CE31 - Physique Subatomique, Sciences de l'Univers, Structure et Histoire de la Terre

Pristine — A survey of the first Galactic stars – Pristine

Pristine — A survey of the first Galactic stars

The most efficient path to understanding the formation of the Milky Way into a disk galaxy during the first few billion years of the universe is to search for, map, and study the dynamics of primordial stars as these stars are the only remaining witnesses of these very early times.

Understanding the formation of the Milky Way from the distribution and the orbits of the oldest stars.

The most metal-poor stars (less than a thousandth of the Sun's abundance in iron) are also the oldest stars as they are expected to have formed within the first 2 billion years of the age of the universe. However, they are also very rare (less than one star in a thousand) et exceedingly hard to isolate from the many more foreground stars from the Milky Way disk. But these are also the stars with the largest potential to inform us on the very early universe: they keep the chemical imprint of the very first stars and their orbits are a direct window onto the assembly of the Milky Way.<br /><br />The Pristine project builds on a wide, dedicated narrow-band photometric survey conducted to efficiently discover such rare stars and build the largest sample of primordial stars ever assembled. The Pristine ANR project is part of the international collaboration and aims to use this unique sample of stars to study how the Milky Way assembled in its infancy, what type of structures the Milky Way absorbed over time to build itself, and how the primordial stars enriched the interstellar medium et formed the first chemical elements.

The Pristine collaboration aims to build the largest sample of primordial stars ever assembled. Pristine builds on a wide, dedicated narrow-band photometric survey conducted with the wide-field imager MegaCam on the Canada-France-Hawaii Telescope (CFHT). Any very metal-poor star will show weak Ca H&K features setting them apart from more metal-rich stars of the same color. Combined with more common broadband photometry (from the Sloan Digital Sky Survey; SDSS), we have demonstrated that the Ca H&K photometry we gathered efficiently weeds out the 99%-contamination of «classic« MW stars with higher metallicity.
With the observations in hand, we can easily select the scarce and under-represented very metal-poor stars that can then be studied in exquisite detail with a dedicated, systematic spectroscopic follow-up program of the most interesting stars.

We have shown, through the study of the spatial distribution and kinematics of the currently known primordial stars, that an unexpectedly large number of these primordial stars are located in the disk of the Milky Way. Theoretically, it was expected that they would instead be distributed on random orbit around the Milky Way. This work, led by then PhD student Federico Sestito, has a significant impact on our understanding of the assembly of the Galaxy and has triggered numerous reactions and follow-up studies in the astronomical community.
Sestito et al. (2019, MNRAS 484, 2166), Sestito et al. (2020, MNRAS 497L, 7), Sestito et al. (2021, MNRAS 500, 3750)

Our collaboration with the large spectroscopic survey WEAVE that groups together numerous European partners, including a large French partner, ensures that we will soon build the largest sample of confirmed primordial stars. From these thousands of unique stars, combined with the astrometric data of the Gaia satellite, we will study the origin of the primordial stars and unveil their assembly at the onset of the formation of the Milky Way. In parallel, the analysis of the chemical composition of those stars will allow us to study the traces left by the very first stars, how they differ from current stars, and how they enriched the early universe.

Bonifacio, P. et al., 2019, MNRAS 487, 3797. The Pristine survey - V. A bright star sample observed with SOPHIE.

Aguado, David S. et al. 2019, MNRAS 490, 2241. The Pristine survey - VI. The first three years of medium-resolution follow-up spectroscopy of Pristine EMP star candidates.

Starkenburg, Else et al., 2019, MNRAS 490, 5757. The Pristine survey - VII. A cleaner view of the Galactic outer halo using blue horizontal branch stars.

Longeard, Nicolas et al., 2020, MNRAS 491, 356. The Pristine Dwarf-Galaxy survey - II. In-depth observational study of the faint Milky Way satellite Sagittarius II.

Arentsen, A. et al., 2020, MNRAS 491L, 11. The Pristine Inner Galaxy Survey (PIGS) I: tracing the kinematics of metal-poor stars in the Galactic bulge.

Sestito, Federico et al., 2020, MNRAS 497L, 7. The Pristine survey - X. A large population of low-metallicity stars permeates the Galactic disc.

Venn, Kim A. et al., 2020, MNRAS 492, 3241. The Pristine survey - IX. CFHT ESPaDOnS spectroscopic analysis of 115 bright metal-poor candidate stars.

Youakim, K. et al., 2020, MNRAS 492, 4986. The Pristine Survey - VIII. The metallicity distribution function of the Milky Way halo down to the extremely metal-poor regime.

Caffau, E. et al., 2020, MNRAS 493, 4677. The Pristine survey XI: the FORS2 sample.

Arentsen, Anke et al., 2020, MNRAS 496, 4964. The Pristine Inner Galaxy Survey (PIGS) II: Uncovering the most metal-poor populations in the inner Milky Way.

+ 14 other publications in refereed, international journals.

The lowest metallicity stars are also the oldest ones and they carry the imprint of the first supernovae. Since the very first stars are likely short-lived and inaccessible to us, the lowest metallicity stars are also those that can inform us most on the first generation of stars, how they enriched their environment, and produced the first structures that, through hierarchical formation, built up the primordial constituents of the galaxies we observe today. These stars are exceedingly rare and few of them are known today. In order to efficiently improve on this situation, we have put in place the Pristine international collaboration. Focussed around a wide narrow-band photometric survey conducted at the Canada-France-Hawaii Telescope and a large dedicated spectroscopic campaign, Pristine is many times more efficient than previous attempts at finding the precious low-metallicity stars. From the detailed study of these stars, we will: hunt for the most extreme low-metallicity stars; place constraints on star formation during the earliest epoch of the universe; reliably unveil the properties of the faintest known dwarf galaxies that orbit the Milky Way and are promising cosmological probes; decompose the Milky Way into its main components to place our Galaxy in the global context of galaxy formation and evolution; deconstruct the stellar halo of the Milky Way into its constituent substructures, which will then be used to constrain the mass and shape of the Milky's Way potential.

In order to reliably achieve these significant goals, we request funding to support Pristine in France and ensure the project is staffed adequately to yield high-impact scientific returns in the field of very low metallicity stars. Our team is built around experts of the field in Nice (OCA/Lagrange), Paris (GEPI), and Strasbourg (Observatoire astronomique de Strasbourg) and represents a large part of the full Pristine collaboration. To complement this team, we ask for funding for a PhD student and two postdoctoral researchers who will be spread over the three French nodes of our project, along with funds to support the team and secure the visibility and active partnership of the French team members within the full international Pristine collaboration.

We wish to emphasize that the effort already invested by the current team members into preparing the survey (successful telescope time proposals, data acquisition, reduction, and calibration, start of the spectroscopic follow-up campaign) means that the Pristine project is a low risk but high return project if it were to be supported by the ANR. It builds on large facilities and surveys with a significant French involvement (CFHT, Gaia, WEAVE) and it promises numerous high impact papers to be published in the high-visibility fields of Galactic archaeology and near-field cosmology in which France is a world leader.

Project coordination

Nicolas Martin (Observatoire astronomique de Strasbourg)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.


ObAS Observatoire astronomique de Strasbourg
LAGRANGE Laboratoire J-L. Lagrange
GEPI Galaxies, Etoiles, Physique, Instrumentation

Help of the ANR 475,651 euros
Beginning and duration of the scientific project: December 2018 - 48 Months

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