# A stellar stream remnant of a globular cluster below the metallicity floor

Nicolas Martin, Kim Venn, David Aguado, Else Starkenburg, Jonay González Hernández, Rodrigo A Ibata, Piercarlo Bonifacio, Elisabetta Caffau, Federico Sestito, Anke Arentsen, et al.

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Nicolas Martin, Kim Venn, David Aguado, Else Starkenburg, Jonay González Hernández, et al. A stellar stream remnant of a globular cluster below the metallicity floor. Nature, 2022, 601 (7891), pp.45-48. 10.1038/s41586-021-04162-2. hal-03524785

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# A stellar stream remnant of a globular cluster below the metallicity floor

https://doi.org/10.1038/s41586-021-04162-2

Received: 14 June 2021

Accepted: 19 October 2021

Published online: 5 January 2022

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Nicolas F. Martin<fnref n="1" />,²,³, Kim A. Venn³, David S. Aguado⁴,⁵,⁶, Else Starkenburg⁷, Jonay I. González Hernández⁵,⁸, Rodrigo A. Ibata<fnref n="1" />, Piercarlo Bonifacio⁹, Elisabetta Caffau⁹, Federico Sestito³, Anke Arentsen<fnref n="1" />, Carlos Allende Prieto⁵,⁸, Raymond G. Carlberg¹⁰, Sébastien Fabbro³,¹¹, Morgan Fouesneau², Vanessa Hill¹², Pascale Jablonka⁹,¹³, Georges Kordopatis¹², Carmela Lardo¹⁴, Khyati Malhan¹⁵, Lyudmila I. Mashonkina¹⁶, Alan W. McConnachie¹¹, Julio F. Navarro³, Rubén Sánchez-Janssen¹⁷, Guillaume F. Thomas⁵,⁸, Zhen Yuan<fnref n="1" /> & Alessio Mucciarelli¹⁴,¹⁸

Stellar ejecta gradually enrich the gas out of which subsequent stars form, making the least chemically enriched stellar systems direct fossils of structures formed in the early Universe<fnref n="1" />. Although a few hundred stars with metal content below 1,000th of the solar iron content are known in the Galaxy²⁻⁴, none of them inhabit globular clusters, some of the oldest known stellar structures. These show metal content of at least approximately 0.2% of the solar metallicity ([Fe/H] ≥ −2.7). This metallicity floor appears universal⁵,⁶, and it has been proposed that protogalaxies that merged into the galaxies we observe today were simply not massive enough to form clusters that survived to the present day⁷. Here we report observations of a stellar stream, C-19, whose metallicity is less than 0.05% of the solar metallicity ([Fe/H] = −3.38 ± 0.06 (statistical) ± 0.20 (systematic)). The low metallicity dispersion and the chemical abundances of the C-19 stars show that this stream is the tidal remnant of the most metal-poor globular cluster ever discovered, and is significantly below the purported metallicity floor: clusters with significantly lower metallicities than observed today existed in the past and contributed their stars to the Milky Way halo.

The C-19 stellar stream is a grouping of stars in the Milky Way halo that share a common orbital motion around the Galaxy and was discovered through the application of the STREAMFINDER algorithm⁸ to the astrometric data of Gaia Early Data Release 3 (ref. ⁹). The stellar structure has a very low density and extends over approximately 15° on the sky. The reality of the stellar stream was confirmed via the photometric metallicities (solar-scaled iron content, [Fe/H]) of the Pristine survey¹⁰, which revealed the coherent and extremely low metallicity of candidate member stars. Especially for the brighter stars, a regime with little expected contamination, the Pristine photometric metallicities exhibit a very coherent metallicity at an extremely metal-poor level. Selecting stars with Gaia proper motions similar to those of the STREAMFINDER sample yields the colour-magnitude diagram (CMD) of stars associated with the structure at high significance, shown in Fig. 1. The CMD of the stream displays a sparsely populated but well-defined red giant branch that extends into a main-sequence turn-off at the faint end of the data, along with a clear, and very blue, horizontal branch. These features are typical of old and metal-poor globular clusters. Correcting for stars that

are fainter than the magnitude limit of the sample, we estimate that the total luminosity and stellar mass of the C-19 progenitor are at least 3.9 × 10³ L⊙ and 0.8 × 10⁴ M⊙, respectively, at the favoured heliocentric distance of 18 kpc (Methods). This can only be a lower limit as the C-19 stream may extend over a larger region of the sky but with lower densities that prevent its detection at this stage. This mass is, however, quite typical for halo globular clusters¹¹.

We performed spectroscopic observations of eight member stars with two telescopes and two spectrographs (Gemini and the Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES), and Gran Telescopio Canarias (GTC) and the Optical System for Imaging and low-Intermediate-Resolution Integrated Spectroscopy (OSIRIS)) to determine the nature of the C-19 progenitor and refine the orbit of the stream (Methods). The brightest of those stars was also observed and analysed in the very metal-poor star sample obtained with the Large Sky Area Multi-Object Fibre Spectroscopic Telescope (LAMOST)³. The spectra of all eight of our Pristine-selected very metal-poor member stars ([Fe/H] < −2.0) yield radial velocities that are in agreement with

<footnote n="1">¹Université de Strasbourg, CNRS, Observatoire astronomique de Strasbourg, UMR 7550, Strasbourg, France. ²Max-Planck-Institut für Astronomie, Heidelberg, Germany. ³Department of Physics and Astronomy, University of Victoria, Victoria, British Columbia, Canada. ⁴Institute of Astronomy, University of Cambridge, Cambridge, UK. ⁵Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain. ⁶Dipartimento di Fisica e Astronomia, Università degli Studi di Firenze, Sesto Fiorentino, Italy. ⁷Kapteyn Astronomical Institute, University of Groningen, Groningen, the Netherlands. ⁸Departamento de Astrofísica, Universidad de La Laguna, La Laguna, Tenerife, Spain. ⁹GEPI, Observatoire de Paris, Université PSL, CNRS, Meudon, France. ¹⁰Department of Astronomy & Astrophysics, University of Toronto, Toronto, Ontario, Canada. ¹¹NRC Herzberg Astronomy & Astrophysics, Victoria, British Columbia, Canada. ¹²Université Côte d'Azur, Observatoire de la Côte d'Azur, CNRS, Laboratoire Lagrange, Nice, France. ¹³Laboratoire d'astrophysique, École Polytechnique Fédérale de Lausanne, Observatoire, Versoix, Switzerland. ¹⁴Dipartimento di Fisica & Astronomia “Augusto Righi”, Alma Mater Studiorum, Università di Bologna, Bologna, Italy. ¹⁵The Oskar Klein Centre, Department of Physics, Stockholm University, Stockholm, Sweden. ¹⁶Institute of Astronomy, Russian Academy of Sciences, Moscow, Russia. ¹⁷UK Astronomy Technology Centre, Royal Observatory, Edinburgh, UK. ¹⁸INAF – Osservatorio di Astrofisica e Scienza dello Spazio di Bologna, Bologna, Italy. ¹⁹e-mail: nicolas.martin@astro.unistra.fr</footnote>
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**Fig. 1 | Properties of the C-19 member stars.** *a*, CMD of the C-19 stream in Gaia magnitudes BP, RP and G. The 56 stars are colour-coded by their photometric metallicity as measured in the Pristine survey (small symbols) or their spectroscopic metallicity (large symbols) from spectra obtained with LAMOST (star symbols), Gemini–GRACES (circles) or GTC–OSIRIS (squares). The Pristine metallicities unambiguously show that C-19 is very metal-poor and helps to remove the few contaminants with higher metallicities ([Fe/H]$_{Pristine}$ > −2.5 shown with red, smaller symbols and removed in *b*, *c*; see Methods). Hollow circles denote stars with no metallicity measurement but that have a proper motion consistent with the C-19 stream in this region of the sky. These stars show the clear and very blue horizontal branch of the system and also include a few probable members along the red giant branch. The grey track is a 13-Gyr

PARSEC isochrone with the lowest available metallicity ([M/H] = -2.2), at the favoured heliocentric distance of 18 kpc. Despite a metallicity that is significantly greater than the spectroscopic metallicity of C-19, this isochrone provides a good match to the member stars because of the low sensitivity of the Gaia colour to metallicity changes in this metal-poor regime. *b*, Distribution of C-19 candidate member stars on the sky, in Galactic coordinates. A blue horizontal branch star candidate, located at (l, b) = (94.6°, -52.7°) and notably detached from the rest of the stream stars, is not represented here but could indicate that C-19 is at least 15° longer than shown here. *c*, Radial velocities, $v_r$, of confirmed C-19 stars with spectra. In *b*, *c*, the dot-dashed grey lines represent the calculated track of the C-19 orbit (Methods). In *a–c*, the error bars represent the uncertainties, calculated as $\pm 1\sigma$ Gaussian standard deviations.

the predictions from STREAMFINDER and therefore confirm that they are stream members. A ninth star, with a significantly greater Pristine metallicity, has a discrepant radial velocity and is confirmed to be an outlier. The kinematics of the eight member stars show that C-19 follows an orbit deep in the potential well of the Milky Way, with a pericentre of approximately 7 kpc and an apocentre of approximately 27 kpc, on a plane that is almost polar with respect to the Milky Way plane (Methods).

Spectroscopic metallicities were determined for all eight confirmed C-19 member stars (Methods). These metallicities are displayed in Fig. 2, and high-resolution spectra for three stars from the Gemini–GRACES facility are shown in Fig. 3. These results unambiguously confirm the extremely low metallicity of C-19, with mean metallicity[Fe/H] = -3.38 ± 0.06 (statistical) ± 0.20 (systematic). Such a low global metallicity has so far never been observed for any stellar system in the Milky Way, in its surroundings or beyond.

We are able to place a stringent limit on the unresolved metallicity dispersion of these eight stars, with $\sigma_{[Fe/H]}$ < 0.18 at the 95% confidence level. This strongly suggests that the C-19 progenitor was a globular cluster, and not a dwarf galaxy, because the latter would exhibit a large metallicity dispersion stemming from multiple bursts of star formation$^{12,13}$. In addition, the mean metallicity of C-19 is significantly below the mean metallicity expected for dwarf galaxies at this luminosity$^{14}$.

A dispersion in the sodium abundances provides even more compelling evidence that C-19 is a disrupted globular cluster. The high-resolution spectra for three member stars of C-19 were used to determine high-precision chemical abundances (Methods). We find the

[Na/Mg] ratios vary by a factor of 3 between these stars. These variations are typical of stars in ancient globular clusters$^{15,16}$, most probably due to exposure to high-temperature hydrogen burning, and possibly due to multiple or extended star formation in early epochs. These chemical signatures are rarely seen in Galactic halo stars or in low-luminosity dwarf galaxies$^{17}$. These sodium variations can even be seen directly from the difference in Na I line strengths in Fig. 3.

For extremely metal-poor stars ([Fe/H] < -3.0), the heavy neutron-capture element abundances, such as those of barium, provide another chemical signature of their origins. At these metallicities, stars that form in low-luminosity dwarf galaxies typically have very low [Ba/Fe] ratios$^{18}$, other than a few which have been enriched by a rare rapid-neutron capture process, such as a compact binary merger$^{19}$ (for example, Reticulum II (ref. 20) or Tucana III (ref. 21)). Given the signal-to-noise ratio of the GRACES spectra, we can measure barium only in one star (Pristine_355.13+27.98), where [Ba/Fe] is near the solar ratio, consistent with typical values in globular cluster stars$^{22}$. We can also rule out highly enriched barium abundances in the two other stars observed with GRACES. Carbon provides another clue to the origins of the C-19 stars, because 40% of field halo stars in the Galaxy are carbon-rich in the metallicity range considered$^{23}$. None of the six C-19 stars with carbon measurements from our OSIRIS spectra is enhanced ([C/Fe] < 0.70 in all cases, although the uncertainties are relatively large). Although this will have to be confirmed through more accurate measurements, it may further suggest that the C-19 progenitor formed from gas enriched differently from that of typical extremely metal-poor halo stars$^{24}$. Overall, the chemical signatures of our sample
<figure><img src="image_4.png" /><figcaption>**Fig. 2 |** *Metallicity properties of C-19 and its stars observed by spectroscopy.)* a, Spectroscopic metallicities [Fe/H] calculated for eight members of C-19. The different colours and symbols denote spectra obtained with the different telescopes, instruments and/or surveys listed at the top. The two brightest stars were observed with two different facilities each. **b,** Probability distribution functions of the mean and dispersion of the metallicities of C-19 stars, assuming they follow a Gaussian distribution. The individual uncertainties in the measurements are taken into account and stars observed twice had their measurements combined, weighted by their</figcaption></figure>

individual uncertainties. **c, d,** The marginalized probability distribution functions for the two parameters mean (**c**) and dispersion (**d**). The dotted lines in **c** represent the uncertainties (Gaussian standard deviation) on the derived metallicity, and in **d** the dotted line highlights the 95% confidence level (CL). **e,** Metallicity distribution of all known globular clusters of the Milky Way¹¹. The metallicity of the C-19 progenitor is highlighted in red, along with the metallicity of the recently discovered Phoenix stream⁶, the lowest-metallicity stellar stream from a globular cluster known to date.

of stars reinforce our conclusion that the progenitor of the C-19 stream was a globular cluster with metallicity significantly below the current globular cluster metallicity floor.

The extremely low metallicity of the C-19 progenitor helps reduce discrepancies in our understanding of the metallicity distribution function of the Milky Way stellar halo. Assuming that the contribution of globular clusters to the Milky Way halo is the same at all

metallicities, there is a dearth of globular clusters with [Fe/H] < -2.5, with none observed but approximately five expected from the metallicity of halo stars²⁵. Our discovery of a cluster stream, significantly below that metallicity floor, in addition to other recently discovered globular cluster stellar streams that are close to the metallicity floor⁶,²⁶, shows that more metal-poor clusters formed at early times but did not survive their tidal interactions with the Milky Way.

<figure><img src="image_5.png" /></figure>

<figure><img src="image_6.png" /><figcaption>**Fig. 3 |** *High-resolution GRACES spectra for three members of C-19.)* a, The spectral range around the region of the Mg triplet. **b,** The spectral region around the Na doublet. All three stars have very similar stellar parameters, showing similarities in the Mg I and Fe II absorption features, but a significantly lower Na I line strength in Pristine_355.27+27.74. The spectra are labelled with the target name, along with the inferred effective temperature, log surface</figcaption></figure>

gravity and signal-to-noise ratio of the spectrum at 600 nm. The strong lines labelled 'ISM' in **b** correspond to sodium absorption by the interstellar medium and are unrelated to the C-19 stars. The deeper lines of the standard star HD 122563, which has similar stellar parameters³² and [Fe/H] = -2.8, confirm that the C-19 stars are markedly more metal-poor.
Intriguingly, the recent realization that a globular cluster with metallicity [Fe/H] = -2.9 currently orbits the Andromeda galaxy²⁷ could also indicate that such low-metallicity structures may have survived until the present day in other environs.

The orbit of C-19 has an apocentre of only approximately 27 kpc, which suggests a very early accretion onto the Milky Way, when the Milky Way's potential well was shallower than it is now. However, if this is the case, it is puzzling that the C-19 stellar stream can still be identified as a coherent structure to this day. The relatively short orbital time (less than 0.5 Gyr) and the expected presence of baryonic or dark matter substructures that should further dynamically heat the stellar stream²⁸ would both work to disperse the stream into the smooth Milky Way stellar halo. This process may well have already happened, and could explain the stream's full width (approximately 600 pc) and radial velocity dispersion (approximately 7 km s⁻¹ from the three GRACES stars that have accurate velocities and are located close together); both are notably larger than what is usually measured for globular cluster streams (less than 100 pc and 1–3 km s⁻¹, respectively²⁹,³⁰). Alternatively, or in addition, the C-19 orbit crossing the disk multiple times over the last 1 Gyr (Methods) could have also heated the stream. It will be fascinating to explore the processes that could have shielded the C-19 progenitor for long enough so that its stream is still visible today. For example, it may have been born in its own host galaxy, which partly shielded C-19 from destructive interactions with the Milky Way. Hunting for host stars on orbits similar to that of the C-19 stream could help to constrain this scenario.

Finally, the C-19 progenitor provides an intriguing window into the formation of globular clusters at very early times. Galaxies are expected to grow hierarchically, and to be enriched gradually as more and more stars form and evolve. The very existence of C-19 proves that globular clusters must have been able to form in the lowest-metallicity environments as the first galactic structures were assembling. The time evolution of scaling relations between the mass of a galaxy and its metallicity, built from galaxy-evolution simulations⁷,³¹, implies that the C-19 progenitor must have formed within at most the first 2–3 Gyr after the formation of the Universe. After that period, no galaxy is expected to still host sufficiently low metallicity gas to have formed a system like C-19. Furthermore, the same scaling relations between metallicity and stellar mass indicate that potential C-19 hosts should have had extremely low masses (less than about 4.5 × 10⁴ M☉), regardless of the formation time³¹. Given our mass estimate of 0.8 × 10⁴ M☉ or greater, the star-formation event that led to the C-19 progenitor must have represented a large fraction of all stars present in the host galaxy at that time. This is compelling evidence that structures such as C-19 provide strong constraints on galaxy and globular cluster formation models at the earliest times.

## Online content

Any methods, additional references, Nature Research reporting summaries, source data, extended data, supplementary information, acknowledgements, peer review information; details of author contributions and competing interests; and statements of data and code availability are available at https://doi.org/10.1038/s41586-021-04162-2.

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# Methods

## Discovery of the C-19 stream

The C-19 stream was discovered⁸ as a grouping of stars along the same orbit by application of the STREAMFINDER algorithm³³ to Early Data Release 3 of the European Space Agency's Gaia space mission⁹. A list of all those stars is given in Extended Data Table 1. The C-19 stars span approximately 15° on the sky, centred near (l, b) = (107°, -37°), as shown in Fig. 1b. The STREAMFINDER analysis favours a heliocentric distance of approximately 18 kpc for this system, confirmed by the combined orbital analysis of the full sample of probable members (see later). As can be seen in Fig. 1, the full width of the stream is approximately 2°, or approximately 600 pc at the favoured distance. This is large for a typical globular cluster stream, indicating that C-19 was probably heated during its orbiting of the Milky Way and/or that it was already tidally disrupted in its (putative) host dwarf galaxy³⁴,³⁵.

The Pristine survey¹⁰ overlaps the region where C-19 stars are found by the STREAMFINDER algorithm, and their combined analysis indicates that C-19 has a very coherent metallicity signal³⁶ in the extremely low metallicity range ([Fe/H]$_{Pristine}$ < -3.0), as shown in Extended Data Fig. 1, although contamination creeps in for faint magnitude ($G_0$ > 19.0, represented by small symbols in Extended Data Fig. 1; see also Fig. 1). The mean metallicity of those stars, determined from a σ-clipping procedure, converges on $\langle[\text{Fe/H}]_{\text{Pristine}}\rangle = -3.51 \pm 0.05$. From the photometric metallicities measured with the Pristine survey, it can already be concluded that C-19 is probably the most metal-poor stellar structure ever discovered. Pristine metallicities can also be used to flag significantly more metal-rich outliers with $[\text{Fe/H}]_{\text{Pristine}} > -2.5$, as rejected by the σ-clipping procedure (stars shown in red in Fig. 1).

## Mass

We first note that the mass of the C-19 stream that we estimate here from its discovered member stars can only be a lower limit of the true mass of C-19 because it is unlikely that we have discovered the full track of the stream and its stars may extend much further than detected here. To estimate the mass of C-19, we summed the fluxes of all likely member stars shown in the CMD in Fig. 1. These correspond to Pristine-selected stars with $[\text{Fe/H}]_{\text{Pristine}} < -2.5$, complemented with all stars that share the proper motion of the C-19 stars in this part of the sky and have a Gaia parallax measurement that is consistent with zero (hollow circles in Fig. 1). All those stars have colour-magnitude properties that are very consistent with the C-19 CMD.

The total observed flux from all these stars with $G_0 < 18.0$, where we are quite confident the data are complete and suffer from little contamination, is $2.2 \times 10^3 L_\odot$. Correcting for fainter stars by using the luminosity function³⁷ associated with the isochrone shown in Fig. 1, we find that the total luminosity of C-19 is at least $3.9 \times 10^3 L_\odot$. Model mass-to-light ratios of very old stellar populations³⁸ imply $M/L = 2-3$ and, consequently, that the mass of C-19 is at least approximately $0.8 \times 10^4 M_\odot$.

We also used an alternative method to derive the total flux and mass of C-19, relying this time on star counts. We determined the typical luminosity of a system with the age, metallicity and distance properties of the isochrone displayed in Fig. 1 and that hosts the number of stars observed in C-19 above $G_0 = 18.0$ (18 stars). This analysis yields a slightly smaller average total luminosity ($2.7 \times 10^3 L_\odot$) but with a long tail towards higher fluxes that includes the value determined previously.

## Heliocentric distance

For each star that STREAMFINDER highlights as a probable member of a stream, the algorithm determines a most likely heliocentric distance, based on the local distribution of stars on the sky, in proper motion space and in CMD space. Although these distances are not individually precise, they provide, as an ensemble, a first guess for the distance to a stream. In the case of C-19, STREAMFINDER assigns a heliocentric

distance of between 16 and 22 kpc for significant members, with no obvious distance gradient in the region over which C-19 was discovered.

We confirmed this rough distance estimate using the blue horizontal branch stars of C-19 that are shown in the CMD as triangles in Fig. 1a. Following equation (7) of Deason et al.³⁹, we calculated an average distance of 17.5 kpc from these seven blue horizontal branch stars. It should, however, be noted that the relation between the colour and the absolute magnitude of blue horizontal branch stars has been calibrated only for systems with $[\text{Fe/H}] > -2.3$ and that this relation could be biased for C-19 stars that are in the extremely metal-poor regime.

An independent measure of the heliocentric distance is obtained from Gaia parallaxes when those are used to fit for the best C-19 orbit (see later). In this case, the favoured orbit has a distance of $20.9 \pm 0.3$ kpc ($\pm 1\sigma$ Gaussian uncertainties) but relies on parallaxes that are, individually, very small, and could therefore be prone to systematic biases, despite the average parallax zero-point offset being taken into account⁴⁰.

As a consequence, we kept the distance constraint loose when estimating the chemical abundances of C-19 stars and checked the results for distances of 16, 18 and 20 kpc. A distance of 18 kpc is marginally favoured to minimize the difference between the Fe I and Fe II abundances. This distance is compatible with the most metal-poor PARSEC isochrone, as shown in Fig. 1, and this is the distance we assume for C-19 for the analysis presented in this article. Further studies and the potential discovery of stars beyond the currently known extent of C-19 may help refine the distance to the stream.

## Orbit

We calculated the orbit of the C-19 stream using a procedure detailed elsewhere⁸,⁴¹ that was developed for STREAMFINDER detections. The sample of stars is the same as before: probable members identified by STREAMFINDER without stars that show $[\text{Fe/H}]_{\text{Pristine}} > -2.5$. Extended Data Figure 2 shows the best orbit in two cases: when the (small) Gaia parallax information of probable stream stars is used, which results in a distance of $20.9 \pm 0.3$ kpc (dashed line Extended Data Fig. 2) or when the distance is anchored at 18 kpc, as favoured by the system's blue horizontal branch stars and the consistency of the Fe I versus Fe II abundances (dotted line). Both solutions use the proper motions and spatial locations of all C-19 members identified by STREAMFINDER, along with the radial velocity of the eight stars for which we obtained spectra (see later). Over the approximately 15° covered by C-19, these two orbital solutions give very similar results. As shown in Extended Data Fig. 2d, determining the velocity gradient of C-19 using more accurate radial velocities will help pin down the distance to the system.

For both solutions, C-19 is on a remarkably polar orbit that is almost perpendicular to the Galactic plane and remains within the inner confines of the Milky Way with a pericentre of 7.0 kpc and an apocentre of 27.4 kpc for the distance-anchored orbit (10.8 kpc and 27.4 kpc, respectively, for the orbital solution constrained by the Gaia parallaxes). The C-19 orbit is slightly prograde and does not share the same plane as any of the growing number of stellar streams on polar orbits (Sagittarius, Cetus or LMS-1).

## Spectroscopy

Spectra were obtained for eight stars in C-19 and one star that, as expected from the Pristine metallicities, turned out to be a field contaminant (Extended Data Table 2). These include high-resolution spectra (R approximately 45,000) for three C-19 stars observed with GRACES⁴²,⁴³, at the 8.1-m Gemini-North telescope on Mauna Kea on Hawaii (United States), and medium-resolution spectra (R approximately 2,400) for seven targets (including one observed with GRACES) observed with the OSIRIS spectrograph mounted on the 10.4-m GTC at Roque de los Muchachos Observatory on La Palma (Spain). Examination of the LAMOST database also revealed a good spectrum of the brightest star associated with C-19 (Pristine_355.32+27.59), which we also observed with GTC-OSIRIS.
GRACES spectra

The Gemini–GRACES spectra were taken as part of a Large and Long Program (GN-2020B-LP-102, principal investigator K.A.V.) between 22 December 2020 and 6 January 2021. The two-fibre (object plus sky) mode was used, and the data were initially reduced with the Gemini Open-source Pipeline for ESPaDOnS Reduction and Analysis (OPERA)⁴⁴. Additional steps were taken to improve the final one-dimensional spectra, including increasing the signal-to-noise ratio in the overlapping order region by combining overlapping orders, inversely weighting the flux values by the noise values and improving the continuum normalization using an asymmetric σ-clipping routine³². The full spectral range is from approximately 4,500 Å to approximately 1 µm; however, light below 4,800 Å is severely limited by poor transmission through the very long optical fibre. Radial velocities were determined from a cross-correlation with a high signal-to-noise ratio Gemini–GRACES spectrum of the metal-poor standard star HD 122563. The target information, exposure times, radial velocities, and signal-to-noise ratio of the final combined spectra are listed in Extended Data Table 2.

Effective temperatures were derived from the $G_{BP} - G_{RP}$ colour, corrected for reddening⁴⁵,⁴⁶. We used a colour-temperature calibration based on the infrared flux method temperatures⁴⁷, updated for the Gaia photometry Early Data Release 3 (ref. ⁴⁸). Surface gravities were computed from the Stefan–Boltzmann equation for an assumed distance of 18 kpc and a stellar mass of 0.8 $M_{\odot}$. Finally, we computed bolometric corrections using a new grid of ATLAS models (A.M. et al., manuscript in preparation). Our analysis of 30 similar metal-poor red giants with Gemini–GRACES spectra showed that the typical uncertainties in these stellar parameters are ±100 K in effective temperature and ±0.1 dex in log gravity³². We use these uncertainties here as well. Microturbulence was initially determined from the relationship with surface gravity for giants⁴⁹; however, raising the initial values slightly (+0.2 km s⁻¹) helped to flatten the slope observed in the Fe I line abundances versus their equivalent width measurements. These stellar parameters are listed in Extended Data Table 3. Obtaining precision stellar parameters for extremely metal-poor benchmark stars on the red giant branch is currently a serious challenge³²,⁵⁰,⁵¹. The uncertainties on the stellar parameters and our previous analysis of similar stars and data imply that the quoted [Fe/H] metallicities have systematic uncertainties at the level of ±0.2 dex.

Chemical abundances were determined with the one-dimensional local thermodynamic equilibrium (LTE) ATLAS12 (ref. ⁵²) model atmospheres and the MOOG spectrum synthesis code⁵³,⁵⁴. The spectral lines examined are listed in Extended Data Table 4, including their equivalent width measurements and atomic data from linemake⁵⁵. The abundance uncertainties were estimated from the stellar parameter uncertainties, added in quadrature with an equivalent width measurement uncertainty. The latter were estimated from the standard deviation in the Fe I abundances divided by the square root of the number of lines of species X; that is, $\sigma(\text{Fe I})/\sqrt{N_X}$.

Departures from LTE are known to affect several atomic species (that is, Na I, Ca I, Cr I and Fe I) in metal-poor red giant stars due to the impact of the stellar radiation field on the statistical populations. Non-LTE (NLTE) corrections were determined from the INSPECT⁵⁶ and MPIA⁵⁷⁻⁶¹ databases, and privately by L.I.M. These are reported for individual lines in Extended Data Table 4. As all three stars have very similar stellar parameters, the NLTE corrections are similar for all three stars. They are applied in Extended Data Table 3. Only Fe II is unaffected by NLTE corrections, and we find it is also much less sensitive to uncertainties in the stellar parameters than the other elements (particularly Fe I). Calibrations for the extremely metal-poor benchmark stars on the red giant branch⁵⁰ also reveal Fe I is far more sensitive to uncertainties in their stellar parameters. For these reasons, we use log(Fe II/H) as representative of the metallicity in the three GRACES stars (regardless that it is determined from only one or two lines, compared with Fe I).

All abundances are scaled relative to the solar values⁶², such that $[X/Fe] = [\log(X/H) - \log(Fe/H)]_* - [\log(X/H) - \log(Fe/H)]_o$.

OSIRIS spectra

Director's Discretionary Time (GTC12-20BDDT) was used to collect spectra for fainter C-19 stars with the OSIRIS spectrograph at the GTC over the period from 19 January 2021 to 24 January 2021 for a total of approximately 6 h, including overheads. OSIRIS was used in longslit mode, with the 2500U grating to cover a wavelength range of 3,440–4,610 Å, with a 1.0–arcsec slit and 2 × 2 binning, leading to a resolving power, *R*, of approximately 2,400. For the five fainter targets, two observing blocks of 1,500 s each were used, and only one observing block of 600 and 300 s was used for the two brighter objects (Extended Data Table 2).

The data reduction was performed with the onedspec package within the IRAF environment⁶³. The individual spectra were normalized with a third-order cubic spline with the continuum task in the IRAF environment. A high-quality spectrum of the extremely metal-poor, bright star G64-12 from previous GTC–OSIRIS observation programmes⁶⁴,⁶⁵ was used as a template spectrum to derive the radial velocities of the C-19 candidate stars. We first cross-correlated the brightest target in our sample, Pristine_355.32+27.59, for which there is also a LAMOST spectrum, with the spectrum of G64-12. This was used to build a cross-correlation function with the IRAF task fxcor over the spectral range from 4,000 to 4,400 Å to minimize possible distortions of the cross-correlation function peak due to interstellar medium Ca II features. This resulted in a radial velocity of –191 ± 14 km s⁻¹ (Extended Data Table 2), consistent with the value of –193.8 km s⁻¹ (no uncertainty) from the LAMOST catalogue³.

Given the similarity in spectral type among the C-19 candidates, the remaining spectra were cross-correlated with the spectrum of Pristine_355.32+27.59. For each star, the weighted mean of the radial velocities from each individual spectrum was calculated along with the corresponding uncertainty. The final radial velocities of all C-19 stars are given in Extended Data Table 2, with the radial velocity uncertainties computed as the quadratic addition of the radial velocity uncertainty of each star and that of Pristine_355.32+27.59. The sample also includes an obvious outlier, Pristine_354.77+32.68, with velocity –49 ± 18 km s⁻¹. This star also has high metallicity measured by the Pristine survey ([Fe/H]$_{Pristine}$ = –0.7), confirming that it is not a member of the C-19 structure. The mean radial velocity for the confirmed C-19 members in the OSIRIS sample is –187.3 km s⁻¹, with a standard deviation of approximately 8.0 km s⁻¹, consistent with the results for the targets observed from Gemini–GRACES spectra.

Stellar parameters were determined with the same methods as for the Gemini–GRACES sample above; these are shown in Extended Data Table 5. Chemical abundances were determined with the FORTRAN FERRE code⁶⁶. The bluest and noisy parts of the spectra (λ < 3,800 Å) were removed and the spectra were normalized with a running mean filter with a width of 30 pixels⁶⁴,⁶⁵. For the model spectra, a grid of synthetic spectra previously computed⁶⁷ with the ASSeT code⁶⁸ was used, but was enlarged towards lower effective temperatures. Microturbulence $\xi = 2$ km s⁻¹) and α-element abundances ([α/Fe] = +0.4) were fixed, whereas carbon abundance ranged over a factor of 5 ([C/M] = –1.0 to +4.0, where [M/H] represents the global metallicity). It is possible to derive a carbon abundance using the G band near 4,300 Å in these OSIRIS spectra, with [C/Fe] values listed in Extended Data Table 5.

The FERRE code is able to look for the best fit by using the Boender–Timmer–Rinnoy Kan global algorithm⁶⁹ to minimize the solution function. In addition, the overall metallicity, [M/H], derived with a spectral grid that assumes [α/M] = +0.4, can be refined by deriving a representative α-abundance, [Ca/H], based on the Ca H and K resonance lines at 3,933 and 3,968 Å. We therefore used a FERRE option called ‘FErre Spectral WIndows’ (FESWI), which applies spectral masks in wavelength ranges with specific chemical information. In
this case, we selected a spectral window of 50 pixels around the two
resonance lines and ran FERRE to fit the spectrum for [Ca/H]. From
there, we derived the iron abundance of each star by simply applying
[Fe/H] = [M/H] - [Ca/M] + 0.4. The uncertainties on the resulting [Fe/H]
values were determined from the quadratic sum of the uncertainties
on [M/H] and [Ca/H]. Extended Data Figure 3 shows the OSIRIS spectra
(black lines) together with the initial fits derived with FERRE (blue lines).
The mean metallicity of the C-19 members from the OSIRIS sample
alone is [Fe/H] = -3.33 and the stream has an unresolved metallicity
dispersion.

As a sanity check, we also analysed the OSIRIS spectra with FERRE,
but in a mode without any assumption on the stellar parameters.
Although it can be difficult to determine these directly from the spec-
tra of extremely low metallicity stars that, apart from Ca H and K lines
and Balmer lines, have only relatively weak features, the results yielded
by this analysis are on the whole similar to those from the analysis with
fixed stellar parameters.

**Data availability**

The data used in this article are listed in Extended Data Tables 1–5.

Code availability

The codes used for the analysis were not designed to be made public
but can be requested from the corresponding author.

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**Acknowledgements** N.F.M., R.A.I., A.A. and Z.Y. gratefully acknowledge support from the French National Research Agency (ANR)-funded project Pristine (ANR-18-CE31-0017) along with funding from Centre National de la Recherche Scientifique (CNRS)-INSU through the Programme National de Cosmologie et Galaxies and through CNRS grant PICS07708 and from the European Research Council under the European Union's Horizon 2020 research and innovation programme (grant agreement no. 834148). K.A.V. is grateful for funding through the National Science and Engineering Research Council of Canada (NSERC) Discovery Grants and CREATE programmes. E.S. acknowledges funding through VIDI grant 'Pushing Galactic Archaeology to its Limits' (project number VI.vidi.193.093), which is funded by the Dutch Research Council (NWO). J.I.G.H. acknowledges financial support from Spanish Ministry of Science and Innovation (MICINIC) project AYA2017-86389-P. and also from the Spanish MICINIC under the 2013 Ramón y Cajal programme (RYC-2013-14875). G.F.T. acknowledges support from the Agencia Estatal de Investigación of the Spanish MICINIC (grant number FJC2018-037323-I). We thank the Canada-France-Hawaii Telescope (CFHT) staff for performing the Pristine observations in queue mode, for their reactivity in adapting the schedule and for answering questions during the data-reduction process. We are also grateful to the High Performance Computing Centre of the University of Strasbourg and its staff for very generous time allocation and for their support during the development of the STREAMFINDER project. This research used the SIMBAD database<sup>70</sup>, managed and run at CDS, Strasbourg, France. This research used the VizieR catalogue access tool<sup>71</sup>, CDS, Strasbourg, France. This work is based on observations obtained with MegaPrime-MegaCam, a joint project of the CFHT and CEA-DAPNIA, at the CFHT, which is operated by the National Research Council of Canada, the Institut National des Science de l'Univers of the French CNRS and the University of Hawaii. ESPaDOnS is a collaborative project funded by France (CNRS, MENESR, OMP, and LATT), Canada (NSERC), the CFHT and the European Space Agency. Data were reduced with use of the CFHT-developed OPERA data-reduction pipeline. We recognize and acknowledge the very significant cultural role and reverence that the summit of Mauna Kea has always had within the Indigenous Hawaiian community. We are very fortunate to have had the opportunity to conduct observations from this mountain. This work is based on observations obtained with Gemini Remote Access to CFHT ESPaDOnS Spectrograph (GRACES), as part of the Gemini Large and NoLLab, 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 is based on observations made with the GTC telescope, at the Spanish
Observatorio del Roque de los Muchachos of the Instituto de Astrofísica de Canarias, under Director's Discretionary Time. This work was partly based on data obtained with the instrument OSIRIS, built by a consortium led by the Instituto de Astrofísica de Canarias in collaboration with the Instituto de Astronomía of the Universidad Autónoma de México. OSIRIS was funded by GRANTECAN and the National Plan of Astronomy and Astrophysics of the Spanish Government. This work used data from the European Space Agency 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. Guo Shoujing Telescope (LAMOST) is a national major scientific project of the Chinese Academy of Sciences. Funding for the project was provided by the National Development and Reform Commission. LAMOST is operated and managed by the National Astronomical Observatories, Chinese Academy of Sciences.

**Author contributions** N.F.M. is a co-leader of the Pristine survey, led the discovery of the C-19 stream, coordinated the spectroscopic observations and co-led the writing of the manuscript. K.A.V. led the GRACES spectroscopic follow-up and the analysis of the resulting spectra, and co-led the writing of the manuscript. D.S.A. led the analysis of the OSIRIS spectra and the

writing of the corresponding section of the manuscript. E.S. is a co-leader of the Pristine survey and derived the Pristine photometric metallicities. J.I.G.H. coordinated the OSIRIS follow-up, performed the radial velocity analysis of these spectra and was greatly involved in writing this part of the manuscript. R.A.I. led the STREAMFINDER analysis and derived the orbit of the stream. P.B., E.C. and F.S. derived stellar and orbital parameters for the stars with spectroscopic follow-up. All other authors helped in the development of the Pristine survey and all authors assisted in the development and writing of the paper. A.M. derived the relations used to infer the stellar parameters of the spectroscopically observed stars.

**Competing interests** The authors declare no competing interests.

**Additional information**

**Supplementary information** The online version contains supplementary material available at https://doi.org/10.1038/s41586-021-04162-2.

**Correspondence and requests for materials** should be addressed to Nicolas F. Martin.

**Peer review information** *Nature* thanks the anonymous reviewers for their contribution to the peer review of this work. Peer reviewer reports are available.

**Reprints and permissions information** is available at http://www.nature.com/reprints.
<figure><img src="image_7.png" /><figcaption>**Extended Data Fig. 1** | Pristine photometric information for all stars of the C-19 stream selected by STREAMFINDER and present in the Pristine survey.</figcaption></figure>

Large symbols represent stars with $G_0 < 19.0$, for which the STREAMFINDER selection is very reliable, and small symbols represent stars fainter than this limit, which are more likely contaminated in the STREAMFINDER catalogue (see also Fig. 1). The lines represent model expectations as determined from the spectral libraries and filter response curves, without an assumption on whether the star is a dwarf or a giant¹⁰. Both model lines and data points are

color-coded by their $[Fe/H]_{Pristine}$ metallicities. Most C-19 candidate members are located in the region that corresponds to metallicities below $[Fe/H] = -3.0$ (above the blue line). For the large data points, we specifically used a photometric metallicity model tailored to giant stars. Near the tip of the red giant branch, it deviates significantly from the generic model represented by the coloured lines and explains the higher metallicity of the right-most point compared with the models.
<figure><img src="image_8.png" /><figcaption>**Extended Data Fig. 2** | Favourite orbital solutions for the C-19 stream.</figcaption></figure>

The dashed line shows the orbit of C-19 constrained using the proper motions (red symbols in **a**, **b**) of C-19 members identified by STREAMFINDER, their Gaia parallaxes (**c**), their radial velocities when available (**d**) and their location on the sky compared with the distribution of extinction⁴⁵ (**e**). The orbit determined

without using the Gaia parallax information but instead anchoring the distance at 18 kpc is represented by the dotted line. **f**, **g** The two orbits, integrated for ±1 Gyr, projected on the Galactic plane, and in the R-z plane. The thick red lines correspond to the part of the orbits that overlaps the observed C-19 member stars.
<figure><img src="image_9.png" /><figcaption>Extended Data Fig. 3 | Spectra of the C-19 member stars observed with OSIRIS, normalized using a running mean filter after removing the velocity signal in the rest frame (black lines), together with the best fit (blue lines)</figcaption></figure>

derived by adopting a fitting procedure. The metallicity, [Fe/H], computed from [M/H] and [Ca/H] is also indicated for each target (see the text for more details).
Extended Data Table 1 | List of potential C-19 members from the STREAMFINDER sample

<table><thead><tr><th>GalaID<br>(EDR3)</th><th>RA<br>(ICRS)</th><th>Dec<br>(ICRS)</th><th>G_0<br>(ICRS)</th><th>d_G<br>(ICRS)</th><th>(BP-RP)_0<br>(BP-RP)</th><th>d<br>(BP-RP)</th><th>Parallax</th><th>d_Parallax</th><th>mu alpha**<br>(mas/yr)</th><th>d Mu alpha*<br>(mas/yr)</th><th>mu delta<br>(mas/yr)</th><th>d Mu delta<br>(mas/yr)</th><th>[Fe/H]_Pristine</th><th>Note</th></tr></thead><tbody><tr><td>1912706716827846528</td><td>352,0630548</td><td>34,1942811</td><td>19,282</td><td>19,282</td><td>19,282</td><td>19,282</td><td>19,282</td><td>19,282</td><td>1,805</td><td>1,805</td><td>1,805</td><td>1,805</td><td>1,805</td><td>---</td></tr><tr><td>1911964791293941760</td><td>352,7665670</td><td>31,2648067</td><td>17,459</td><td>0.003</td><td>0.996</td><td>0.010</td><td>0.082</td><td>0.653</td><td>0.863</td><td>0.62</td><td>0.62</td><td>0.62</td><td>0.62</td><td>---</td></tr><tr><td>286246202636892544</td><td>353,7994502</td><td>21,2648067</td><td>17,459</td><td>0.003</td><td>0.938</td><td>0.016</td><td>0.125</td><td>0.100</td><td>1,144</td><td>1,144</td><td>1,144</td><td>1,144</td><td>1,144</td><td>---</td></tr><tr><td>2862600927030372224</td><td>353,8760338</td><td>20,3940474</td><td>19,067</td><td>0.004</td><td>0.792</td><td>0.044</td><td>0.210</td><td>0.297</td><td>1,163</td><td>1,163</td><td>1,163</td><td>1,163</td><td>1,163</td><td>---</td></tr><tr><td>2872378543071230464</td><td>353,8805901</td><td>32,9557055</td><td>19,739</td><td>0.004</td><td>0.772</td><td>0.073</td><td>-0.349</td><td>0.427</td><td>1,381</td><td>1,381</td><td>1,381</td><td>1,381</td><td>1,381</td><td>---</td></tr><tr><td>191260027035914888</td><td>353,9078460</td><td>36,105041</td><td>19,132</td><td>0.004</td><td>0.841</td><td>0.064</td><td>0.169</td><td>0.302</td><td>1,385</td><td>1,385</td><td>1,385</td><td>1,385</td><td>1,385</td><td>---</td></tr><tr><td>286235947674483840</td><td>353,9983456</td><td>20,8001111</td><td>19,223</td><td>0.004</td><td>0.756</td><td>0.056</td><td>-0.352</td><td>0.272</td><td>1,305</td><td>1,305</td><td>1,305</td><td>1,305</td><td>1,305</td><td>---</td></tr><tr><td>2864548542298645260</td><td>354,1179097</td><td>21,1058495</td><td>19,035</td><td>0.004</td><td>0.820</td><td>0.041</td><td>0.006</td><td>0.236</td><td>1,368</td><td>1,368</td><td>1,368</td><td>1,368</td><td>1,368</td><td>---</td></tr><tr><td>2872375338815387136</td><td>354,1856575</td><td>32,7555069</td><td>19,792</td><td>0.005</td><td>0.564</td><td>0.083</td><td>-0.456</td><td>0.473</td><td>1,376</td><td>1,376</td><td>1,376</td><td>1,376</td><td>1,376</td><td>---</td></tr><tr><td>2862456439206381312</td><td>354,1924982</td><td>21,1427394</td><td>19,388</td><td>0.004</td><td>0.623</td><td>0.056</td><td>0.369</td><td>0.333</td><td>0.974</td><td>0.974</td><td>0.974</td><td>0.974</td><td>0.974</td><td>---</td></tr><tr><td>286010071845069056</td><td>354,3366907</td><td>19,552089</td><td>19,169</td><td>0.004</td><td>0.721</td><td>0.049</td><td>-0.349</td><td>0.285</td><td>1,347</td><td>1,347</td><td>1,347</td><td>1,347</td><td>1,347</td><td>---</td></tr><tr><td>2828149927361546624</td><td>354,4114566</td><td>23,8164958</td><td>19,914</td><td>0.005</td><td>0.713</td><td>0.114</td><td>1.495</td><td>0.546</td><td>1,093</td><td>1,093</td><td>1,093</td><td>1,093</td><td>1,093</td><td>---</td></tr><tr><td>2872756203686469120</td><td>354,1656880</td><td>32,7555069</td><td>19,792</td><td>0.005</td><td>0.658</td><td>0.076</td><td>-0.271</td><td>0.395</td><td>1,275</td><td>1,275</td><td>1,275</td><td>1,275</td><td>1,275</td><td>---</td></tr><tr><td>28727731607833206272</td><td>354,4218696</td><td>22,6125320</td><td>19,520</td><td>0.004</td><td>0.755</td><td>0.069</td><td>-0.304</td><td>0.402</td><td>1,392</td><td>1,392</td><td>1,392</td><td>1,392</td><td>1,392</td><td>---</td></tr><tr><td>2864770364985943424</td><td>354,4223451</td><td>26,6375212</td><td>19,114</td><td>0.004</td><td>0.777</td><td>0.049</td><td>0.078</td><td>0.323</td><td>1,403</td><td>1,403</td><td>1,403</td><td>1,403</td><td>1,403</td><td>---</td></tr><tr><td>2871586031407465728</td><td>354,4571315</td><td>32,3977802</td><td>19,668</td><td>0.004</td><td>0.844</td><td>0.062</td><td>-0.106</td><td>0.367</td><td>1,245</td><td>1,245</td><td>1,245</td><td>1,245</td><td>1,245</td><td>---</td></tr><tr><td>2828159930612041344</td><td>354,6358014</td><td>23,7941231</td><td>19,369</td><td>0.003</td><td>0.919</td><td>0.014</td><td>0.072</td><td>0.100</td><td>1,266</td><td>1,266</td><td>1,266</td><td>1,266</td><td>1,266</td><td>---</td></tr><tr><td>2879312333975462400</td><td>354,5012423</td><td>22,1880743</td><td>19,514</td><td>0.004</td><td>0.707</td><td>0.069</td><td>0.337</td><td>0.383</td><td>1,151</td><td>1,151</td><td>1,151</td><td>1,151</td><td>1,151</td><td>---</td></tr><tr><td>28282409806398817752</td><td>354,5752008</td><td>32,6125320</td><td>19,520</td><td>0.004</td><td>0.777</td><td>0.062</td><td>-0.123</td><td>0.328</td><td>1,182</td><td>1,182</td><td>1,182</td><td>1,182</td><td>1,182</td><td>---</td></tr><tr><td>287110121979671040</td><td>354,5752008</td><td>30,6963267</td><td>18,497</td><td>0.003</td><td>0.878</td><td>0.028</td><td>0.315</td><td>0.186</td><td>1,222</td><td>1,222</td><td>1,222</td><td>1,222</td><td>1,222</td><td>---</td></tr><tr><td>2865228517736519168</td><td>354,5832758</td><td>27,3150892</td><td>19,740</td><td>0.005</td><td>0.606</td><td>0.073</td><td>-0.507</td><td>0.439</td><td>1,121</td><td>1,121</td><td>1,121</td><td>1,121</td><td>1,121</td><td>---</td></tr><tr><td>2828159930612041344</td><td>354,6358014</td><td>23,7941231</td><td>19,369</td><td>0.003</td><td>0.919</td><td>0.014</td><td>0.072</td><td>0.100</td><td>1,266</td><td>1,266</td><td>1,266</td><td>1,266</td><td>1,266</td><td>---</td></tr><tr><td>2872770584153320192</td><td>354,6605429</td><td>33,3859327</td><td>19,297</td><td>0.004</td><td>0.710</td><td>0.045</td><td>0.256</td><td>0.272</td><td>1,576</td><td>1,576</td><td>1,576</td><td>1,576</td><td>1,576</td><td>---</td></tr><tr><td>2864329185945227904</td><td>354,6617901</td><td>25,0942850</td><td>19,311</td><td>0.004</td><td>0.738</td><td>0.046</td><td>0.530</td><td>0.284</td><td>1,233</td><td>1,233</td><td>1,233</td><td>1,233</td><td>1,233</td><td>---</td></tr><tr><td>2871966806028483584</td><td>354,7700213</td><td>32,6894334</td><td>17,741</td><td>0.003</td><td>0.905</td><td>0.016</td><td>0.018</td><td>0.136</td><td>1,427</td><td>1,427</td><td>1,427</td><td>1,427</td><td>1,427</td><td>---</td></tr><tr><td>2868052548930201984</td><td>354,7700213</td><td>32,7555069</td><td>19,489</td><td>0.004</td><td>0.881</td><td>0.006</td><td>-0.109</td><td>0.401</td><td>1,294</td><td>1,294</td><td>1,294</td><td>1,294</td><td>1,294</td><td>---</td></tr><tr><td>2868021350287794560</td><td>354,8314453</td><td>29,7995017</td><td>17,538</td><td>0.003</td><td>0.923</td><td>0.013</td><td>-0.178</td><td>0.101</td><td>1,256</td><td>1,256</td><td>1,256</td><td>1,256</td><td>1,256</td><td>---</td></tr><tr><td>2862226221186789376</td><td>354,8642536</td><td>20,5547948</td><td>19,742</td><td>0.005</td><td>0.720</td><td>0.083</td><td>0.168</td><td>0.472</td><td>1,020</td><td>1,020</td><td>1,020</td><td>1,020</td><td>1,020</td><td>---</td></tr><tr><td>28522595903209614336</td><td>354,8710332</td><td>24,4749727</td><td>19,141</td><td>0.003</td><td>0.805</td><td>0.041</td><td>-0.065</td><td>0.313</td><td>1,276</td><td>1,276</td><td>1,276</td><td>1,276</td><td>1,276</td><td>---</td></tr><tr><td>2827766454649949832</td><td>354,876137</td><td>33,964777</td><td>19,391</td><td>0.004</td><td>0.853</td><td>0.061</td><td>-0.073</td><td>0.338</td><td>1,196</td><td>1,196</td><td>1,196</td><td>1,196</td><td>1,196</td><td>---</td></tr><tr><td>2871914163585750272</td><td>354,8830318</td><td>32,5154698</td><td>19,383</td><td>0.004</td><td>0.667</td><td>0.052</td><td>-0.256</td><td>0.285</td><td>1,542</td><td>1,542</td><td>1,542</td><td>1,542</td><td>1,542</td><td>---</td></tr><tr><td>2823040806334830592</td><td>354,9002104</td><td>19,7876803</td><td>19,406</td><td>0.004</td><td>0.747</td><td>0.066</td><td>0.097</td><td>0.360</td><td>1,245</td><td>1,245</td><td>1,245</td><td>1,245</td><td>1,245</td><td>---</td></tr><tr><td>2865245816507418496</td><td>354,9213839</td><td>27,4455205</td><td>19,078</td><td>0.004</td><td>0.771</td><td>0.055</td><td>0.345</td><td>0.317</td><td>1,469</td><td>1,469</td><td>1,469</td><td>1,469</td><td>1,469</td><td>---</td></tr><tr><td>2866151046649949832</td><td>354,9615142</td><td>28,4659617</td><td>13,800</td><td>0.004</td><td>1.358</td><td>0.005</td><td>0.004</td><td>0.024</td><td>1,231</td><td>1,231</td><td>1,231</td><td>1,231</td><td>1,231</td><td>---</td></tr><tr><td>2864431371806333440</td><td>355,9088699</td><td>25,3505201</td><td>19,346</td><td>0.004</td><td>0.727</td><td>0.054</td><td>0.417</td><td>0.347</td><td>1,496</td><td>1,496</td><td>1,496</td><td>1,496</td><td>1,496</td><td>---</td></tr><tr><td>2866119435690240256</td><td>355,0691475</td><td>28,0094903</td><td>19,476</td><td>0.004</td><td>0.831</td><td>0.080</td><td>0.252</td><td>0.345</td><td>1,441</td><td>1,441</td><td>1,441</td><td>1,441</td><td>1,441</td><td>---</td></tr><tr><td>2865364834487899008</td><td>355,1326832</td><td>27,3919597</td><td>15,052</td><td>0.003</td><td>1.095</td><td>0.005</td><td>0.055</td><td>0.032</td><td>1,236</td><td>1,236</td><td>1,236</td><td>1,236</td><td>1,236</td><td>---</td></tr><tr><td>2862664453062267776</td><td>355,1364844</td><td>21,3505987</td><td>19,399</td><td>0.004</td><td>0.727</td><td>0.075</td><td>0.500</td><td>0.368</td><td>1,173</td><td>1,173</td><td>1,173</td><td>1,173</td><td>1,173</td><td>---</td></tr><tr><td>28628881812767837440</td><td>355,1706311</td><td>21,8067445</td><td>19,552</td><td>0.004</td><td>0.593</td><td>0.074</td><td>0.344</td><td>0.348</td><td>1,187</td><td>1,187</td><td>1,187</td><td>1,187</td><td>1,187</td><td>---</td></tr><tr><td>2872994922416754944</td><td>355,2060212</td><td>34,6340794</td><td>18,481</td><td>0.004</td><td>0.849</td><td>0.039</td><td>0.078</td><td>0.197</td><td>1,654</td><td>1,654</td><td>1,654</td><td>1,654</td><td>1,654</td><td>---</td></tr><tr><td>2852422883765640320</td><td>355,2483495</td><td>25,3717030</td><td>19,495</td><td>0.004</td><td>0.689</td><td>0.053</td><td>0.322</td><td>0.341</td><td>1,289</td><td>1,289</td><td>1,289</td><td>1,289</td><td>1,289</td><td>---</td></tr><tr><td>28229842854827324162</td><td>355,2549449</td><td>19,2284211</td><td>19,599</td><td>0.005</td><td>0.855</td><td>0.082</td><td>-0.098</td><td>0.446</td><td>1,037</td><td>1,037</td><td>1,037</td><td>1,037</td><td>1,037</td><td>---</td></tr><tr><td>28278492492567538656</td><td>355,2671146</td><td>22,9677418</td><td>17,524</td><td>0.003</td><td>0.933</td><td>0.012</td><td>-0.040</td><td>0.099</td><td>1,271</td><td>1,271</td><td>1,271</td><td>1,271</td><td>1,271</td><td>---</td></tr><tr><td>2865256628300500352</td><td>355,2755507</td><td>27,7483341</td><td>15,611</td><td>0.003</td><td>1.066</td><td>0.005</td><td>0.024</td><td>0.039</td><td>1,217</td><td>1,217</td><td>1,217</td><td>1,217</td><td>1,217</td><td>---</td></tr><tr><td>2865251577418971392</td><td>355,3224059</td><td>27,5993570</td><td>13,996</td><td>0.003</td><td>1.282</td><td>0.005</td><td>0.024</td><td>0.020</td><td>1,222</td><td>1,222</td><td>1,222</td><td>1,222</td><td>1,222</td><td>---</td></tr><tr><td>2852418524375994112</td><td>355,3553606</td><td>25,2993957</td><td>19,615</td><td>0.004</td><td>0.823</td><td>0.061</td><td>0.201</td><td>0.377</td><td>1,073</td><td>1,073</td><td>1,073</td><td>1,073</td><td>1,073</td><td>---</td></tr><tr><td>2826217216068446336</td><td>355,3689432</td><td>20,9285651</td><td>19,584</td><td>0.003</td><td>0.680</td><td>0.069</td><td>-0.244</td><td>0.367</td><td>1,148</td><td>1,148</td><td>1,148</td><td>1,148</td><td>1,148</td><td>---</td></tr><tr><td>2852197625617728384</td><td>355,4056343</td><td>24,2832897</td><td>17,168</td><td>0.003</td><td>0.957</td><td>0.009</td><td>-0.047</td><td>0.100</td><td>1,196</td><td>1,196</td><td>1,196</td><td>1,196</td><td>1,196</td><td>---</td></tr><tr><td>2852399381704345600</td><td>355,5377628</td><td>25,0715186</td><td>19,597</td><td>0.004</td><td>0.818</td><td>0.060</td><td>0.275</td><td>0.388</td><td>1,231</td><td>1,231</td><td>1,231</td><td>1,231</td><td>1,231</td><td>---</td></tr><tr><td>2823165364682996992</td><td>355,6829549</td><td>20,3234309</td><td>19,450</td><td>0.005</td><td>0.563</td><td>0.094</td><td>0.805</td><td>0.441</td><td>1,072</td><td>1,072</td><td>1,072</td><td>1,072</td><td>1,072</td><td>---</td></tr><tr><td>282732457652743168</td><td>354,1805918</td><td>22,0364990</td><td>16,945</td><td>0.003</td><td>0.606</td><td>0.009</td><td>-0.061</td><td>0.074</td><td>1,206</td><td>1,206</td><td>1,206</td><td>1,206</td><td>1,206</td><td>---</td></tr><tr><td>2852215046005045248</td><td>355,1653168</td><td>24,4487976</td><td>16,981</td><td>0.003</td><td>0.069</td><td>0.009</td><td>0.253</td><td>0.080</td><td>3,221</td><td>3,221</td><td>3,221</td><td>3,221</td><td>3,221</td><td>---</td></tr><tr><td>28516131922380927232</td><td>359,0995336</td><td>25,0727130</td><td>17,606</td><td>0.003</td><td>0.094</td><td>0.216</td><td>0.079</td><td>0.079</td><td>2,016</td><td>2,016</td><td>2,016</td><td>2,016</td><td>2,016</td><td>---</td></tr><tr><td>28218132236662858240</td><td>354,5801622</td><td>23,6484716</td><td>16,762</td><td>0.003</td><td>0.193</td><td>0.009</td><td>0.074</td><td>0.068</td><td>1,184</td><td>1,184</td><td>1,184</td><td>1,184</td><td>1,184</td><td>---</td></tr><tr><td>2864750818590938752</td><td>354,8195599</td><td>26,4910224</td><td>16,846</td><td>0.003</td><td>0.166</td><td>0.009</td><td>0.007</td><td>0.069</td><td>1,247</td><td>1,247</td><td>1,247</td><td>1,247</td><td>1,247</td><td>---</td></tr><tr><td>286776330865175376</td><td>355,2655209</td><td>29,4259271</td><td>16,949</td><td>0.003</td><td>0.184</td><td>0.009</td><td>-0.034</td><td>0.088</td><td>1,305</td><td>1,305</td><td>1,305</td><td>1,305</td><td>1,305</td><td>---</td></tr><tr><td>275680618428977288</td><td>356,0119714</td><td>6,4344992</td><td>16,749</td><td>0.003</td><td>0.216</td><td>0.011</td><td>-0.056</td><td>0.075</td><td>1,308</td><td>1,308</td><td>1,308</td><td>1,308</td><td>1,308</td><td>---</td></tr></tbody></table>

When available the photometric metallicities from the Pristine survey are provided in this column. We note that, by construction¹⁰, the Pristine photometric metallicities do not go below $[Fe/H]_{Pristine} = -4.0$ and that the lessened sensitivity of the Pristine narrowband in this regime means that although stars can be flagged to have $[Fe/H] < -3.0$, the actual photometric metallicity is less accurate in this regime than for stars with $[Fe/H] > -3.0$. 'HB': candidate horizontal branch star. 'Non-member': considered a non-member on the basis of the Pristine metallicity ($[Fe/H]_{Pristine} > -2.5$) and not used in the analysis.
Extended Data Table 2 | Summary of observations for the C-19 candidate stars

<table><thead><tr><th>Name</th><th>RA (deg)</th><th>DEC (deg)</th><th>Gaia G</th><th>[Fe/H]</th><th>Pristine</th><th>t_exp</th><th>S/N<sup>+</sup>(a)</th><th>v_r (km/s)</th><th>Observatory</th><th>Comment</th></tr></thead><tbody><tr><td>Pristine_354.77+30.25</td><td>354.7701</td><td>+30.2509</td><td>15.69</td><td>-3.38</td><td></td><td>1x2,400</td><td>50</td><td>-186.7 +/- 2.2</td><td>Gemini/GRACES</td><td></td></tr><tr><td>Pristine_355.13+27.98</td><td>355.1327</td><td>+27.9819</td><td>15.39</td><td>-3.12</td><td></td><td>3x2,400</td><td>100</td><td>-194.4 +/- 2.0</td><td>Gemini/GRACES</td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td>1x600</td><td>45</td><td>-198 +/- 16</td><td>GTC/OSIRIS</td><td></td></tr><tr><td>Pristine_355.27+27.74</td><td>355.2755</td><td>+27.7483</td><td>15.82</td><td>-3.41</td><td></td><td>2x2,400</td><td>55</td><td>-197.3 +/- 2.1</td><td>Gemini/GRACES</td><td></td></tr><tr><td>Pristine_353.79+21.26</td><td>353.7994</td><td>+21.2648</td><td>17.63</td><td>-3.71</td><td></td><td>2x1,500</td><td>47</td><td>-179 +/- 17</td><td>GTC/OSIRIS</td><td></td></tr><tr><td>Pristine_354.63+23.79</td><td>354.6358</td><td>+23.7941</td><td>17.49</td><td>-3.25</td><td></td><td>2x1,500</td><td>49</td><td>-178 +/- 16</td><td>GTC/OSIRIS</td><td></td></tr><tr><td>Pristine_355.26+22.96</td><td>355.2671</td><td>+22.9677</td><td>17.60</td><td>-3.11</td><td></td><td>2x1,500</td><td>44</td><td>-192 +/- 17</td><td>GTC/OSIRIS</td><td></td></tr><tr><td>Pristine_355.32+27.59</td><td>355.3223</td><td>+27.5993</td><td>14.19</td><td>-2.63</td><td></td><td>1x300</td><td>62</td><td>-191 +/- 14</td><td>GTC/OSIRIS</td><td></td></tr><tr><td></td><td></td><td></td><td></td><td></td><td></td><td></td><td>40</td><td>-194</td><td>LAMOST</td><td>Li et al. (2018)</td></tr><tr><td>Pristine_355.40+24.28</td><td>355.4056</td><td>+24.2832</td><td>17.28</td><td>-3.78</td><td></td><td>2x1,500</td><td>38</td><td>-184 +/- 16</td><td>GTC/OSIRIS</td><td></td></tr><tr><td>Pristine_354.77+32.68</td><td>354.7700</td><td>+32.6894</td><td>17.89</td><td>-0.55</td><td></td><td>2x1,500</td><td>39</td><td>-49 +/- 18</td><td>GTC/OSIRIS</td><td>non-member</td></tr></tbody></table>

*Signal-to-noise ratio (S/N) at 600 nm (Gemini-GRACES) and 420 nm (GTC-OSIRIS).
Extended Data Table 3 | Spectroscopic parameters and one-dimensional LTE chemical abundances for the Gemini–GRACES spectra

<table><thead><tr><th>Parameter</th><th>Pristine_354.77+30.25</th><th>Pristine_355.13+27.98</th><th>Pristine_355.27+27.74</th></tr></thead><tbody><tr><td>T<sub>eff</sub> (K)</td><td>4,928 +/- 100</td><td>4,881 +/- 100</td><td>4,958 +/- 100</td></tr><tr><td>log g</td><td>1.83 +/- 0.10</td><td>1.64 +/- 0.10</td><td>1.89 +/- 0.10</td></tr><tr><td>xi (km/s)</td><td>2.13 +/- 0.10</td><td>2.20 +/- 0.10</td><td>2.12 +/- 0.10</td></tr><tr><td>[FeI/H]</td><td>-3.21 +/- 0.17</td><td>-3.30 +/- 0.15</td><td>-3.15 +/- 0.17</td></tr><tr><td>[FeI/H]_NLTE</td><td>-3.03 +/- 0.17</td><td>-3.12 +/- 0.15</td><td>-2.97 +/- 0.17</td></tr><tr><td>[FeII/H]</td><td>-3.42 +/- 0.12</td><td>-3.45 +/- 0.11</td><td>-3.42 +/- 0.17</td></tr><tr><td>[NaI/FeII]</td><td>+0.73 +/- 0.18</td><td>+0.77 +/- 0.17</td><td>+0.34 +/- 0.15</td></tr><tr><td>[NaI/FeII]_NLTE</td><td>+0.34 +/- 0.18</td><td>+0.37 +/- 0.17</td><td>+0.05 +/- 0.15</td></tr><tr><td>[MgI/FeII]</td><td>+0.27 +/- 0.17</td><td>+0.33 +/- 0.14</td><td>+0.38 +/- 0.14</td></tr><tr><td>[MgI/FeII]_NLTE</td><td>+0.33 +/- 0.17</td><td>+0.39 +/- 0.14</td><td>+0.42 +/- 0.14</td></tr><tr><td>[NaI/MgI]</td><td>+0.46 +/- 0.18</td><td>+0.44 +/- 0.16</td><td>-0.04 +/- 0.15</td></tr><tr><td>[NaI/MgI]_NLTE</td><td>+0.01 +/- 0.18</td><td>-0.02 +/- 0.16</td><td>-0.39 +/- 0.15</td></tr><tr><td>[CaI/FeII]</td><td>+0.36 +/- 0.20</td><td>+0.36 +/- 0.08</td><td>+0.28 +/- 0.13</td></tr><tr><td>[CaI/FeII]_NLTE</td><td>+0.56 +/- 0.20</td><td>+0.56 +/- 0.08</td><td>+0.48 +/- 0.13</td></tr><tr><td>[CrI/FeII]</td><td>-0.16 +/- 0.16</td><td>-0.17 +/- 0.15</td><td>-0.05 +/- 0.15</td></tr><tr><td>[BaII/FeII]</td><td>&lt;0.52</td><td>-0.22 +/- 0.17</td><td>&lt;0.39</td></tr></tbody></table>

The uncertainties of Fe I measurements correspond to the line-to-line scatter in the Fe I abundances only, whereas uncertainties on [Fe II/H] and [X/Fe] combine the measurement uncertainties (as $\sigma/\sqrt{N}$) and the uncertainties due to the stellar parameters, added in quadrature. NLTE corrections are listed in Extended Data Table 2; the averaged NLTE corrections are applied here, other than for Na I, for which individual line calculations per star were applied. We adopt the metallicities for these stars from [Fe II/H].
Extended Data Table 4 | Spectral lines and atomic data used for the chemical abundances for the Gemini–GRACES spectra

<table><thead><tr><th>Wavelength<br>(\AA)</th><th>Elem<br>(---)</th><th>xi<br>(eV)</th><th>log gf<br>(---)</th><th>Pristine_355.1+27.9<br>(m\AA)</th><th>Pristine_354.7+30.2<br>(m\AA)</th><th>Pristine_355.2+27.7<br>(m\AA)</th><th>NLTE_corr^a<br>(m\AA)</th><th>NLTE_corr^b<br>(m\AA)</th></tr></thead><tbody><tr><td>4,890.755</td><td>FeI</td><td>2.88</td><td>-0.38</td><td>30</td><td>w</td><td>55</td><td>+0.18</td><td>+0.22</td></tr><tr><td>4,891.492</td><td>FeI</td><td>2.85</td><td>-0.14</td><td>40</td><td>39</td><td>w</td><td>+0.18</td><td>+0.21</td></tr><tr><td>4,918.994</td><td>FeI</td><td>2.86</td><td>-0.37</td><td>n</td><td>w</td><td>43</td><td>---</td><td>+0.22</td></tr><tr><td>4,920.502</td><td>FeI</td><td>2.83</td><td>0.06</td><td>58</td><td>60</td><td>57</td><td>+0.23</td><td>+0.20</td></tr><tr><td>5,012.068</td><td>FeI</td><td>0.86</td><td>-2.6</td><td>50</td><td>55</td><td>70</td><td>---</td><td>+0.18</td></tr><tr><td>5,041.072</td><td>FeI</td><td>0.96</td><td>-3.09</td><td>28</td><td>30</td><td>37</td><td>---</td><td>+0.18</td></tr><tr><td>5,041.756</td><td>FeI</td><td>1.48</td><td>-2.20</td><td>36</td><td>n</td><td>45</td><td>---</td><td>+0.18</td></tr><tr><td>5,051.635</td><td>FeI</td><td>0.91</td><td>-2.76</td><td>55</td><td>n</td><td>57</td><td>---</td><td>+0.18</td></tr><tr><td>5,083.339</td><td>FeI</td><td>0.96</td><td>-2.84</td><td>40</td><td>w</td><td>33</td><td>---</td><td>+0.18</td></tr><tr><td>5,123.700</td><td>FeI</td><td>1.01</td><td>-3.06</td><td>14</td><td>w</td><td>23</td><td>---</td><td>+0.18</td></tr><tr><td>5,171.596</td><td>FeI</td><td>1.48</td><td>-1.72</td><td>54</td><td>58</td><td>70</td><td>---</td><td>+0.18</td></tr><tr><td>5,194.942</td><td>FeI</td><td>1.56</td><td>-2.02</td><td>44</td><td>n</td><td>n</td><td>+0.20</td><td>+0.18</td></tr><tr><td>5,202.336</td><td>FeI</td><td>2.17</td><td>-1.87</td><td>16</td><td>n</td><td>w</td><td>+0.20</td><td>+0.19</td></tr><tr><td>5,216.274</td><td>FeI</td><td>1.61</td><td>-2.08</td><td>27</td><td>n</td><td>n</td><td>+0.20</td><td>+0.18</td></tr><tr><td>5,227.190</td><td>FeI</td><td>1.56</td><td>-1.23</td><td>94</td><td>90</td><td>80</td><td>---</td><td>+0.14</td></tr><tr><td>5,232.940</td><td>FeI</td><td>2.94</td><td>-0.19</td><td>51</td><td>48</td><td>58</td><td>+0.20</td><td>+0.22</td></tr><tr><td>5,266.555</td><td>FeI</td><td>2.99</td><td>-0.49</td><td>42</td><td>w</td><td>w</td><td>---</td><td>+0.22</td></tr><tr><td>5,269.537</td><td>FeI</td><td>0.86</td><td>-1.33</td><td>135</td><td>120</td><td>129</td><td>+0.10</td><td>+0.08</td></tr><tr><td>5,281.700</td><td>FeI</td><td>3.04</td><td>-1.02</td><td>10</td><td>w</td><td>w</td><td>+0.19</td><td>+0.21</td></tr><tr><td>5,324.179</td><td>FeI</td><td>3.21</td><td>-0.11</td><td>32</td><td>b</td><td>n</td><td>+0.21</td><td>+0.25</td></tr><tr><td>5,328.039</td><td>FeI</td><td>0.91</td><td>-1.47</td><td>112</td><td>109</td><td>113</td><td>---</td><td>+0.10</td></tr><tr><td>5,328.532</td><td>FeI</td><td>1.56</td><td>-1.85</td><td>51</td><td>37</td><td>38</td><td>---</td><td>+0.18</td></tr><tr><td>5,371.489</td><td>FeI</td><td>0.96</td><td>-1.64</td><td>102</td><td>112</td><td>112</td><td>---</td><td>+0.12</td></tr><tr><td>5,397.128</td><td>FeI</td><td>0.91</td><td>-1.98</td><td>91</td><td>77</td><td>93</td><td>+0.17</td><td>+0.15</td></tr><tr><td>5,405.775</td><td>FeI</td><td>0.99</td><td>-1.85</td><td>99</td><td>105</td><td>96</td><td>+0.16</td><td>+0.14</td></tr><tr><td>5,429.696</td><td>FeI</td><td>0.96</td><td>-1.88</td><td>100</td><td>93</td><td>101</td><td>---</td><td>+0.14</td></tr><tr><td>5,434.524</td><td>FeI</td><td>1.01</td><td>-2.13</td><td>70</td><td>71</td><td>60</td><td>+0.20</td><td>+0.16</td></tr><tr><td>5,446.917</td><td>FeI</td><td>0.99</td><td>-1.91</td><td>96</td><td>105</td><td>91</td><td>+0.15</td><td>+0.15</td></tr><tr><td>5,455.609</td><td>FeI</td><td>1.01</td><td>-2.09</td><td>77</td><td>81</td><td>74</td><td>---</td><td>+0.16</td></tr><tr><td>5,497.516</td><td>FeI</td><td>1.01</td><td>-2.83</td><td>34</td><td>25</td><td>22</td><td>---</td><td>+0.18</td></tr><tr><td>5,501.465</td><td>FeI</td><td>0.96</td><td>-3.05</td><td>30</td><td>w</td><td>w</td><td>---</td><td>+0.18</td></tr><tr><td>5,506.779</td><td>FeI</td><td>0.99</td><td>-2.79</td><td>33</td><td>36</td><td>34</td><td>---</td><td>+0.18</td></tr><tr><td>5,572.842</td><td>FeI</td><td>3.39</td><td>-0.28</td><td>19</td><td>n</td><td>n</td><td>+0.20</td><td>+0.24</td></tr><tr><td>5,586.756</td><td>FeI</td><td>3.37</td><td>-0.11</td><td>23</td><td>34</td><td>27</td><td>+0.20</td><td>+0.25</td></tr><tr><td>6,136.615</td><td>FeI</td><td>2.45</td><td>-1.40</td><td>16</td><td>w</td><td>n</td><td>+0.18</td><td>+0.21</td></tr><tr><td>6,137.691</td><td>FeI</td><td>2.59</td><td>-1.40</td><td>11</td><td>w</td><td>n</td><td>+0.17</td><td>+0.20</td></tr><tr><td>6,191.558</td><td>FeI</td><td>2.43</td><td>-1.60</td><td>18</td><td>n</td><td>n</td><td>+0.20</td><td>+0.21</td></tr><tr><td>6,230.722</td><td>FeI</td><td>2.56</td><td>-1.28</td><td>22</td><td>27</td><td>n</td><td>+0.20</td><td>+0.20</td></tr><tr><td>6,252.555</td><td>FeI</td><td>2.40</td><td>-1.69</td><td>11</td><td>w</td><td>n</td><td>+0.20</td><td>+0.20</td></tr><tr><td>4,923.922</td><td>FeII</td><td>2.89</td><td>-1.21</td><td>73</td><td>70</td><td>n</td><td>-0.01</td><td>-0.01</td></tr><tr><td>5,018.435</td><td>FeII</td><td>2.89</td><td>-1.35</td><td>59</td><td>56</td><td>58</td><td>-0.00</td><td>-0.01</td></tr><tr><td>5,889.951</td><td>NaI</td><td>0.00</td><td>0.12</td><td>152</td><td>135</td><td>126</td><td>-0.33</td><td>-0.40</td></tr><tr><td>5,895.924</td><td>NaI</td><td>0.00</td><td>-0.18</td><td>137</td><td>145</td><td>106</td><td>-0.33</td><td>-0.33</td></tr><tr><td>5,172.684</td><td>MgI</td><td>2.71</td><td>-0.40</td><td>133</td><td>133</td><td>145</td><td>+0.08</td><td>+0.02</td></tr><tr><td>5,183.604</td><td>MgI</td><td>2.72</td><td>-0.18</td><td>157</td><td>143</td><td>150</td><td>+0.06</td><td>-0.04</td></tr><tr><td>5,528.405</td><td>MgI</td><td>4.34</td><td>-0.62</td><td>20</td><td>21</td><td>20</td><td>+0.11</td><td>+0.15</td></tr><tr><td>6,102.723</td><td>CaI</td><td>1.88</td><td>-0.89</td><td>11</td><td>n</td><td>n</td><td>+0.27</td><td>+0.14</td></tr><tr><td>6,122.217</td><td>CaI</td><td>1.88</td><td>-0.41</td><td>20</td><td>25</td><td>b</td><td>+0.25</td><td>+0.14</td></tr><tr><td>6,162.173</td><td>CaI</td><td>1.90</td><td>0.10</td><td>32</td><td>32</td><td>32</td><td>+0.24</td><td>+0.14</td></tr><tr><td>6,439.075</td><td>CaI</td><td>2.52</td><td>0.47</td><td>20</td><td>24</td><td>23</td><td>+0.21</td><td>+0.20</td></tr><tr><td>5,204.498</td><td>CrI</td><td>0.94</td><td>-0.19</td><td>53</td><td>27</td><td>50</td><td>+0.56</td><td>---</td></tr><tr><td>5,206.023</td><td>CrI</td><td>0.94</td><td>0.02</td><td>39</td><td>48</td><td>45</td><td>+0.56</td><td>---</td></tr><tr><td>5,208.409</td><td>CrI</td><td>0.94</td><td>0.17</td><td>33</td><td>48</td><td>41</td><td>+0.55</td><td>---</td></tr><tr><td>4,934.100</td><td>BaII</td><td>0.00</td><td>-1.16</td><td>26</td><td>&lt;40</td><td>&lt;35</td><td>---</td><td>+0.20</td></tr></tbody></table>

Equivalent widths are in milliångströms. As these three stars have very similar stellar parameters, a single NLTE abundance correction estimate is shown per line from three sources (where NLTE = log(X/H)NLTE - log(X/H)NLTE).

When lines could not be measured in all three stars, they are noted as 'n', 'w' or 'b', which refer to 'noisy', 'weak' and 'blended'.

aNLTE correction estimates from the INSPECT database (http://www.inspect-stars.com) and/or MPIA database (http://nlte.mpa.de).

bNLTE corrections from the private calculations of L.I.M.⁷². Only the Na I NLTE corrections show some star-to-star variations due to the differences in the line strengths (the average from the three stars is reported here).
Extended Data Table 5 | Stellar parameters and abundances of C-19 stars observed with GTC-OSIRIS

<table><thead><tr><th>Name<br>(---)</th><th>T_eff<br>(K)</th><th>log g<br>(cm/s<sup>2</sup>)</th><th>[M/H]</th><th>[Ca/H]</th><th>[Fe/H]</th><th>[C/Fe]</th></tr></thead><tbody><tr><td colspan="7">With stellar parameters based on Gaia photometry (used in the paper):</td></tr><tr><td>Pristine_353.79+21.26</td><td>5,230 +/- 100</td><td>2.73 +/- 0.10</td><td>-3.51 +/- 0.15</td><td>-3.21 +/- 0.11</td><td>-3.41 +/- 0.19</td><td>+0.48 +/- 0.32</td></tr><tr><td>Pristine_354.63+23.79</td><td>5,280 +/- 100</td><td>2.71 +/- 0.10</td><td>-3.49 +/- 0.17</td><td>-3.20 +/- 0.10</td><td>-3.38 +/- 0.20</td><td>+0.61 +/- 0.63</td></tr><tr><td>Pristine_355.13+27.98</td><td>4,881 +/- 100</td><td>1.64 +/- 0.10</td><td>-3.37 +/- 0.14</td><td>-3.02 +/- 0.12</td><td>-3.32 +/- 0.18</td><td>-0.11 +/- 0.82</td></tr><tr><td>Pristine_355.26+22.96</td><td>5,248 +/- 100</td><td>2.76 +/- 0.10</td><td>-3.45 +/- 0.16</td><td>-3.17 +/- 0.11</td><td>-3.34 +/- 0.19</td><td>+0.44 +/- 0.36</td></tr><tr><td>Pristine_355.32+27.59</td><td>4,570 +/- 100</td><td>1.07 +/- 0.10</td><td>-3.23 +/- 0.19</td><td>-2.92 +/- 0.10</td><td>-3.15 +/- 0.18</td><td>-0.60 +/- 0.46</td></tr><tr><td>Pristine_355.40+24.28</td><td>5,193 +/- 100</td><td>2.60 +/- 0.10</td><td>-3.45 +/- 0.15</td><td>-3.11 +/- 0.11</td><td>-3.39 +/- 0.18</td><td>+0.16 +/- 0.23</td></tr><tr><td colspan="7">Pure spectroscopic stellar parameters:</td></tr><tr><td>Pristine_353.79+21.26</td><td>5,319 +/- 180</td><td>1\downarrowarrow</td><td>-3.50 +/- 0.12</td><td>---</td><td>---</td><td>---</td></tr><tr><td>Pristine_354.63+23.79</td><td>5,253 +/- 340</td><td>1\downarrowarrow</td><td>-3.56 +/- 0.15</td><td>---</td><td>---</td><td>---</td></tr><tr><td>Pristine_355.13+27.98</td><td>4,874 +/- 105</td><td>1\downarrowarrow</td><td>-3.37 +/- 0.12</td><td>---</td><td>---</td><td>---</td></tr><tr><td>Pristine_355.26+22.96</td><td>5,290 +/- 186</td><td>1\downarrowarrow</td><td>-3.48 +/- 0.14</td><td>---</td><td>---</td><td>---</td></tr><tr><td>Pristine_355.32+27.59</td><td>5,089 +/- 106</td><td>1\downarrowarrow</td><td>-3.02 +/- 0.18</td><td>---</td><td>---</td><td>---</td></tr><tr><td>Pristine_355.40+24.28</td><td>5,344 +/- 142</td><td>5\uparrowarrow</td><td>-3.61 +/- 0.11</td><td>---</td><td>---</td><td>---</td></tr></tbody></table>

Up and down arrows for log g values represent cases where FERRE reached the limit of the spectral model grid.