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

Federico Sestito1, Christian R. Hayes2, Kim A. Venn1, Jaclyn Jensen1, Alan W.
McConnachie2,1, John Pazder2,1, Fletcher Waller1, Anke Arentsen3, Pascale
Jablonka4,5, Nicolas F. Martin6,7, Tadafumi Matsuno8, Julio F. Navarro1, Else
Starkenburg8, Sara Vitali9, John Bassett10, Ruben Diaz10, Michael L. Edgar11,
Veronica Firpo10, Manuel Gomez-Jimenez10, Venu Kalari10, Sam Lambert2, Jon
Lawrence12, Gordon Robertson12, Roque Ruiz-Carmona10, Ricardo Salinas10, Kim
M. Sebo13, and Sudharshan Venkatesan12
1 Department of Physics and Astronomy, University of Victoria, PO Box 3055,
STN CSC, Victoria BC V8W 3P6, Canada
2 NRC Herzberg Astronomy & Astrophysics, 5071 West Saanich Road, Victoria, BC
V9E 2E7, Canada
3 Institute of Astronomy, University of Cambridge, Madingley Road, Cambridge
CB3 0HA, UK
4 Laboratoire d’astrophysique, École Polytechnique Fédérale de Lausanne
(EPFL), Observatoire, CH-1290 Versoix, Switzerland
5 GEPI, Observatoire de Paris, Université PSL, CNRS, 5 Place Jules Janssen,
F-92195 Meudon, France
6 Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR
7550, F-67000 Strasbourg, France
7 Max-Planck-Institut für Astronomie, Königstuhl 17, D-69117 Heidelberg,
Germany
8 Kapteyn Astronomical Institute, University of Groningen, Landleven 12,
NL-9747AD Groningen, the Netherlands
9 Instituto de Estudios Astrofísicos, Facultad de Ingeniería y Ciencias,
Universidad Diego Portales, Av. Ejército Libertador 441, Santiago, Chile
10 Gemini Observatory/NSF’s NOIRLab, Casilla 603, La Serena, Chile
11 Australian Astronomical Observatory
12 Australian Astronomical Optics, Macquarie University, 105 Delhi Rd, North
Ryde NSW 2113, Australia
13 Research School of Astronomy and Astrophysics, College of Science,
Australian National University, Canberra 2611, Australia Email:
<EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

This study focuses on Pristine$\\_180956.78$$-$$294759.8$ (hereafter P180956,
${\rm[Fe/H]}=-1.95\pm 0.02$), 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), which were not accessible in the previous analysis of P180956.
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 of comparable
metallicity in the Milky Way. Our analysis suggests that P180956 bears the
chemical imprints of a small number (2 or 4) of low-mass hypernovae ($\sim
10-15{\rm\,M_{\odot}}$), which are needed to reproduce the abundance pattern
of the light-elements (e.g., [Si, Ti/Mg, Ca] $\sim 0.6$), and one fast-
rotating intermediate-mass supernova ($\sim 300{\rm\,km\ s^{-1}}$, $\sim
80-120{\rm\,M_{\odot}}$). Both types of supernovae explain the high [Sr/Ba] of
P180956 ($\sim 1.2$). The small pericentric ($\sim 0.7{\rm\,kpc}$) and
apocentric ($\sim 13{\rm\,kpc}$) distances and its orbit confined to the plane
($\lesssim 2{\rm\,kpc}$), 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.

###### keywords:

Galaxy: formation - Galaxy: evolution - Galaxy: bulge - Galaxy: abundances -
stars: kinematics and dynamics - stars: Population II

††pubyear: 2023††pagerange: GHOST Commissioning Science Results II: a very
metal-poor star witnessing the early Galactic assembly–References

##  1 Introduction

Low-metallicity stars are among the oldest stars in the Galaxy. Cosmological
simulations suggest that most metal-poor stars formed within $2-3$ Gyr after
the Big Bang, likely in low-mass systems that were accreted early on into the
Galaxy ("building blocks", e.g., Starkenburg et al., 2017a; El-Badry et al.,
2018; Sestito et al., 2021). These merging building blocks contributed stars,
gas, and dark matter to the proto-Milky Way. Because they formed at the bottom
of the potential well of the merging building blocks, these stars are
predicted to occupy the inner regions of the present-day Galaxy (e.g., White &
Springel, 2000; Starkenburg et al., 2017a; El-Badry et al., 2018; Sestito et
al., 2021). Systems accreted later are anticipated to disperse their stars
primarily in the halo (Bullock & Johnston, 2005; Johnston et al., 2008;
Tissera et al., 2012), or possibly the disc (e.g., Abadi et al., 2003; Sestito
et al., 2021; Santistevan et al., 2021). An in-situ component may also form
from the gas deposited by the building blocks, recently called the Aurora
stars (Belokurov & Kravtsov, 2022). This in-situ component may have formed in
a lumpy and chaotic interstellar medium (ISM), possibly resembling the
chemical properties of globular clusters (Belokurov & Kravtsov, 2023).

The most chemically pristine stars in the Milky Way (MW) may have been
enriched by only one or a few supernovae or hypernovae events (e.g., Frebel et
al., 2010a; Ishigaki et al., 2018). The study of the orbital properties and
chemical abundance patterns of pristine stars is essential for understanding
the lives and deaths of the first stars and the assembly history of the Galaxy
(e.g., Freeman & Bland-Hawthorn, 2002; Venn et al., 2004; Tumlinson, 2010;
Wise et al., 2012; Karlsson et al., 2013).

Metal-poor stars in and towards the Galactic bulge can serve as important
tracers of the earliest stages of Galactic assembly, yet their detection is
extremely challenging (e.g., Schlaufman & Casey, 2014; Lamb et al., 2017). The
inner regions of the MW are dominated by a metal-rich population and disrupted
globular clusters (Ness et al., 2013a, 2014; Bensby et al., 2013, 2017;
Schiavon et al., 2017; Schultheis et al., 2019). Furthermore, extreme
interstellar extinction and stellar crowding have made photometric surveys of
bulge metal-poor stars exceedingly difficult.

The Abundances and Radial velocity Galactic Origins Survey (ARGOS, Ness et
al., 2013b) found that $\lesssim 1$ percent of their sample had
[Fe/H]111[Fe/H]$=\log(\rm{N_{Fe}/N_{H}}){\star}-\log(\rm{N_{Fe}/N_{H}})_{\odot}$,
where $\rm{N_{X}}$ represents the number density of element X. $<-1.5$,
resulting in a total of 84 stars. The metallicity-sensitive photometric filter
from the SkyMapper Southern Survey (Bessell et al., 2011; Wolf et al., 2018)
has been used by the Extremely Metal-poor BuLge stars with AAOmega (EMBLA,
Howes et al., 2014, 2015, 2016) survey to select very metal-poor stars (VMPs,
${\rm[Fe/H]}\leq-2.0$). Their high-resolution analysis of 63 VMPs revealed
that the majority resembled chemically metal-poor stars in the Galactic halo,
with the exception of a lack of carbon-rich stars and a larger scatter in
[$\alpha$/Fe] abundances. Additionally, their kinematic analysis found that it
was challenging to distinguish stars that were born in the bulge from those
that are merely in the inner halo. The Chemical Origins of Metal-poor Bulge
Stars (COMBS, Lucey et al., 2019) studied the chemo-dynamical properties of
inner Galactic stars, finding that around $\sim 50$ percent of their sample is
composed of halo interlopers, while their chemical properties resemble those
of the halo (Lucey et al., 2021, 2022).

Similar to the EMBLA survey, the Pristine Inner Galaxy Survey (PIGS, Arentsen
et al., 2020a, b) selected metal-poor targets from the narrow-band photometry
of the Pristine survey (Starkenburg et al., 2017b). The Pristine survey,
conducted at the Canada-France-Hawaii Telescope (CFHT), utilises the CaHK
filter in combination with broad-band photometry to provide a highly efficient
method of identifying low-metallicity stars ($\sim$56 percent success rate at
${\rm[Fe/H]}\leq-2.5$, Youakim et al., 2017; Aguado et al., 2019; Venn et al.,
2020; Lucchesi et al., 2022). Around $\sim$12,000 inner Galaxy metal-poor
candidates selected by PIGS were observed with low-/medium-resolution
spectroscopy using the AAOmega spectrograph on the Anglo Australian Telescope
(AAT). The results of these observations showed $\sim$80 percent efficiency in
identifying VMP stars towards the bulge (Arentsen et al., 2020b) and the
Sagittarius dwarf galaxy (Vitali et al., 2022) using the Pristine metallicity-
sensitive filter for initial selection. Interestingly, within PIGS, Mashonkina
et al. (2023) reports the serendipitous discovery of the first r- and s-
processes rich Carbon-enhanced star (CEMP$-$r/s) in the inner Galaxy.

In a recent study, Sestito et al. (2023b) analysed high-resolution spectra of
17 metal-poor stars selected from the PIGS survey taken with the Gemini Remote
Access to CFHT ESPaDOnS Spectrograph (GRACES, Chene et al., 2014; Pazder et
al., 2014). Their findings, consistent with Howes et al. (2016), indicate that
the chemo-dynamical properties of the VMP population in the inner Galaxy
resemble that of the halo, suggesting a common origin from disrupted building
blocks. Sestito et al. (2023b) report stars with chemical abundances
compatible with those of disrupted second-generation globular clusters (GC),
one with exceptionally low metallicity (${\rm[Fe/H]}\sim-3.3$), well below the
metallicity floor of GCs (${\rm[Fe/H]}\sim-2.8$, Beasley et al., 2019). This
provides further evidence that extremely metal-poor structures (EMPs,
${\rm[Fe/H]}\leq-3.0$) can form in the early Universe (see also Martin et al.,
2022, on the discovery of the disrupted EMP globular cluster, C-19).

In this paper, we present a new analysis of the inner Galactic very metal-poor
Pristine$\\_180956.78$$-$$294759.8$ (P180956), from spectra taken during the
commissioning of the new Gemini High-resolution Optical SpecTrograph (GHOST,
Pazder et al., 2020). This star was previously analysed in the PIGS/GRACES
analysis (Sestito et al., 2023b), but it can be observed from either the
northern hemisphere (GRACES) or the south (GHOST, at Gemini South). P180956 is
selected for this work for its unusually low [Na, Ca/Mg] and [Ba/Fe] ratios
and for its high eccentric orbit that remains confined close to the Milky Way
plane. The new GHOST spectrograph has also been used in the analysis of two
stars in the Reticulum II ultra-faint dwarf galaxy (Hayes et al., 2023), and
the presentation of the spectrum of the r-process rich standard star HD 222925
(Hayes et al., 2022; McConnachie et al., 2022). GHOST’s wide spectral coverage
and its high efficiency in the blue region are crucial to detect species that
are tracers of the early chemical evolution (e.g., Al, Mn, Sr, Eu) and that
were not accessible with GRACES spectrograph.

The instrument setup and data reduction are described in Section 2. Sections 3
and 4 discuss the model atmospheres analysis and the chemical abundance
analyses, respectively. Orbital parameters are reported in Section 5. The
results are discussed in Section 6, focusing on the type of supernovae that
polluted the formation site of P180956, and its origin in an ancient dwarf
system, which may have resembled today’s ultra-faint dwarfs. Conclusions are
presented in Section 7.

##  2 Data

###  2.1 GHOST observations

The target (G $=13.50$ mag), P180956, was initially observed as part of the
PIGS photometric survey using MegaCam at the Canada-France-Hawaii Telescope
(CFHT). Its small pericentric distance (r${}_{\rm{peri}}\sim 0.7{\rm\,kpc}$),
the apocentre ($\sim 13{\rm\,kpc}$), the limited maximum excursion from the
plane (Z${}_{\rm{max}}\sim 1.8{\rm\,kpc}$), and its high eccentricity
($\epsilon\sim 0.90$) imply that it was likely accreted during the early
stages of Galactic assembly (Sestito et al., 2023b). A low/medium-resolution
(R $\sim 1,300$ and R $\sim 11,000$) spectrum of P180956 was obtained using
AAT/AAOmega (Arentsen et al., 2020b), and also with the Gemini Remote Access
to CFHT ESPaDOnS Spectrograph (GRACES, R $\sim 40,000$, Chene et al., 2014;
Pazder et al., 2014) as part of LLP-102 (PI K.A. Venn). The GRACES spectrum
was analysed by Sestito et al. (2023b) along with 16 other VMP stars in the
Galactic bulge; however, due to limitations in the throughput of the 300-metre
optical fiber, the bluest spectral regions were not accessible for that study.
The GRACES spectral analysis was limited to a range of $4900-10000$ Å only.
Thus, P180956 was chosen as a commissioning target for the Gemini High-
resolution Optical SpecTrograph (GHOST, Pazder et al., 2020). The instrument’s
high efficiency in the blue spectral region makes it ideal for detecting
spectral features of additional chemical elements which allow a deeper
investigation of the origins of P180956.

The target was observed on September 12th, 2022, during the second
commissioning run of GHOST. Three acquisitions, each lasting 600 seconds, were
conducted. The instrument was configured in the standard resolution (R $\sim
50,000$) single object mode, employing a spectral and spatial binning of 2 and
4, respectively. This specific setup and exposure times were chosen to enable
the detection of spectral lines from species in the bluer regions ($\sim$4000
Å) of the spectrum that were not accessible with GRACES, i.e., C, Al, Si, Sc,
V, Mn, Co, Cu, Zn, Sr, Y, La, Eu. Table 1 reports the Gaia DR3 source ID and
photometry, the reddening from Green et al. (2019) along with the total
exposure time and the number of exposures.

Table 1: Log of the observations. The Pristine name, the short name, the
source ID, G, and BP$-$RP from Gaia DR3, the reddening from the 3D map of
Green et al. (2019), the total exposure time, the number of exposures, the
SNR, and the radial velocity are reported. The SNR is measured as the ratio
between the median flux and its standard deviation in three spectral regions,
close to the Eu ii 4129 Å, in the Mg ib Triplet and in the Na i Doublet.

Pristine name | source ID | G | BP$-$RP | E(B$-$V) | Texp | Nexp | SNR | RV
---|---|---|---|---|---|---|---|---
|  | (mag) | (mag) | (mag) | (s) |  | @Eu ii, @Mg ib, @Na i | (${\rm\,km\ s^{-1}}$)
Pristine$\\_180956.78$$-$$294759.8$ | 4050071013878221696 | 13.50 | 1.55 | 0.53 | 1800 | 3 | 16, 50, 65 | $262.976\pm 0.363$

###  2.2 Data reduction

The acquired spectra were processed using the GHOST Data Reduction pipeline
(GHOSTDR, Ireland et al., 2018; Hayes et al., 2022), which is integrated into
the DRAGONS suite (Labrie et al., 2019). This
pipeline222https://github.com/GeminiDRSoftware/GHOSTDR generates 1D spectra
for the blue and red arms, which were wavelength calibrated, order-combined,
and sky-subtracted. Subsequently, the spectra were individually normalized
through polynomial fitting, and barycentric corrections were applied. A radial
velocity correction was performed using the Doppler
code333https://github.com/dnidever/doppler, employing a VMP template
synthesized with
Turbospectrum444https://github.com/bertrandplez/Turbospectrum2019 (Plez, 2012)
and a MARCS555https://marcs.astro.uu.se model atmosphere (Gustafsson et al.,
2008). The radial velocity measured from the blue arm is RV $=263.034\pm
0.020{\rm\,km\ s^{-1}}$, while from the red arm is RV $=262.918\pm
0.362{\rm\,km\ s^{-1}}$. Their average, RV $=262.976\pm 0.363{\rm\,km\
s^{-1}}$, is in agreement by $0.8\sigma$ with the measurement from the GRACES
analysis (RV $=262.45\pm 0.54{\rm\,km\ s^{-1}}$, Sestito et al., 2023b) and by
$2.3\sigma$ with the value from the low-/medium-resolution PIGS campaign (RV
$=264.86\pm 0.73{\rm\,km\ s^{-1}}$, Arentsen et al., 2020a). The lack of RV
variation likely rules out the possibility that the object is in a binary
system. The continuum points in the observations were identified using
spectral templates and fitted via an iterative sigma clipping method to obtain
a normalised spectrum. Finally, the blue and red output spectra were merged,
taking into account the signal-to-noise ratio (SNR) in the overlapping
regions.

Figure 1 showcases the reduced spectrum of P180956. In the top panel, a
comparison is made with the GRACES observation in the $4400-5200$ Å region.
Both instruments have similar spectral (high) resolution; however, the SNR of
the GRACES spectrum (black line) deteriorates below $5000$ Å, a region where
the number of spectral lines clearly increases, as seen in the GHOST spectrum
(blue). The Balmer line H$-\epsilon$, the Ca ii H&K lines, along with two Al i
lines, two Ti i, five Fe i lines, and one Co i line are shown in the central
panel. The Mg ib Triplet region also contains several Fe and Ti lines, as
shown in the bottom panel. Table 1 reports the SNR measured close to the Eu ii
4129 Å line, the Mg ib Triplet region, and to the Na i Doublet.

Figure 1: GHOST spectrum of P180956. Top panel: Wide spectral region,
$4400-5200$ Å, also showing the GRACES spectrum (black line). Central panel:
blue region $3900-4000$ Å. Bottom panel: Mg ib Triplet. Spectral lines used
for the chemical abundance analysis are marked.

##  3 Model Atmospheres Analysis

###  3.1 Stellar parameters

The stellar parameters used in this study are adopted from the GRACES analysis
by Sestito et al. (2023b, T${}_{\rm{eff}}=5391\pm 133$ K, logg $=1.87\pm
0.10$). To briefly summarize, the effective temperature is estimated based on
the color-temperature relationship derived by Mucciarelli et al. (2021). This
relationship is based on the Infrared Flux Method introduced by González
Hernández & Bonifacio (2009) and adapted to the Gaia EDR3 photometry. The
surface gravity is determined by applying the Stefan-Boltzmann equation,
inferred assuming a flat mass distribution between 0.5 to 0.8
${\rm\,M_{\odot}}$. These calculations rely on several factors: 1) the de-
reddened photometry666Extinction is from the 3D map of Green et al. (2019). To
convert from the E(B-V) map to Gaia extinction coefficients, the $\rm
A_{V}/E(B-V)=3.1$ (Schultz & Wiemer, 1975) and the $\rm A_{G}/A_{V}=0.85926$,
$\rm A_{BP}/A_{V}=1.06794$, $\rm A_{RP}/A_{V}=0.65199$ relations (Marigo et
al., 2008; Evans et al., 2018) are used.; 2) the distance to the star, which
is estimated to be $3.30\pm 0.27$ kpc (Sestito et al., 2023b); and 3) a
metallicity (taken as ${\rm[Fe/H]}=-2.0\pm 0.1$, Sestito et al., 2023b).
Uncertainties on the stellar parameters are derived using a Monte Carlo
simulation.

A first estimate of the microturbulence velocity (vmicro) was calculated using
the relation provided by Mashonkina et al. (2017a). However, that value was
slightly revised to $1.5{\rm\,km\ s^{-1}}$ to achieve a flatter distribution
in the abundances from the Fe i lines, A(Fe i) vs. the reduced equivalent
width.

###  3.2 Spectral lines and atomic data

The spectral line list is generated with linemake (Placco et al., 2021),
including lines with hyper-fine structure corrections (Sc, V, Mn, Co, and Cu),
molecular bands (CH in the 4300 Å region), and isotopic corrections (Sr, Ba,
Eu).

An initial measurement of the equivalent width is performed using DAOSPEC
(Stetson & Pancino, 2008), which automatically fits Gaussian profiles to
spectra following the input line list. Given the signal-to-noise ratio of our
spectrum, lines weaker than 15 mÅ are rejected, and lines stronger than 100 mÅ
are further examined with non-Gaussian measurements. The equivalent widths are
then used with the MOOG777https://www.as.utexas.edu/~chris/moog.html spectrum
synthesis code (Sneden, 1973; Sobeck et al., 2011) to determine the chemical
abundances assuming Local Thermodynamic Equilibrium (LTE). The spherical
MARCS888https://marcs.astro.uu.se model atmospheres (Gustafsson et al., 2008;
Plez, 2012) are used for the chemical abundance analysis in this paper, which
yield to ${\rm[Fe/H]}=-1.95\pm 0.02$. The chemical abundances of Sc, Cu, Y,
Ba, La, and Eu are determined using the synth mode within MOOG.

The list of spectral lines used for the chemical abundance analysis, their EWs
or abundances from synthesis are reported as supplementary online material.

###  3.3 Uncertainties on the chemical abundances

MOOG provides estimates of the chemical abundances A(X) along with their line-
to-line scatter, $\delta_{\rm A(X)}$. The total abundance uncertainties in
this paper are calculated by combining the line-to-line scatter ($\delta_{\rm
EW}$) with the uncertainties resulting from variations in the stellar
parameters ($\delta_{\rm T_{eff}}$, $\delta_{\rm logg}$), in quadrature. The
final uncertainty for element X is given by $\sigma_{\rm A(X)}=\delta_{\rm
A(X)}/\sqrt{{\rm N_{X}}}$. In case there is only one spectral line or synth
mode is employed, the dispersion in Fe i lines is considered as the typical
dispersion.

##  4 Chemical abundance analysis

The blue wavelength coverage of GHOST permits an analysis of the spectral
lines of many elements in metal-poor stars, including a range of $\alpha$-,
odd-Z, Fe-peak, and neutron-capture process elements.

###  4.1 Carbon

Carbon was first inferred from the low-/medium-resolution campaign of PIGS
(Arentsen et al., 2020a), showing the star is Carbon-normal, [C/Fe] $=0.17\pm
0.24$. Figure 2 shows the synthesis on the CH bands in the $4300$ Å region
(top panel) and the residuals (bottom panel). The synthesis is made with synth
mode of MOOG, yielding a [C/Fe] $=0.2\pm 0.1$, in agreement with the previous
measurement.

Figure 2: Carbon synthesis. Top panel: The observed spectrum of P180956 is
marked with a blue line, while synthetic spectra are denoted by black ([C/Fe]
$=0.0$), red ([C/Fe] $=0.2$, best fit), and olive ([C/Fe] $=0.5$) lines.
Bottom panel: Residuals of the fits. Horizontal dashed lines marks the null
difference (blue) and the dispersion around the continuum ($\pm 0.05$ dex,
black). The residuals of the best fit (red line) in the regions of Carbon
bands are within or similar to the level of the continuum dispersion.

###  4.2 $\alpha$-elements

The $\alpha$-elements producing detectable lines in this star are Mg, Si, Ca,
and Ti. The A(Mg i) is from two lines of the Mg i Triplet ($\lambda\lambda
5172.684,5183.604$Å) and from other 5 lines for which the SNR is high. Si i is
detected from the $4102.936$ Å line, which is blended with the wing of the
broader H$\delta$ line. The A(Ca i) is inferred from 17 spectral lines, from
4200 Å to 6500 Å. The Ca Triplet has been excluded since it shows strong lines
($>140$ mÅ). Ti i and Ti ii are present with 17 and 30 lines, respectively.

###  4.3 Odd-Z elements

Four odd-Z elements are detectable in the GHOST spectrum of this object, Na,
Al, K and Sc. Na i is present with the Na i Doublet ($\lambda\lambda
5889.951,5895.924$ Å). The Na i D lines from the interstellar medium are not
affecting the stellar component, in agreement with Sestito et al. (2023b). The
combination of the wide spectral coverage of GHOST and its efficiency in the
blue region of the spectrum allows us to detect the 2 lines of Al i
$\lambda\lambda 3944.006,3961.520$ Å. K i is observable with two lines at
$\lambda\lambda 7664.899,7698.965$ Å, which are usually close to water vapour
lines of the Earth’s atmosphere. In this case, the K i spectral lines are well
separated from the telluric lines. Sc is present with 6 Sc ii lines from 4200
to 5700 Å.

###  4.4 Fe-peak elements

The Fe-peak elements that are observable in the GHOST spectra of this target
are Fe, V, Cr, Mn, Co, Ni, Cu and Zn. A(Fe i) and A(Fe ii) are measured from
188 and 15 lines, respectively. V i is present with the 4379.230 Å line. Cr i
and Cr ii are detected from 8 and 1 line (4558.650 Å), respectively. A(Mn i)
is from 7 lines between 4000 to 4850 Å. A(Co i) is measured from 4 lines in
the blue from 3800 to 4150 Å. Ni i is detected from the $\lambda\lambda
5476.904,6643.630$ Å lines. An upper limit on A(Cu i) is obtained with the
synthesis on the $\lambda\lambda 5105.541,5782.132$ Å lines, [Cu/Fe] $<-0.2$.
Only line 4810.528 Å of Zn i is present in this GHOST spectrum.

###  4.5 Neutron-capture process elements

The blue coverage of the GHOST instrument is essential to detect the neutron-
capture process elements in the spectrum of this very metal-poor star, namely
Sr, Y, Ba, La, and Eu. Sr ii is observed from the $\lambda\lambda
4077.709,4215.519$ lines. A(Y ii) is measured with the synth mode in MOOG from
the $\lambda\lambda 4398.010,4900.119,4883.682,5200.409$ Å, giving an upper
limit of [Y/Fe] $<0.0$. The synthesis of Ba ii is made from 4 lines,
$\lambda\lambda 4554.029,4934.076,6141.730,6496.910$ Å, which provide a value
in agreement with the GRACES analysis, [Ba/Fe] $\sim-1.5$. La ii produces very
weak lines, $\lambda\lambda 4920.980,5290.820,5301.970,5303.530$ Å, resulting
in a high upper limit, [La/Fe] $<0.5$. The Eu ii lines, $\lambda\lambda
4129.700,4205.050$ Å, are hard to detect, giving an upper limit of [Eu/Fe]
$<0.0$. Other species of neutron-capture elements have been inspected, e.g.,
Nd and Tb. They present lines at the level of the noise, resulting in
uninformative and very high upper limits.

###  4.6 Non-Local Thermodynamic Equilibrium corrections

The atmosphere of VMP stars are affected by non-local thermodynamic
equilibrium (NLTE) effects, which can be large for some species. NLTE
corrections have been applied to Fe i and Fe ii (Bergemann et al., 2012a), Mg
i (Bergemann et al., 2017), Si i (Bergemann et al., 2013), Ca i (Mashonkina et
al., 2017a), Ti i and Ti ii (Bergemann, 2011), Cr i (Bergemann & Cescutti,
2010), Mn i (Bergemann et al., 2019), and Co i (Bergemann et al., 2010) using
the MPIA data base999https://nlte.mpia.de. For Na i (Lind et al., 2012) and Sr
ii (Bergemann et al., 2012b), the INSPECT101010http://www.inspect-stars.com
webtool was used. Note that NLTE corrections for Sr ii are not available for a
star with the stellar parameters of P180956, therefore, the closest parameters
logg $=2.3$ (vs. $1.87\pm 0.10$) and v${}_{\rm{micro}}=1.0{\rm\,km\ s^{-1}}$
(vs. $1.5\pm 0.1{\rm\,km\ s^{-1}}$) provide a negligible correction of
$\sim-0.01$ dex. Similarly, Ba ii NLTE corrections, adopted from Mashonkina &
Belyaev (2019), are not available for this star. The closest match in their
online database111111http://www.inasan.ru/~lima/pristine/ba2/ gives a minor
correction of $\lesssim 0.05$. NLTE corrections for K are obtained from
Ivanova & Shimanskiĭ (2000), which also include hyperfine structure
corrections. We highlight that NLTE corrections on Fe, Ti, and Cr are helpful
to obtain the ionisation balance among these species, i.e., A(X i) $\approx$
A(X ii).

Table 2 reports the chemical abundances ratios in LTE, [X/H]LTE, their
uncertainties, the number of lines used, and the average NLTE corrections,
$\Delta_{\rm{NLTE}}$.

Table 2: Chemical abundances [X/H] in LTE, their uncertainties,
$\sigma_{\rm{[X/H]}}$, already divided by the square root of the number of
lines, the number of spectral lines used Nlines, and the NLTE corrections
$\Delta_{\rm{NLTE}}=\rm{[X/H]}_{\rm{NLTE}}-\rm{[X/H]}_{\rm{LTE}}$.

Species | [X/H]LTE | $\sigma_{\rm{[X/H]}}$ | Nlines | $\Delta_{\rm{NLTE}}$
---|---|---|---|---
Fe i | $-1.95$ | $0.02$ | 188 | $+0.148$
Fe ii | $-1.64$ | $0.04$ | 15 | $+0.004$
C | $-1.80$ | $0.10$ | $-$ | $-$
Na i | $-2.38$ | $0.05$ | 2 | $-0.457$
Mg i | $-1.94$ | $0.09$ | 7 | $+0.089$
Al i | $-2.20$ | $0.25$ | 2 | $-$
Si i | $-1.25$ | $0.10$ | 1 | $+0.040$
K i | $-1.40$ | $0.13$ | 2 | $-0.300$
Ca i | $-2.03$ | $0.05$ | 17 | $+0.173$
Sc ii | $-1.23$ | $0.07$ | 6 | $-$
Ti i | $-1.81$ | $0.04$ | 17 | $+0.509$
Ti ii | $-1.27$ | $0.03$ | 30 | $-0.038$
V i | $-2.07$ | $0.10$ | 1 | $-$
Cr i | $-2.23$ | $0.04$ | 8 | $+0.382$
Cr ii | $-1.87$ | $0.10$ | 1 | $-$
Mn i | $-2.40$ | $0.05$ | 7 | $+0.381$
Co i | $-1.59$ | $0.21$ | 4 | $+0.583$
Ni i | $-1.82$ | $0.04$ | 2 | $-$
Cu i | $<-2.20$ | $-$ | 2 | $-$
Zn i | $-1.66$ | $0.10$ | 1 | $-$
Sr ii | $-2.25$ | $0.01$ | 2 | $-0.011$
Y ii | $<-2.00$ | $-$ | 4 | $-$
Ba ii | $-3.50$ | $0.10$ | 4 | $<0.05$
La ii | $<-1.50$ | $-$ | 4 | $-$
Eu ii | $<-2.00$ | $-$ | 2 | $-$

###  4.7 Comparison with the GRACES spectral analysis

One of the reasons that this star P180956 was selected as a GHOST
commissioning target was its unusually low [Na/Mg], [Ca/Mg], and [Ba/Fe]
ratios, either in LTE or NLTE. Thus, we examine our abundance results from
this improved GHOST spectrum in comparison to those from the more limited
GRACES spectrum.

In general, the chemical abundance results from the GHOST spectra are similar
to those from the previous GRACES analysis, as seen in Figure 3, both in LTE.
The agreement is excellent for the majority of the elements ($\lesssim
2\sigma$), as expected given that we have adopted the same stellar parameters
and model atmospheres analysis methods. Furthermore, EW measurements of lines
in common to both analyses are similar, to within $\sim 5-10$ percent. The
better quality of the GHOST spectrum at all wavelengths and the larger number
of lines produce a smaller line-to-line dispersion, and, therefore, smaller
uncertainties on [X/H].

Our Fe i abundance from GHOST is determined from 188 lines, whereas the GRACES
analysis included only 63 lines, in which the mean Fe i abundance changes by
only 0.05 dex, and the line-to-line scatter improves by a factor of $\sim
1.5$.

Larger differences ($\gtrsim 3\sigma$) are found for Mg i, Ti ii, and Ni i,
primarily due to the addition of many new spectral lines, which alter the
average A(X), and the lower noise around spectral lines.

The revised ratios are [Na/Mg]${}_{\rm{NLTE}}=-0.99\pm 0.10$ and
[Ca/Mg]${}_{\rm{NLTE}}=-0.01\pm 0.10$. Comparing the other $\alpha-$elements
to Mg, this star shows [Si/Mg]${}_{\rm{NLTE}}=+0.64\pm 0.10$ and
[Ti/Mg]${}_{\rm{NLTE}}=+0.55\pm 0.10$. Thus, P180956 exhibits very low
[Na/Fe], slightly sub-solar [Mg, Ca/Fe], enhanced [Si, Ti/Mg], in addition to
a very low Ba-content. This is a somewhat unusual chemical abundance pattern
for a typical MW halo or bulge star.

Figure 3: GHOST vs. GRACES chemical abundances, [X/H]. GRACES abundances are
from Sestito et al. (2023b). Both datasets are in LTE. Points are slightly
offset in the horizontal direction to better show their errorbars.
Uncertainties on GHOST data are often smaller than the size of the marker.

###  4.8 Comparisons with the MW halo and bulge

The chemical abundances of P180956 are compared to a compilation of stars in
the MW bulge and halo in Figure 4. The panels in the figure are arranged in
order of increasing proton number of the species. The MW bulge compilation
(small light blue circles) includes results from Howes et al. (2014, 2015,
2016), Koch et al. (2016), Reggiani et al. (2020), and Lucey et al. (2022).
The MW halo compilation (small grey squares) consists of stars obtained from
the Stellar Abundances for Galactic Archaeology
database121212http://sagadatabase.jp (SAGA, Suda et al., 2008), with high-
resolution analysis and no lower or upper limits on the measurements. Both the
compilations are from LTE analyses.

The chemical ratios of [Na, Mg, Ca, Sr, Ba/Fe] and upper limit for [Eu/Fe] in
P180956 are situated at the lower end of the distribution observed in the MW
halo and bulge. This is particularly evident for [Ba/Fe], which is nearly 2
dex lower than the majority of stars at the same [Fe/H]. In contrast, [Si, Ti,
Sc, Co/Fe] ratios in P180956 are slightly enhanced compared to the literature
compilations for the MW. Other upper limits (Cu, Y, La) do not provide
significant constraints on the abundances.

Figure 4: Chemical abundances. Blue and red circles represent the chemical
abundances of P180956 in LTE and NLTE-corrected, respectively. Blue triangles
denote upper limits for P180956. Small light blue circles represent the bulge
compilation (Howes et al., 2014, 2015, 2016; Koch et al., 2016; Reggiani et
al., 2020; Lucey et al., 2022), while small grey squares correspond to the
halo compilation from the SAGA database (Suda et al., 2008). The uncertainties
on [X/Fe] for P180956 are often smaller than the symbol size. The scarcity of
literature stars in the [K/Fe] panel may be attributed to the difficulty in
measuring K spectral lines in the optical range, as they can be blended with
telluric water vapour lines.

##  5 Galactic Orbit

Orbital parameters are derived using Galpy code (Bovy, 2015), integrating the
orbit for 1 Gyr in the future and in the past. Uncertainties are determined
through a Monte Carlo simulation (1000 iterations) on the input quantities,
drawn from a Gaussian distribution. Two Galactic gravitational potentials are
adopted and the relative orbits are displayed in Figure 5. One potential
corresponds to the model used in Sestito et al. (2023b), which includes a
rotating bar (red line); the second is without the bar as in Sestito et al.
(2019).

In both cases, P180956 exhibits a slightly prograde orbit (vertical angular
momentum L${}_{\rm{z}}\sim 300$ kpc ${\rm\,km\ s^{-1}}$), which is confined to
the Milky Way plane (Z${}_{\rm{max}}\sim 2{\rm\,kpc}$). The pericentric
(R${}_{\rm{peri}}\sim 0.7{\rm\,kpc}$) and apocentric (R${}_{\rm{apo}}\sim
13{\rm\,kpc}$) distances indicate that the star’s trajectory takes it very
close to the Galactic centre before venturing far beyond the Sun’s position,
resulting in an orbit characterised by high eccentricity ($\epsilon\sim 0.9$).
These results are in agreement within $1\sigma$ with the previous orbital
analysis from Sestito et al. (2023b).

Table 3 reports pericentric and apocentric distances, the eccentricity, the
maximum excursion from the plane, the vertical component of the angular
momentum as calculated using both gravitational potentials.

Figure 5: Galactic orbit of P180956. Left panel: Height from the plane Z vs.
in-plane projected distance RXY. Central panel: Y vs. X Galactic positions.
Right panel: Z vs. X Galactic positions. Red and blue lines represent the
orbit integrated in a gravitational potential with and without the presence of
a rotating bar as in Sestito et al. (2023b) and as in Sestito et al. (2019),
respectively. The red, black and green circles mark the position of P180956,
of the Galactic centre (GC), and of the Sun at the present day.  Table 3:
Pericentric and apocentric distances (Rperi, Rapo), the eccentricity
($\epsilon$), the maximum excursion from the plane (Zmax), and the vertical
component of the angular momentum (Lz) are reported.

Quantity | Bar | No Bar
---|---|---
Rperi (kpc) | $0.67\pm 0.21$ | $0.73\pm 0.18$
Rapo (kpc) | $12.82\pm 1.06$ | $12.67\pm 0.74$
$\epsilon$ | $0.90\pm 0.03$ | $0.89\pm 0.02$
Zmax (kpc) | $2.01\pm 0.35$ | $2.02\pm 0.33$
Lz (kpc ${\rm\,km\ s^{-1}}$) | $307.0\pm 83.0$ | $303.0\pm 85.0$

##  6 Discussion

P180956 has peculiar kinematics that indicate that the star is confined to the
Milky Way plane on a very eccentric orbit, reaching both the very inner region
of the MW and a position well beyond the Sun (see Figure 5).

Furthermore, P180956 showed a very low amount of [Ba/Fe], [Ca/Mg], and
[Na/Mg]. These signatures stand out when compared to MW halo stars, and
resemble the abundances of stars found at present in ultra-faint dwarf
galaxies. Low chemical abundance ratios in the elements listed above have been
interpreted as a sign of contributions from a small number of low-mass
supernovae type II (SNe II) in the past, e.g., the so-called "one-shot" model
(Frebel et al., 2010a). Therefore, the interpretation of the chemo-dynamical
properties from the GRACES spectrum implied that this star may have formed in
an ancient dwarf galaxy accreted very early in the MW formation. In the
following subsections, a revised and more thorough discussion on the origin of
this star and on the properties of its formation site is presented.

###  6.1 The yields of the supernovae progenitors

The wavelength coverage of GHOST and the SNR of the observed spectra allow the
detection of up to 18 chemical species and four meaningful upper limits, for a
total of 13 elements beyond those available in the previous GRACES spectral
analysis. Comparing this extensive set of chemical abundances with the
predicted yields of supernovae from theoretical models can be used for
insights into the nucleosynthetic processes that occurred at the formation
site of P180956. To accomplish this, the online tool
StarFit131313https://starfit.org is used. The yields of best fit are obtained
by combining the supernovae II yields of the best solutions chosen from a pool
of theoretical models. A total of ten models have been selected, encompassing
various types of supernovae events, i.e., hypernovae, core-collapse, rotating
massive stars, neutron stars mergers, and pair-instability SNe.

The theoretical yields [X/H] from contributing SNe are compared with the
observational data in the top panel of Figure 6. The scaled solar abundances
ratios (black line) are from Asplund et al. (2009), shifted to match the
[Fe/H] of P180956. These fail to reproduce our data, except for Fe (by
design), Na, and Mg. The solar abundances pattern predicts a net decrease in
the yields from Si, which is not seen in P180956.

The best fit solution (magenta line) from StarFit consists in five SNe II
events originating from Population III stars. This fit is a mixture of three
low-mass hypernovae141414A hypernova explosion has a typical energy of at
least a factor $\sim 10$ greater than a classical supernova type II. with M
$\sim 10{\rm\,M_{\odot}}$ (Heger & Woosley, 2010; Heger et al., 2012), one
around M $\sim 15{\rm\,M_{\odot}}$ (Limongi & Chieffi, 2018), and one
intermediate-mass ($\sim 80{\rm\,M_{\odot}}$) fast-rotating ($\sim
300{\rm\,km\ s^{-1}}$) supernova (Grimmett et al., 2018). The second best fit
(orange line) solution is composed of three events, two low-mass hypernovae
with M $\sim 10-17{\rm\,M_{\odot}}$ (Heger & Woosley, 2010; Heger et al.,
2012) and one fast-rotating ($\sim 300{\rm\,km\ s^{-1}}$) SN with M $\sim
120{\rm\,M_{\odot}}$ (Limongi & Chieffi, 2018).

The difference between the observed [X/H] and the theoretical yields from the
best (magenta circles) and second best (orange squares) fits are displayed in
the bottom panel of Figure 6. Both solutions provide a difference below
$\lesssim 0.3$ in absolute value for the majority of the species.

We want to emphasise that the use of StarFit is more as an illustration of the
range of events that might be needed to explain the chemistry of this star.
StarFit suggests an interpretation that contributions from high-mass
supernovae ($>140{\rm\,M_{\odot}}$, e.g., pair-instability) and neutron-star
mergers are ruled out for this star. The former would produce a strong odd-
even effect in the yields (Takahashi et al., 2018; Salvadori et al., 2019),
i.e., very low [Na, Al/Mg] ($\sim-1.3$) and high [Ca/Mg] ($\gtrsim+0.6$); the
latter would produce an enrichment in neutron-capture elements (e.g., Cowan et
al., 2021). Both scenarios are in contrast with the observed chemical
properties of P180956. StarFit suggests that the main contributing SNe II are
in the low-mass range. Specifically, hypernovae are necessary to produce the
high [Si, Ti/Mg] ratios. These findings align with the scenario proposed by
Ishigaki et al. (2018), where low-mass hypernovae ($\lesssim
40{\rm\,M_{\odot}}$) are the primary sources of enrichment of the interstellar
medium during the early stages of chemical evolution. In addition, the
presence of one fast-rotating intermediate-mass supernova is needed to well
reproduce the pattern of the heavy elements, i.e., Sr, Ba.

Figure 6: Supernovae yields. Red and blue circles are NLTE and LTE [X/H] of
P180956, respectively. Upper limits of Cu, Y, and Eu are marked with empty
downward blue triangles. Top panel: Black dots and line represent the solar
scaled abundances from Asplund et al. (2009) shifted to match the [Fe/H] of
P180956. Magenta and orange dots and lines represent the theoretical yields
from the best (5 SNe II events) and second best fit (3 SNe II events),
respectively. Lower limits of Sc and Cu in the theoretical models are denoted
with empty upward triangles. Bottom panel: The difference between the observed
[X/H] and the yields predicted by the best (magenta circles) and second best
(orange squares) fit from StarFit. Upper limits in the data and lower limits
in the modelled yields are removed. Uncertainties on the data are summed in
quadrature to $0.15$ for the theoretical yields. The horizontal dashed line
marks the null difference.

###  6.2 In-situ vs. accreted diagnostics

What if this star formed in-situ, i.e., in the proto-disc but after the early
Galactic assembly? The period between the early MW assembly and the formation
of the disc, dubbed "Aurora" (Belokurov & Kravtsov, 2022), has been proposed
to be very chaotic, forming bound massive clusters, chemically similar to
globular clusters (Belokurov & Kravtsov, 2023). This implies that some Aurora
stars would be enriched in N, Na, and Al, while others would resemble the
"normal" halo stars (Belokurov & Kravtsov, 2023). P180956 is Na-poor, Al-
normal, and r-process poor, with the latter being very rare for halo stars at
that metallicity (see Figure 4). This chemical pattern rules out the "Aurora"
as the origin for P180956.

Other studies have suggested [Mg/Mn] vs. [Al/Fe] as a diagnostic to
differentiate accreted stars from in-situ stars (e.g., Das et al., 2020; Horta
et al., 2021). Figure 7 illustrates this chemical space, including APOGEE DR17
(Abdurro’uf et al., 2022) stars for comparison. Dashed black lines delineate
three regions where accreted, in-situ low-$\alpha$, and in-situ high-$\alpha$
stars are more likely to be found (e.g., Das et al., 2020; Horta et al.,
2021). In the LTE case, P180956 lies close to the centre of the accreted
"blob", while considering NLTE corrections, the star falls into the in-situ
low-$\alpha$ region.

Recently, Horta et al. (2023) provide a summary of the chemo-dynamical
properties of known accreted structures based on the latest Gaia and APOGEE
data releases. Their Figure 13 displays the [Mg/Mn] vs. [Al/Fe] space for all
the accreted known structures, showing that some of their stars have [Mg/Mn]
$\sim 0.0$ or even negative (e.g., Sagittarius, Sequoia, Gaia-Sausage-
Enceladus). Therefore, part of the in-situ low-$\alpha$ region, is actually
accreted. The accreted region in Figure 7 is here tentatively extended
(accreted low-$\alpha$), by lengthening the dashed green line, which suggests
that P180956 is of accreted origin. Offsets between the NLTE infra-red
(APOGEE) and the NLTE-corrected optical analyses are estimated to be
$\Delta\rm{[Mg/Mn]}=+0.15\pm 0.18$ and $\Delta\rm{[Al/Fe]}=-0.11\pm 0.12$
(Jönsson et al., 2020). These corrections, not applied in Figure 7, would move
P180956 more closely to the accreted low-$\alpha$ "blob".

Figure 7: [Mg/Mn] vs. [Al/Fe] space. P180956 data are shown in blue and red in
LTE and NLTE-corrected ([Mg/Mn] only), respectively. Small grey dots are
APOGEE DR17 MW stars selected to have SNR $>70$ and ${\rm[Fe/H]}<-0.7$, which
are derived from an NLTE analysis. The three regions delimited by the dashed
black lines, accreted and in-situ low-/high-$\alpha$ are defined following Das
et al. (2020) and Horta et al. (2021). The accreted region has been prolonged
following the dashed green line, in which accreted low-$\alpha$ stars should
lie.

Can P180956 be associated with any of the recently discovered accreted
structures? The system that bears the closest resemblance to P180956 in terms
of kinematical properties (see Horta et al., 2023) is Gaia-Sausage-Enceladus
(GSE, e.g., Helmi et al., 2018; Belokurov et al., 2018). The high eccentric
orbit of GSE ($0.93\pm 0.06$) and its mean pericentric and apocentric
distances ($r_{\rm{peri}}=0.61\pm 1.03{\rm\,kpc}$ and $r_{\rm{apo}}=17.15\pm
5.22{\rm\,kpc}$) are compatible with P180956’s orbital parameters within
$1\sigma$ (see Table 3). However, stars of GSE reach a maximum height from the
plane that is typically greater than the one of our target
($Z_{\rm{max}}=9.84\pm 6.14{\rm\,kpc}$ vs. $Z_{\rm{max}}=2.01\pm
0.35{\rm\,kpc}$).

Helmi et al. (2018) demonstrate that the GSE population has lower
[$\alpha$/Fe] ratios compared to stars in the MW halo. Similarly, Hayes et al.
(2018) provide evidence for a low-$\alpha$ population, showing that its stars
exhibit [Mg, Si/Fe]${}_{\rm{NLTE}}\sim 0.25-0.30$ at the [Fe/H] of P180956.
However, these ratios are not compatible with the values of our target, which
have [Mg/Fe]${}_{\rm{NLTE}}=-0.05\pm 0.09$ and [Si/Fe]${}_{\rm{NLTE}}=+0.59\pm
0.10$. Thus, the chemo-dynamical properties of P180956 differ from those of
GSE and other known accreted structures.

###  6.3 Origin in an ultra-faint dwarf galaxy

A comparison of the neutron-capture elements [Sr/Fe] and [Ba/Fe] vs. [Fe/H]
for P180956, stars from ultra-faint dwarf (UFDs), from classical dwarf
galaxies (DGs), and Milky Way halo is presented in the left panels of Figure
8. It is evident that P180956 exhibits lower [Sr, Ba/Fe] (and [Eu/Fe] from
Figure 4) ratios than the majority of MW halo stars at the same metallicity.
The [Sr/Fe] of P180956 is similar to the bulk value of DGs, while UFDs have
lower values. On the opposite behaviour is [Ba/Fe], where P180956’s ratio is
similar to those in the low-end distribution of UFDs. The low-abundance of
neutron-capture processes elements has been interpreted as the contribution of
low-mass supernovae and the absence of neutron stars mergers events (e.g.,
Cowan et al., 2021). Furthermore, the combinations of stochasticity in the
production of neutron-capture elements with the inability of UFDs to retain
metals can explain the low [Sr, Ba, Eu/Fe] observed in these systems (e.g.,
Venn et al., 2012; Ji et al., 2019).

The distribution of [Sr/Ba] vs. [Ba/Fe] is shown in the right panel of Figure
8. Halo stars exhibit a downward trend as [Ba/Fe] increases (Mashonkina et
al., 2017b), albeit mostly clumped around [Sr/Ba] $\sim 0.3$ (Ji et al.,
2019). Both UFDs and DGs present a wide distribution in [Sr/Ba] (up to $\sim
1.5$ dex), with UFDs populate a distinct region from the majority of MW halo
stars and of DGs (Mashonkina et al., 2017b; Roederer, 2017; Ji et al., 2019;
Reichert et al., 2020; Sitnova et al., 2021). P180956 has a relatively high
[Sr/Ba] ratio, close to the upper-end of the distributions of the MW halo,
Coma Berenices (UFD), and Sculptor (DG), while its [Ba/Fe] is typical of an
UFD’s star.

Did this star originate in an UFD or in classical DG? At the metallicity of
P180956, DGs would likely involve contributions from asymptotic giant branch
stars (AGBs) and, in some cases, SNe Ia. AGBs would produce Ba via s-process
nucleosynthesis (e.g., Pignatari et al., 2008; Cescutti & Chiappini, 2014),
reaching solar values. Additional contribution from SNe Ia would lower the
overall [$\alpha$/Fe] ratios to solar or sub-solar values. Given P180956 has a
very low-Ba ([Ba/Fe]${}_{\rm{NLTE}}\sim-1.7$) and the inhomogeneity in the
$\alpha$-elements ([Si, Ti/ Mg, Ca] $\sim 0.5$), the contributions of AGBs and
SNe Ia are likely ruled out, and, as well, the DG scenario.

How to explain the high [Sr/Ba]? Mashonkina et al. (2017b) discuss that sub-
solar [Sr/Ba] implies that both elements are produced solely by r-process,
while solar- and supersolar- [Sr/Ba] indicate the involvement of s-processes
in the Sr production. Various kinds of supernovae events have been proposed to
explain the relative excess of Sr compared to other neutron-capture elements
(Mashonkina et al., 2017b, and references therein). Hypernovae (Izutani et
al., 2009) and s-process nucleosynthesis in low-metallicity fast-rotating
supernovae (Pignatari et al., 2008; Banerjee et al., 2018; Grimmett et al.,
2018; Limongi & Chieffi, 2018) are also listed among these, which are always
invoked by the best fit models from StarFit (see Section 6.1). Thus, the
interpretation that P180956 originated in a UFD is further supported by the
new chemical abundance analysis and preferred over the DG hypothesis.

Figure 8: Neutron-capture elements. Left panel: [Sr/Fe] vs. [Fe/H]. Central
panel: [Ba/Fe] vs. [Fe/H]. Empty symbols denote upper limits on the vertical
axis. Right panel: [Sr/Ba] vs. [Ba/Fe]. The red circle marks the NLTE-
corrected chemical ratios of P180956. MW stars are from the SAGA database
(Suda et al., 2008); Coma Berenice (ComBer) stars are from Frebel et al.
(2010b) and Waller et al. (2023); Bootes I (BooI) stars are from Feltzing et
al. (2009), Norris et al. (2010), Gilmore et al. (2013), Ishigaki et al.
(2014), and Frebel et al. (2016); Gru I stars are from Ji et al. (2019);
Tucana II (TucII) stars are from Ji et al. (2016) and Chiti et al. (2018);
Segue 1 (Seg1) stars are from Frebel et al. (2014); Hercules (Her) stars are
from Koch et al. (2008), Koch et al. (2013), and François et al. (2016);
Sculptor (Scl) and Ursa Minor (UMi) stars are from Mashonkina et al. (2017b).

###  6.4 P180956, witness of the early Galactic assembly

Above we have outlined the most likely origin for P180956 as from an accreted
UFD. Two new questions now arise whether the star was brought in during the
early Galactic assembly or later on and if its system was isolated or brought
in with one of the massive known accreted satellites.

P180956 was also selected for GHOST commissioning because of its peculiar
orbital parameters, as it remains relatively close to the MW plane (see Figure
5). Recently, the presence of this population has been observed from the ultra
metal-poor regime (UMP, ${\rm[Fe/H]}\leq-4.0$, Sestito et al., 2019) to the
disc’s metallicity (Sestito et al., 2020; Di Matteo et al., 2020; Venn et al.,
2020; Cordoni et al., 2021; Mardini et al., 2022), finding that the majority
of them moves in a prograde orbit. While VMP "planar" stars have been
dynamically detected in various investigations, thorough and detailed analyses
of their chemical properties are scarce.

The existence of this population has also been explored through high-
resolution cosmological zoom-in simulations (Sestito et al., 2021; Santistevan
et al., 2021). The simulations151515Sestito et al. (2021) utilised the NIHAO-
UHD simulations (Buck et al., 2019), while Santistevan et al. (2021) employed
the FIRE simulations (Hopkins et al., 2018). predict the presence of a
"planar" population. While previous observational studies focused on the
origin of low-eccentricity stars, Sestito et al. (2021) also discussed that
the more eccentric members of the planar population are likely brought in
during the early-assembly phase. This is because, during that epoch, the
gravitational potential of the forming proto-Galaxy is still shallow, allowing
for the deposit of accreted systems into the inner regions. The relatively
small excursion from the MW plane, the pericentric and apocentric distances,
in addition to the high eccentricity suggest that the star was brought in
during the early-assembly phase.

In Section 6.2, we rule out that P180956 formed in one of the known accreted
structures, however, given 1) that some of its orbital parameters are similar
to those of GSE (except for Zmax), 2) its formation likely in an UFD-like
system (see Section 6.3), and 3) its early accretion to the MW, two scenarios
on the origin of P180956 are proposed. The first is that P180956 originate in
one of the many low-mass building blocks that formed the proto-MW, which had
chemical and physical properties similar to those of present UFDs; the second
is that its progenitor system was an UFD satellite of GSE, that has been
brought in into the inner Galaxy during the infall of GSE.

##  7 Conclusions

This work represents the second high-resolution spectroscopic analysis
utilising the GHOST spectrograph mounted at Gemini South, following the
detailed analysis of two r-process rich stars in the Reticulum II ultra-faint
dwarf galaxy (Hayes et al., 2023). The spectra of P180956, a star with unique
chemo-dynamical properties, were observed during the second commissioning run
of the instrument in September 2022. Previously, the star was observed with
GRACES at Gemini North and analysed as part of the Pristine Inner Galaxy
Survey (Arentsen et al., 2020a; Sestito et al., 2023b). In this study, we
conducted a comprehensive analysis of the chemo-dynamical properties of
P180956, leading to the following results:

  1. 1.

The high efficiency and wide spectral coverage of the GHOST instrument (see
Figure 1) enabled the detection of approximately 20 atomic species (see Figure
4), providing crucial insights into the origin and chemical properties of
P180956 and its formation site.

  2. 2.

The larger number of species and spectral lines available with GHOST
facilitated a more accurate and thorough analysis compared to GRACES (see
Figure 3) that usually allows the detection of only $\sim 10$ atomic species
(Kielty et al., 2021; Jeong et al., 2023; Sestito et al., 2023b, a; Waller et
al., 2023).

  3. 3.

Theoretical models of supernovae yields suggest that the formation site of
P180956 experienced pollution from 2 to 4 low-mass hypernovae ($\lesssim
15{\rm\,M_{\odot}}$) and one intermediate-mass ($80-120{\rm\,M_{\odot}}$)
fast-rotating ($\sim 300{\rm\,km\ s^{-1}}$) supernova (see Figure 6).

  4. 4.

These combinations of supernovae events resulted in a composition of
$\alpha$-elements with solar Ca and Mg abundances and enhanced Si and Ti
abundances (see Figures 4 and 6).

  5. 5.

The specific combination of supernovae yields led to low abundances of
neutron-capture elements (Sr, Ba, Eu), with a relatively high [Sr/Ba] ratio
(see Figure 8). This can be explained with the additional s-process channels
for Sr production that occur in hypernovae and fast-rotating supernovae.

  6. 6.

The low amount amount of neutron-capture elements (see Figures 4 and 8) is
rare in the MW halo and typical of stars in ultra-faint dwarf galaxy systems.

  7. 7.

The kinematical properties of P180956 (see Figure 5) suggests the star was
likely accreted during the early-assembly phase of the Milky Way. Its [Mg/Mn]
ratio is also indicative of its accreted origin (see Figure 7).

  8. 8.

None of the known accreted structures exhibit chemo-dynamical properties
resembling perfectly those of P180956. Only Gaia-Sausage-Enceladus has similar
eccentricity, apocentric and pericentric distances to P180956.

  9. 9.

P180956 originated in an ancient system chemically similar to present ultra-
faint dwarf galaxies, either accreted alone or dragged in with Gaia-Sausage-
Enceladus as its satellite.

The advent of the GHOST high-resolution spectrograph has been invoked by
various chemo-dynamical investigations targeting the MW and its satellite
systems (e.g., Sestito et al., 2023b, a; Waller et al., 2023). This study,
along with Hayes et al. (2023), demonstrates that the combination of the
Gemini South’s large aperture and GHOST’s high efficiency and wide spectral
coverage is ideal for investigating low-metallicity stars in the Milky Way and
nearby systems. The synergy between GHOST and the Gaia satellite will
undoubtedly propel Galactic Archaeological studies forward.

## Acknowledgements

We acknowledge and respect the lkwŋn peoples on whose traditional territory
the University of Victoria stands and the Songhees, Esquimalt and
$\underaccent{\bar}{\rm W}$SÁNEĆ peoples whose historical relationships with
the land continue to this day.

FS thanks the Dr. Margaret "Marmie" Perkins Hess postdoctoral fellowship for
funding his work at the University of Victoria. KAV thanks the National
Sciences and Engineering Research Council of Canada for funding through the
Discovery Grants and CREATE programs. AA acknowledges support from the Herchel
Smith Fellowship at the University of Cambridge and a Fitzwilliam College
research fellowship supported by the Isaac Newton Trust. NFM gratefully
acknowledge support from the French National Research Agency (ANR) funded
project “Pristine” (ANR-18-CE31-0017) along with funding from the European
Research Council (ERC) under the European Unions Horizon 2020 research and
innovation programme (grant agreement No. 834148). TM acknowledges the Spinoza
Grant from the Dutch Research Council (NWO) for supporting his research. ES
acknowledges funding through VIDI grant "Pushing Galactic Archaeology to its
limits" (with project number VI.Vidi.193.093) which is funded by the Dutch
Research Council (NWO). This research was supported by the International Space
Science Institute (ISSI) in Bern, through ISSI International Team project 540
(The early Milky Way).

This work is based on observations obtained with Gemini South/GHOST, during
the commissioning run of September 2022. Based on observations obtained at the
international Gemini Observatory, a program of NSF’s NOIRLab, which is managed
by the Association of Universities for Research in Astronomy (AURA) under a
cooperative agreement with the National Science Foundation. On behalf of the
Gemini Observatory partnership: the National Science Foundation (United
States), National Research Council (Canada), Agencia Nacional de Investigación
y Desarrollo (Chile), Ministerio de Ciencia, Tecnología e Innovación
(Argentina), Ministério da Ciência, Tecnologia, Inovações e Comunicações
(Brazil), and Korea Astronomy and Space Science Institute (Republic of Korea).

This work has made use of data from the European Space Agency (ESA) mission
Gaia (https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing
and Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium). Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

This research has made use of the SIMBAD database, operated at CDS,
Strasbourg, France (Wenger et al., 2000). This work made extensive use of
TOPCAT (Taylor, 2005).

## Data Availability

The data underlying this article are available in the article and in its
online supplementary material.

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