Evolved stars, multiplicity and planets: Shaping and Being Shaped by Stellar Evolution

The gist: Understanding how planets and stellar companions survive and interact with their host stars as they evolve, or are ultimately destroyed in the process, is essential both for predicting the long-term future of planetary systems like our own and the contribution of stars to galactic chemical evolution.

Stellar Multiplicity and Interactions

One of the most consequential open questions in the field of stellar evolution is how the presence of companions affects the fates of stars This has also been prominently highlighted by observations of evolved systems since the advent of ALMA In fact, one of the important empirical results of the last decade is that the effects of stellar and/or planetary companions on the wind of evolved stars are ubiquitous, even for evolved stars not originally expected to be experiencing or to have experienced interactions with a companion These effects can be spectacular, through mergers and interactions that eject the whole stellar envelope (e.g. Ivanova et al. 2013) and produce complex morphologies and jets (e.g. Sahai et al. 2007; Tafoya et al. 2020), or less spectacular but very consequential through enhancing mass loss (Decin 2021; Aydi & Mohamed 2022) The progeny of systems that interacted spans a large number of classes and phenomena: sub-dwarf B stars, Barium stars, blue stragglers, ellipsoidal variables, novae, luminous red novae, systems with white dwarf (WD) companions This non-exhaustive list spans interactions occurring at every stage of stellar evolution.

Mass Transfer Regimes and Orbital Evolution

When stars evolve to become red (super)giants, their sizes increase dramatically, potentially leading to mass transfer, tidal interactions, and common envelope evolution that reshape the configuration of a binary or multiple system Although mass loss, tidal torques, and angular momentum redistribution are tightly coupled, we lack empirical characterization of how these processes drive orbital evolution beyond the main sequence One clear point of tension is the distributions of orbital distance and eccentricity observed in systems that experienced substantial mass transfer Effectively all samples of binary interaction products, such as Ba stars, post-AGB binaries, WD binaries, CEMP-s stars, and extrinsic S stars, do not match the predictions of current mass-transfer and tidal theory (Abate et al. 2015; Mathieu & Pols 2025; van Son et al. 2026) Possible solutions include better description of the mass-transfer processes that are more stable than currently considered (e.g. Woods & Ivanova 2011; Ge et al. 2020; Temmink et al. 2023), higher accretion efficiency (Mohamed & Podsiadlowski 2007), formation of circumbinary discs (Chen et al. 2020) and orbital shrinking from angular momentum loss (e.g. Jahanara et al. 2005; Saladino et al. 2019) ALMA2040 will completely change this landscape by allowing studies of the key processes in samples of objects in the evolutionary phase where the interaction is the strongest and the most consequential, when one of the components swells to giant dimensions.

Mass transfer from an evolved star to a companion can occur via three distinct regimes depending on how the Roche lobe radius compares to its stellar and dust condensation radii At the widest separations, the wind reaches terminal velocity before encountering the companion and accretion follows the Bondi-Hoyle-Lyttleton prescription (Hoyle & Lyttleton 1939; Bondi & Hoyle 1944), with efficiencies (accreted mass relative to mass lost) of order a few percent If the dust condensation radius is comparable to or larger than the Roche lobe radius, the wind is gravitationally focused by the companion, leading to what is known as Wind Roche Lobe Overflow (WRLOF, Mohamed & Podsiadlowski 2007, 2012) Simulations suggest WRLOF significantly increases the accretion efficiency from a few percent up to tens of percent of the mass lost (Mohamed & Podsiadlowski 2012; Abate et al. 2013) At still smaller separations, the photosphere itself fills the Roche lobe Whether such systems survive as persistent mass transfer binaries or enter common envelope depends on the poorly understood stability of mass transfer from convective giant donors.

Post-Interaction Systems and Transients

The end products of strong binary interactions – dusty (post-)AGB binaries (Fig. 1, left panel), merger remnants, and red transients – are among the most complex and least understood phases of stellar evolution Many post-interaction systems are known or suspected (e.g., Boffin & Jones 2019; Kaminski et al. 2021), with several classes of evolved stars now recognized or suggested to be outcomes of recent binary interaction, such as post-AGB stars and extreme AGB stars (e.g. Jones et al. 2015; Dell’Agli et al. 2021; Khouri et al. 2021, 2025; Van Winckel 2025), newly identified post-RGB objects (e.g., Kamath et al. 2015; Sahai et al. 2017; Olofsson et al. 2019; Moltzer et al. 2025), and red or blue stragglers (e.g. Patrick et al. 2020) Common-envelope and merger events produce luminous red novae transients, but these remain sparsely characterized (Karambelkar et al. 2025) Due to a large amount of circumstellar dust, many of these systems lack optical counterparts and can be studied only at infrared and (sub-)millimeter wavelengths (e.g. Kaminski et al. 2018; Steinmetz et al. 2025) The interaction processes that control the evolution of these systems remain poorly constrained.

A fraction of the systems that interact during the transition from the AGB (RGB) to the postAGB (post-RGB) phase form circumbinary disks (van Winckel 2003) These disks act as reservoirs of mass and angular momentum and continue to interact with the central system: re-accretion feeds discs and jets around the companion (Oomen et al. 2020; Mohorian et al. 2024; Van Winckel 2025), while the binary in turn reshapes the disk and drives disk winds (Verhamme et al. 2024) These disks have cavity edges, spirals, warps, and wind-launching regions on scales of a few to tens of au (angular sizes of a few mas for typical distance of ∼ 2 kpc) An exciting question is whether these disks are potential birth-places for second-generation giant planets Although gravitational fragmentation appears unlikely under present-day postAGB disk conditions (Pourmand et al. 2025), planets could form through core accretion Such planets could be detected in the future through compact continuum emission from circumplanetary material, or indirectly through gaps, dust traps, azimuthal asymmetries, and departures from Keplerian rotation — the same diagnostics that reveal planet–disk interaction in protoplanetary disks (Andrews et al. 2018; Zhang et al. 2018) Distinguishing binary-driven structures from companion-carved cavities requires resolving both dust morphology and gas kinematics on au scales (Corporaal et al. 2023) IRAS 08544−4431 is an immediate benchmark: at ∼ 1.6 kpc, current 10 mas observations probe ∼ 16 au and barely resolve its inner rim, modelled at 16.5 au in diameter (Mohorian et al. accepted; Kluska et al. 2018) ALMA2040 will also be crucial to identify observables which can differentiate between disk-planet interactions with potential planets from the first or second generation.

Planetary Engulfment and Chemical Enrichment

The most substantial advance offered by ALMA2040 will be the possibility of studying the effects of planetary engulfment The theoretical expectation for the evolution of planetary systems is that the large sizes of stars in the giant branches would cause planets in close orbits (within ∼ 2 au Villaver et al. 2014) to be engulfed by the star In fact, the dynamical effect of a potentially engulfed planet has been reported for the nearest AGB star, R Doradus at ∼ 50 pc (Vlemmings et al.

1. textbfProcessing Evolved Stellar System Dynamics for Planetary Fate Prediction: Capabilities include simulating how planets survive engulfment by studying the dynamical effect of a potentially engulfed planet, as suggested by studies of rotation in nearby evolved stars and the prediction that a large fraction of stars evolving through the giant branches is expected to engulf planets.

2. textbfCharacterizing Binary Interaction Physics: The system can now empirically constrain poorly understood processes like mass transfer by studying the distributions of orbital distance and eccentricity observed in systems that experienced substantial mass transfer, addressing the tension where no samples of binary interaction products, such as Ba stars, post-AGB binaries, WD binaries, CEMP-s stars, and extrinsic S stars do not match the predictions of current mass-transfer and tidal theory.

3. textbfDetecting Accretion Signatures in Evolved Systems: The AI can detect the most conspicuous signal of an accreting main-sequence companion: dust emission from its accretion disk by utilizing ALMA2040 data, which would allow for the detection of hundreds of systems and reveal the location and properties of companions that at present can only be inferred.

4. textbfMapping Circumstellar Disk Structure: The system can map complex structures in evolved star outflows, specifically by resolving the clear spiral pattern imprinted in the outflow from the carbon-rich AGB star AFGL 3068 by the presence of a companion on an eccentric orbit, which is crucial for understanding orbital evolution and disk structure.

5. textbfIdentifying Potential Second-Generation Planet Birthplaces: The AI can search for potential birth-places for second-generation giant planets by analyzing structures like circumbinary disks, distinguishing them from binary effects, and searching for diagnostics such as compact continuum emission from circumplanetary material, or indirectly through gaps, dust traps, azimuthal asymmetries.

6. textbfModeling Chemical Enrichment Impacts: The AI can connect binary interactions may affect mass-loss rates and suppress or delay key nucleosynthetic yields by analyzing molecular tracers like SiN, SiC, and NS in outflows to quantify how binary interactions affect mass-loss and nucleosynthetic yields.

Understanding how planets and stellar companions survive and interact with their host stars as they evolve, or are ultimately destroyed in the process, is essential both for predicting the long-term future of planetary systems like our own and the contribution of stars to galactic chemical evolution. In this white paper, we discuss how the next generation ALMA (ALMA2040) will revolutionize our understanding of the key processes by detecting and characterising accreting companions and planetary engulfment signatures in hundreds of systems with evolved stars.