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The non-canonical evolution of stars in clusters StarDance Elena Pancino Introduction to Binary Workshop 2025 (Florence, October 2025) Qui ci metto una striscia fatta di immaginine belle di clusters, stelle rotanti, e interazioni binarie
We do not know how star clusters form We cannot explain their stellar content Clocks Templates Tracers Our astrophysical laboratories Directly or indirectly used in a range of research fields Laboratories MW: 170 globular clusters, >3000 open clusters Elena Pancino –Arcetri –October 2025
Two elephants in the room Gaia-ESO data, Pancino et al. 2017 Multiple populations Are our models and stellar clocks calibrated on true single stars? Since 1976 •Element variations •Hot H-burning •Stellar generations? No scenario works Multiple stellar generations? A different star formation mode? Non-canonical populations Gaia DR3 data, Pancino et al., in prep. Since 1953 •Binary byproducts •Stellar rotation •Also open clusters Not understood 00.1 0.2 0.3 0.4 0.5 BP-RP (mag) 8 9 10 11 G (mag) 100 200 300 vsini (km/s) Elena Pancino –Arcetri –October 2025
Anti-correlations Oldest evidence Bessell & Norris 1976 •Light element variations – CNO + NeNa + MgAl – Hot H burning •Iron-peak and heavy elements vary only in some GC (about 20%) Pancino et al. 2017 Briley et al. 2005 [Fe/H]=-0.5 dex [Fe/H]=-1.5 dex [Fe/H]=-2.5 dex In globular clusters Elena Pancino –Arcetri –October 2025
Multiple populations Oldest evidence Lee et al. 1999 Pancino et al. 2000 Sbordone et al. 2011 Piotto et al. 2009 •Nitrogen variations alter the U-band magnitudes •Helium variations alter the Vand I-band ones Photometry is very efficient to study the phenomenon Bastian & Lardo 2018 Milone et al. 2017 Elena Pancino –Arcetri –October 2025
The current paradigm Observations: 1. Abundance variations in all phases 2. Even in stars that cannot produce them Interpretation: 1. That chemistry comes from outside 2. That chemistry must survive evolution •A large fraction of the star polluted •The stars form from polluted gas These are the two pillars of currently most explored explanations 1st DU RGB bump Stetson et al. 2019 Elena Pancino –Arcetri –October 2025
Non-canonical populations M67 (Credits: Emily Leiner)Star clusters favor dynamical interactions among stars •They contain a variety of non-canonical, or “exotic” stellar populations … … blue, red, yellow stragglers, sub-subgiants, li-rich stars, extended MS TO, split MS, hot subdwarfs, extreme HB stars, overmassive and undermassive stars … … then blue lurkers … and now we also have metal-rich RR Lyrae… It is a real jungle, some even call it a zoo! Elena Pancino –Arcetri –October 2025
Stars which gained mass (Credit: Barry Roal Carlsen) Blue stragglers •Brigther and bluer than TO •Collisions or mass transfer? •Triple systems? •Blue lurkers Overmassive stars •Found with Kepler •In open clusters •Large proportions (Brogaard et al 2021) 10% of the stars in NGC 6791 have undergone mass transfer (Brogaard+2021) Elena Pancino –Arcetri –October 2025
Stars which lost mass Leiner et al. (2017) ESO/L. Calçada, INAF-Padua/S. Zaggia Randall et al. (2016) Red stragglers & sub-subgiants •fainter + redder than SGB/RGB •rotate fast or loosing mass •some are binaries CV, LMXB •in open and globular clusters Hot subdwarfs and extreme HB stars •Core He-burning •Very little envelope left •In the field: all binaries •In GCs: no binaries, giant spots M67 NGC5139 Elena Pancino –Arcetri –October 2025
StarDance team and LOC The name on the badges of the LOC members is written in RED If you have doubts or questions on the workshop, please do feel free to ask any of us and we will do our best to help Pancino Turchi Rani Saracino Reggiani Leitinger Sanna Avdeeva Nedhath Steinbauer Echeveste Alvarez Elena Pancino –Arcetri –October 2025
