Tuesday 14 October 

StarDance project and motivation for the workshop

Elena Pancino (INAF - Osservatorio Astrofisico di Arcetri, Italy) 

The ERC project "StarDance: the non-canonical evolution of stars in clusters" seeks to connect exotic stellar populations such as blue stragglers, red stragglers, hot subdwarfs, and the like, with the byproducts of binary and multiple stellar evolution. In particular, multiple stellar populations in globular clusters display chemical patterns that are compatible with hot hydrongen burning (CNO cycle and hotter, in some cases s-process elements). This chemistry could in principle be produced by binary interactions as well. This founding hypothesis requires a well-designed sample of binaries and exotic stars in star clusters and in the field, the ability to derive their accurate properties and chemistry, and reliable interpretation tools. This workshop gathers experts in all the aspects relevant to the topic, to identify open questions and possible ways to answer them, including the possibility of creating new collaborations between different teams of experts. 

Binaries in globular clusters 

Stefan Dreizler - Fabian Göttgens - Sven Martens (University of Göttingen, Germany) 

Binaries in globular clusters are relevant for a range of astrophysical topics. The high stellar density leads to the formation of peculiar objects like blue stragglers, cataclysmic variables, millisecond pulsars, or binaries composed of white dwarfs, neutron stars and black holes. The latter are sources of gravitational waves detectable in current gravitational wave detectors when merging or in future detectors like LISA over a wide range of orbital periods. Binaries also serve as energy source. The evolution of the cluster dynamics therefore depends on the binary properties.   Probing the binary population in the central regions of globular clusters is intrinsically challenging due to the high stellar density requiring integral field spectrographs like MUSE. We have collected multiple epochs of several globular clusters using this instrument. We will summarize the results of this survey, present comparisons of binary properties with those from cluster simulations, and discuss potential problems resulting from crowding. We also investigated the possibility of detecting triples. 

A speckle view of binary populations 

Maria Jose Rain (ESO, Chile) - Ricardo Salinas (Nicolaus Copernicus Astronomical Center, Poland)

Reaching an image quality close to 20mas, speckle interferometry at 8m class telescopes provides a look into a parameter space not accesible through any other method, be it spectroscopy, astrometry, NIR interferometry or direct imaging; being particularly suited for the study of wide binaries.  In this presentation we will show results from ongoing speckle surveys in a variety of stellar populations, including a search for blue stragglers in triple systems, a search for the elusive companions of RR Lyrae stars; and the formation of hot sub-dwarfs.

The Rotation of Blue Straggler Stars and Friends 

Robert Mathieu (University of Wisconsin-Madison, USA) - Onno Pols (Radboud University, The Netherlands)

Rapid rotation is an observed hallmark of binary evolution products, and an expected outcome of all proposed formation channels for blue stragglers—mass transfer, mergers, and collisions.   The evolution of spin angular momentum during binary interactions has only nascent constraints from observations. More comprehensive stellar rotation distributions for binary evolution products are vital, especially when combined with observed masses, orbital parameters and young ages.  Understanding angular momentum transfer and evolution remains crucial for progress. Theoretical predictions of spin angular momentum gains during mass transfer, and indeed beyond breakup for the observed mass gains, must be resolved. Excess angular momentum must be removed to permit observed mass gains by processes yet to be understood. Very likely such resolution will require deeper understanding of the role of accretion disks and (perhaps coupled) magnetic fields.

Binaries, blue stragglers, stripped stars, their companions: characteristics and evolutionary pathways 

Annapurni Subramaniam (Indian Insitute of Astrophysics, Bangalore) - Vikrant Vinayak Jadhav (HISKP & AIfA, University of Bonn, DE)

Most stars belong to binary systems, characterized by a present-day binary population (PDBP), which includes properties such as period, mass ratio, eccentricity distributions, and the multiplicity fraction. Observational incompleteness and limited statistics pose challenges, yet cluster PDBP provides valuable constraints on the initial conditions. In the first part, we will discuss the binary population of star clusters and their tidal tails. Both clusters and tails have binary fractions of 0.3–0.6, though predicted differences in period and mass distributions are likely obscured by stochasticity and small samples. The second part will focus on the binary products in open clusters. Binary evolution can lead to interactions and the formation of blue straggler stars, which offer crucial insight into binary interactions. Ultraviolet imaging with UVIT/AstroSat revealed diverse hot companions to blue stragglers, including hot subdwarfs and extremely low mass to high mass white dwarfs, pointing to varied mass-transfer histories and evolutionary status in cluster environments. In a related investigation, we studied the population of Be stars in the young open cluster NGC 663, identifying new candidates and characterising their UV and optical properties. These rapidly rotating, mass-losing stars may represent an accretor in a binary, with the companion resulting in a striping star. Together, these studies provide a unified view of blue stragglers, stripped stars, and their companions, offering new constraints on binary evolution in open clusters. Our results emphasise the unique capability of UV observations to unravel complex stellar histories and reveal otherwise hidden binary products.