Exotic populations are families of peculiar stars that ubiquitously inhabit star clusters. They occupy unexpected regions of the color-magnitude diagram of their host system, and their existence challenges canonical stellar evolution across all stages, from the first moments of a star's life to its spectacular death. From blue straggler stars, blue lurkers, and extended main-sequence turn-offs in the main-sequence, up to sub-sub-giants, red stragglers, and extreme horizontal branch stars in the more evolved stages, these stars show significant rotational and chemical peculiarities. For some of these classes of objects, a binary origin is widely recognized; for others, it is still a hypothesis that has yet to be verified.

Our understanding of the role of binaries and binary interactions in shaping the evolution and morphology of star clusters has received a boost in recent years, thanks to dedicated observational studies. However, it is still unclear how close the connection is between binary systems and the appearance of exotic populations in clusters, so further investigations in this direction are needed, especially regarding the low-mass end of binary systems.
In particular, both a theoretical and observational approach are necessary: on one hand, next-generation stellar evolution and population synthesis codes that can test different prescriptions to describe mass transfer in all its forms, as well as provide a more robust treatment of Roche Lobe overflow, common envelope phase, etc..; on the other, large observational samples of well-characterized binary stars with high-quality data, providing crucial information on their orbital and chemical properties. The solution to the open question raised by exotic populations can only be found through collaboration and the conjunction of these two fields.

The EAS Special Session format is particularly well suited for this purpose, as it can bring together a large and diverse community of researchers with complementary expertise. Our goal is to feature talks on representative objects from different areas or population studies, promote a shared vision, spark innovative discussions, and encourage the community to collaborate on these emerging ideas.

Programme

The Special Session will be divided into three main themes, discussed in 3 blocks of 1.5 hr, designed to facilitate the discussion between theorists and observers on each topic:

• Over-massive stars
All those objects (blue stragglers and blue lurkers, O/Be stars, and compact binaries) for which mass accretion is the most favored explanation for the origin of their peculiarities;
• Under-massive stars
The products of consistent and ubiquitous mass loss (extreme horizontal branch stars, hot sub-dwarfs);
• Peculiar objects
Stars whose chemical (Li-rich, N-rich, B-depleted) or morphological (red straggler and sub-sub giants) properties could be explained by the interplay between binary interactions and rotation.

Invited speakers

TBA

Scientific organisers

• Sara Saracino - Chair, INAF Arcetri, Italy
  
• Elisabetta Reggiani - Chair, University of Florence, Italy
  
• Maite Echeveste - INAF Arcetri, Italy
  
• Giuliano Iorio - Institut de Ciències del Cosmos: Barcelona, Spain
  
• Sebastian Kamann - Liverpool John Moore University, UK
  
• Andrew Nine - University of Wisconsin-Madison, USA
  

Contact

• elisabetta.reggiani at unifi.it
  
• sara.saracino at inaf.it