OUTLIERS · Observing Unique sTars and gaLactic chemIcal Evidence of the fiRst relicS

The number of identified primitive low-metallicity stars has steadily grown. Today, we know of stars with less than one ten-millionth of the solar iron content, but no stars completely devoid of metals —known as Population III (Pop III)— have been identified so far. It was commonly accepted that only massive, short-lived Pop III stars formed from metal-free clouds, but recent simulations suggest that their low-mass counterparts (<1M☉) could also form and may still survive today. However, the mass distribution of these first stars remains unknown, though most are expected to be massive, reaching up to 1000M☉. The first supernova (SN) enriched the medium with heavy elements, enabling the formation of long-lived Population II (Pop II) stars from this processed material. These stars exhibit distinct chemical peculiarities that are key to understanding their Pop III progenitors, including mass and SN explosion energy. However, identifying and characterising these exceedingly rare chemically peculiar stars poses significant challenges. Finding long-lived Pop III stars or second-generation stars polluted by very massive Pop III SNe will revolutionise modern astronomy.With OUTLIERS I propose a novel approach that combines an unprecedented observational effort, encompassing my large program with the 10.4m GTC, privileged access to the latest spectroscopic surveys (DESI, 4MOST, WEAVE), and Gaia BP/RP data. Together with state-of-the-art models and my advanced analysis tools (FESWI, PISN-explorer), this approach will increase the number of chemically peculiar stars by over an order of magnitude, allowing us, for the first time, to:-Search for Pop III stars within the largest spectroscopic dataset ever compiled-Identify and characterise second-generation stars to reconstruct the mass and energy of their progenitorsAchieving these objectives will shed light on a critical issue in astrophysics: the formation of the first stars as evidenced by their Pop II descendants.