Wednesday 15 October 

Modelling the evolution of low-mass, close binary systems 

Omar Gustavo Benvenuto - María Alejandra De Vito (UNLP / Instituto de Astrofísica de La Plata - CONICET, Argentina) 

We describe the main features of the evolution of low-mass, close binary systems. We detail the main unavoidable approximations we need to consider to keep the problem tractable in 1-D simulations. We detail our main tool for modelling such systems, enumerating its virtues and limitations. Briefly, our code calculates the mass transfer rate in a self-consistent way; it may consider different magnetic braking prescriptions; irradiation feedback, and evaporation are included. We are in the process of incorporating rotation in our code. Still, common envelope stages and accretion disks are to be included.  Finally, we enumerate some of the relevant astrophysical systems to which it was already applied: helium white dwarfs; low-mass, X-ray binaries;  blue stragglers; and type Ib supernovae. 

Impact of binary evolution and dynamics on the formation of massive black holes

Michela Mapelli - Stefano Torniamenti (MPIA, Heidelberg, Germany)

The LIGO–Virgo–KAGRA observing runs provide crucial insights into the populations of binary compact objects, helping us to address key open questions: What is their mass function? What are the formation channels of merging black holes (BHs)?

In this talk, we discuss several possible scenarios. We explore the uncertainties in massive binary evolution, stellar winds and pair instability that still limit our understanding. Also, we investigate how dynamical processes in dense star clusters can trigger the formation of BHs in the upper mass gap and intermediate-mass BHs via hierarchical mergers and stellar collisions. 

Initial binary properties and dynamical processing in star clusters

Abbas Askar - Grzegorz Wiktorowicz (Nicolaus Copernicus Astronomical Center, PAS, Poland)

Star clusters are natural laboratories for studying binaries, yet a fundamental open question remains: what is the initial binary fraction and distribution of binary parameters such as period, mass ratio, and eccentricity? Cluster dynamics can reshape these initial properties, altering the populations of observable binaries, including exotic systems undergoing mass transfer such as X-ray binaries and, in particular, ultraluminous X-ray sources (ULXs). Observational studies of the number of mass-transferring binaries, overall binary fractions, radial trends, and detailed parameter distributions can help constrain the primordial population, while theoretical work highlights the competing role of uncertain binary evolution processes such as common-envelope evolution. In this contribution we will frame the problem of disentangling initial conditions, dynamical evolution, and binary physics, using ULXs as a key example where both channels may leave distinct signatures. We aim to open a discussion on how best to combine observations and numerical simulations to constrain the properties of binaries in star clusters and better understand their initial conditions.

Solar-mass stars in binaries before and after interaction

Paul Beck - Ana Escorza (Instituto de Astrofisica de Canarias, Spain)

In this talk, we explore the complementarity of asteroseismology, spectroscopy, binary dynamics, and binary population synthesis in the study of low-mass stars in binary systems – both before and after interaction.   On one hand, solar-like oscillations provide unique diagnostics of stellar interiors and evolutionary states. By combining large-scale asteroseismic surveys from Kepler, K2, and TESS with binary catalogs such as Gaia DR3, we can characterize the binary population across evolutionary phases, from the main sequence to the red giant branch and beyond. When integrated with spectroscopic and dynamical information, asteroseismology reveals how tidal interactions and mass transfer affect both stellar structure and orbital evolution. These observational diagnostics are essential for identifying systems undergoing or having undergone interaction. By comparing these data with predictions from binary population synthesis, we uncover the role of star-star interactions in shaping stellar activity, angular momentum evolution, and mass loss. This multi-faceted approach also points to the occurrence of stellar cannibalism during the giant phases and helps us trace the life of stars beyond the interaction phase.  On the other hand, spectroscopic analysis of low-mass stars in binary systems can reveal interactions that occurred in the past between the observed star and a now extinguished companion. This leads in particular to very specific chemical signatures that can then be interpreted in terms of nucleosynthetic processes. Therefore, such binary systems offer a unique window on a very wide range of objects that can otherwise be almost unobservable.  This contribution aims to highlight observational and theoretical challenges that will be crucial for advancing our understanding of low-mass stars in binary systems.

Asteroseismic probes of binary interaction

Nicholas Rui (Princeton University, USA) - Joel Ong (University of Hawaii, USA / University of Sydney, Australia)

Asteroseismology is our most direct probe of stellar interiors, and is uniquely sensitive to signatures of past or ongoing binary and tidal interactions which might otherwise be invisible at the stellar surface. We review seismic signatures of binary interaction, particularly those which exotically modify the stellar structure, rotational configuration, magnetic field, chemical composition, and tidal potential, and describe how each signature informs our knowledge of the binary interaction history. We sketch upcoming prospects for both single-system and population-level characterization.

Seismic Binaries as Laboratories for Stellar Physics

Andrea Miglio - Alessandro Mazzi - Jeppe Sinkbæk Thomsen - Walter van Rossem (Bologna University, Italy)

This presentation, delivered jointly by several members of our Bologna group, will highlight how binaries hosting solar-like oscillators provide stringent tests of stellar physics. By combining dynamical and seismic constraints, we obtain percent-level radii and masses, enabling robust calibration of seismic distance and age scales. Binaries also allow us to identify and interpret products of interaction, and to reconstruct internal structures from asteroseismic data. A key element of our programme is the development of flexible, parameterised models of stellar structure to systematically search for and characterise stars that have undergone non-standard evolution. We further integrate these analyses with population-synthesis predictions and with the study of asteroseismic binaries, i.e. unresolved binaries where both components show oscillations, which provide direct constraints on the properties of their parent populations. The talk will conclude by setting the stage for the discussion we will moderate, where we will address current limitations and the prospects for future space-based seismic studies, including HAYDN.