.78$$-$$294759.8$ (hereafter P180956, $[Fe/H] =-1.95\pm0.02$), 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 ($3700-4800$~\AA) enables the detection of elemental tracers of early supernovae (\eg 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 ($\sim10-15 M_{\odot}$), which are needed to mostly reproduce the abundance pattern of the light-elements (\eg [Si, Ti/Mg, Ca] $\sim0.6$), and one fast-rotating intermediate-mass supernova ($\sim300\kms$, $\sim80-120 M_{\odot}$), which is the main channel contributing to the high [Sr/Ba] ($\sim +1.2$). The small pericentric ($\sim0.7$ kpc) and apocentric ($\sim13$ kpc) distances and its orbit confined to the plane ($\lesssim 2$ 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.