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arXiv · 2610.20714

The Pristine survey XXIX. Data Release 2: Photometric metallicities for 15 millions stars down to $G\sim 20.5$

Zhen Yuan·Anke Ardern-Arentsen·Nicolas F. Martin·Else Starkenburg·Morgan Fouesneau·Piercarlo Bonifacio·Elisabetta Caffau·Vanessa Hill·Georges Kordopatis·Camila Navarrete·Julio Navarro·Samuel Rusterucci·Federico Sestito·Salvatore Taibi·Kim A. Venn·Akshara Viswanathan·HanYuan Zhang·Giuseppina Battaglia
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Abstract

Tailored narrow-band photometric observations are an efficient way to measure the metallicity of Milky Way stars. The Pristine survey has been mapping the northern sky since 2015 using narrow-band $CaHK$ photometry obtained with the MegaCam imager on the Canada-France-Hawaii Telescope (CFHT). Data Release 1 (DR1) provided a high-precision absolute calibration of the survey observed before October 2022 using \Gaia DR3 XP synthetic photometry, yielding photometric metallicities over $\sim$6,500 deg$^2$, but was limited in depth to the availability of \Gaia spectro-photometry ($G \lesssim 17.65$). We present the Pristine Data Release 2 (DR2) that includes all Pristine observations until January 2025, corresponding to an additional 1,500\,deg$^2$ of coverage, and including all faint sources down to $G\sim20.5$. This release aims to exploit the full depth of the Pristine observations to uncover the faint metal-poor halo. We process all images observed from the start of the survey in March 2015 through to January 2025 following the methodology presented in DR1. The core of the calibration utilizes the subset of bright stars with \Gaia synthetic $CaHK_\mathrm{syn}$ magnitudes to train the \texttt{PhotCalib} neural network. We then derive photometric metallicities using the established Pristine ($CaHK, G, G_{\rm BP}, G_{\rm RP}$) to [Fe/H] model. The resulting catalogue covers approximately $\sim$ 8,000 deg$^{2}$ and provides photometric metallicities for 15 millions FGK stars, which show good agreement with recently published, independent Dark Energy Spectroscopic Instrument (DESI) metallicities. With its increased coverage and access to a significantly fainter regime compared to DR1, DR2 significantly increases the volume of the survey. We report more than 200,000 and 6,500 very and extremely metal-poor star candidates, respectively.

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Zhen Yuan, Anke Ardern-Arentsen, Nicolas F. Martin, Else Starkenburg, Morgan Fouesneau, Piercarlo Bonifacio, Elisabetta Caffau, Vanessa Hill, Georges Kordopatis, Camila Navarrete, Julio Navarro, Samuel Rusterucci, Federico Sestito, Salvatore Taibi, Kim A. Venn, Akshara Viswanathan, HanYuan Zhang, Giuseppina Battaglia. 2026-10-14. The Pristine survey XXIX. Data Release 2: Photometric metallicities for 15 millions stars down to $G\sim 20.5$. https://arxiv.org/abs/2610.20714

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