Thursday 16 October

The binary populations of young massive clusters

Sebastian Kamann (Liverpool John Moores University, UK) 

I will present results from recent studies into the the binary population of young massive star clusters in the Magellanic Cloud. Through a combination of MUSE and FLAMES observations, we aim to measure the binary fractions in our target clusters and characterise individual binaries, especially those with massive companions like black holes.

Constraining Binary Evolution Using Hot Subdwarf Stars

Alexey Bobrick (Monash University)

Hot subdwarf stars are nature's gift to astronomers. Because many are double‑lined spectroscopic binaries, their orbital periods, mass ratios, and eccentricities can be measured from radial velocities alone. Evolutionary models and observations further imply that they are recently formed products of binary interaction. For example, well‑characterised double‑lined systems have ascended the red giant branch, lost their envelopes through mass transfer, and ignited helium at the core helium flash. Therefore, by analysing the present‑day properties of such binaries, we can reconstruct in detail what happened during the preceding mass transfer phase. In this talk, I will review the key lessons learned so far: constraints on mass loss efficiencies, the impact of metallicity, signatures of Galactic chemical evolution, the predicted metal‑rich, binary‑made RR Lyrae population, and other products of binary evolution.

Case Studies in Binary Evolution from Open Clusters 

Emily Leiner (Illinois Institute of Technology, Chicago, USA) - Erin Motherway (University of Wisconsin-Madison, USA)

Approximately half of Sun-like stars are in binary or higher order systems, and half of these binaries will interact at some point in their evolution, evolving through mass transfer or a merger. The resulting post-interaction stars and binaries can be identified in several ways, including because they are outliers in color-magnitude diagrams, asteroseismic outliers, rotational outliers, they possess abundance anomalies, or they display excess ultraviolet emission. Once identified, these post-interaction populations reveal the outcomes of a variety of binary evolution pathways. In star cluster environments, key parameters including age, metallicity, and cluster turnoff mass provide important additional constraints on these populations, which enables uniquely detailed reconstructions of their binary evolution histories. Here we will discuss several interesting case studies of post-interaction binaries in star clusters that have evolved through at least one episode of mass transfer or a merger. We will discuss what these case studies reveal about binary evolution pathways, as well as the questions they pose for our understanding of binary evolution physics.

Challenges in RR Lyrae formation via binary evolution channels

Giuliano Iorio (ICCUB, Universitat de Barcelona, Spain) - Cecilia Mateu (UdelaR) - Valentina D'Orazi (INAF / INFN - Roma)

RR Lyrae (RRL) stars are pulsating variables at the intersection of the horizontal branch and the instability strip. Widely recognized as Population II tracers and thanks to their standard candle nature, they have been extensively used to probe the 3D structure of the oldest Milky Way populations across large volumes of the Galaxy.   Several recent independent results point to the existence of intermediate-age RRL, challenging the canonical view of these stars as tracers of exclusively old (>10 Gyr) and metal-poor populations (sub-solar metallicities), a notion based on our understanding of single stellar evolution models. On one hand, metal-rich RRL with metallicities up to solar values are known to exist all over the Galactic disc, with their fast-rotating and cold kinematics implying their association to intermediate-age populations ~2 to 8 Gyr old; while in the Magellanic Clouds’s field and clusters, at sub-solar metallicities, inference of the delay time distribution for RRLs also implies a significant fraction of these stars may be associated to intermediate-age populations in a similar age range. Binary evolution involving mass transfer is an obvious candidate to offer viable evolutionary channels for the production of RRL at these unusual ages and metallicities.  In this session, we aim to discuss possible formation channels of binary-made RRLs, with a focus on the crucial role of mass-transfer stability during the red giant branch phase. This assumption largely determines whether the system emerges as an RRL with a surviving companion or as a single star formed through a merger or common-envelope event. Stable mass transfer, as suggested by several recent studies, can strip the RGB envelope without entering a common envelope, leaving a main-sequence companion. If unstable, the outcome is typically a merger or the production of a compact remnant.  A central part of our discussion will be how to connect these theoretical predictions with observations: What chemical abundance patterns should we expect in binary-made RRLs? How can these be distinguished from other binary products at different evolutionary stages? What are the most promising strategies to identify such systems, given that only one confirmed RRL in a binary is known –a post-RGB object, partially unrelated to the intermediate-age population of interest–  while other candidates await spectroscopic confirmation? We will also address how observational biases may be influencing current detection statistics and what future surveys could reveal.

Disentangling stellar activity, pulsations, and keplerian motions

Deimer Antonio Alvarez Garay - Nicoletta Sanna - Sharmila Rani (INAF - Osservatorio Astrofisico di Arcetri)

Binarity has been proposed as one of the possible formation routes to explain different types of peculiar stars in our Galaxy. So far, various techniques, including observations and modelling, have been used to identify binaries among different kinds of stellar populations. Nevertheless, some stars exhibit unusual observational behaviour, caused either by binarity, stellar activity, or pulsations. Understanding the connections between stellar activity, pulsations, and binarity has become particularly important in the context of stellar evolution. In this talk, we are going to present results on some peculiar stars based on the multiwavelength photometric and optical spectroscopic studies.