Modelling angular momentum transport by mixed modes across stellar evolution

Beatriz Bordadágua (HITS)

Current stellar models that include hydrodynamic processes for the transport of angular momentum (AM) predict core rotation rates in low-mass red giant stars that are orders of magnitude higher than those inferred from asteroseismic observations.
This discrepancy motivates the search for additional transport processes capable of efficiently redistributing AM within radiative interiors. In addition to being a crucial observational probe of the stellar internal structure, mixed gravity-pressure modes may also provide an efficient mechanism for extracting AM from red-giant cores via wave-driven heat transport.

In this seminar, I will discuss the role of mixed modes in the AM transport of low-mass red giants. First, I will focus on modelling mixed-mode amplitudes throughout stellar evolution, accounting for radiative damping across different angular degrees. Next, I will present the impact of mixed modes on rotation rates along the red giant branch and in the red clump. These results point to aspects that remain to be understood within this theoretical framework. Finally, I will show the resulting rotation profiles obtained by combining the mixed-mode mechanism with other transport processes, such as meridional circulation and magnetic fields.

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What do nonlinear terms reveal about δ Scuti stars oscillations?

Miriam Rodriguez Sanchez, Valencia

δ Scuti stars have a complex power spectrum in which numerous radial and nonradial modes are excited. Despite this, nonlinear contributions to the oscillations have generally not been taken into account and most studies have remained within the linear regime due to the complexity of the problem. In this seminar, I will present the challenges and results of studying the nonlinear contributions. This work is based on analyzing the fundamental hydrodynamic equations expanded to second order in perturbations in terms of the displacement vector. The analysis considers no rotation, no magnetic fields, no viscosity, adiabaticity, and no turbulent convection. I evaluate the relative importance of the nonlinear terms by focusing on the simplest case of radial modes and the maximal temporal contribution. I computed these terms using stellar models together with the eigenfunctions of the linear adiabatic oscillation equations. The results show that some nonlinear terms can locally reach magnitudes comparable to the linear contributions of the continuity, momentum, and energy equations. This indicates that nonlinear terms have a non-negligible role in the internal dynamics of δ Scuti oscillations.

Behind the complex red-giant oscillation power spectra

Jeong Yun Choi, HITS

Solar-like oscillations in red giants show characteristic patterns of pressure modes and mixed modes that allow us to probe stellar interiors. However, some red giants exhibit power density spectra (PDS) with irregular mode patterns and unusually low oscillation power. Various physical processes can produce a wide range of complex PDS morphologies, including magnetic fields, structural discontinuities, rapid rotation, and strong coupling of mixed modes. In this talk, I present a new perspective in which some of these complex PDS originate from seismically unresolved oscillation signals, i.e., red-giant asteroseismic binaries (ABs).

First, I present the predicted PDS morphologies of 5,000 artificial ABs to illustrate how seismically unresolved red-giant ABs are expected to appear in the observations. Next, I compare these predictions with six seismically unresolved AB candidates identified in Kepler data. With these six AB candidates, I will show how unresolved oscillations can bias stellar masses and radii by factors of up to ~ 3 and ~ 2, and make unreliable core properties such as period spacings and coupling factors. Finally, I will present a systematic search for red giants with complex PDS and investigate the origins of the complexity, including AB candidates and other physical processes.

Asteroseismic grid-modeling of red giants: precise age constraints for galactic-archeology

Desmond Grossmann, IAC & ULL

Precise stellar ages are crucial for understanding the Galaxy’s dynamical and chemical evolution. Stellar evolution and asteroseismic modeling, combined with precise spectroscopic and asteroseismic observables, can significantly reduce age uncertainties. This is particularly evident when adding additional constraints, such as binarity or individual oscillation frequencies, to the modeling. In this talk, I will discuss how different asteroseismic modeling approaches constrain red giant ages, contribute to the reduction of the uncertainties, and affect systematic offsets between methods.

The project builds on a study of a benchmark red-giant binary, in which combining grid-based modeling with binarity, spectroscopy, and asteroseismology yielded an age precision of 9%, a significant improvement over classical single-star global seismic modeling for red-giant stars. Extending this methodology, I developed an individual-frequency grid-modeling pipeline for red giant stars. Applied to approximately 100 Kepler red giants, close to solar mass and metallicity, this approach achieves mean-age uncertainties of 13% across a significant range of galactic ages, representing a factor-of-two improvement over global seismic modeling. Furthermore, the stellar ages derived from individual-frequency modeling are consistently larger than those derived from global seismic modeling. Given fixed input physics in the models, this indicates a systematic offset between methods even in the solar-parameter regime.

This modeling framework is now being extended to Galactic halo stars, targets poor in metals and enriched in alpha elements that are crucial for understanding the Milky Way’s evolution. Determining precise parameters for these populations will place them into a broader context of Galactic formation and chemical history.

Visibilities of damped mixed modes in red giant stars

Jonas Müller, HITS

The visibility of oscillation modes can be used to draw conclusions about the damping rate in the stellar core. Recent observational studies demonstrated that although the damping rate of the oscillations in the core of many red giants appears to be negligible, other red giants exhibit high core damping rates that are sometimes consistent with the infinite value limit. However, some of these red giants with strongly damped oscillations show a clear mixed mode signature, suggesting that their damping rate in the core must remain finite.

In this talk, I will present an analytical formalism that can be used to express the approximate power spectrum of red giants up to a proportionality factor, while taking into account the influence of damping on the oscillations. This formalism can be used to obtain a quantitative estimate for the visibility of mixed modes and to predict the detectability of the mixed mode signature under different core damping rates.

Then, I will compare the visibilities calculated from theory with observational methods and discuss the effects of potential biases along the red-giant branch. Finally, I will show the evolution of the visibility and detectability of the mixed mode signature while testing different prescriptions for the potential energy loss caused by a strong internal magnetic field in the core of red giant stars.

Toward a unified picture of solar and stellar activity

TOKUNO Takato, University of Tokyo

Photometric surveys from space telescopes now provide light curves for large samples of solar-type stars, enabling systematic searches for starspots and stellar flares. Comparative studies with the Sun are beginning to clarify the similarity in underlying mechanisms and the contrast in characteristic magnitudes. In this talk I summarize recent progress, including my own results, toward a unified picture of solar and stellar activity. The first part focuses on the mechanisms. Similarities revealed by comparative studies of spot and flare properties provide key constraints on the underlying physics. After reviewing the commonalities discussed in previous work, I present my ongoing study on the universality of the spot–flare connection. We tested whether the same spot-to-flare pathway operates from the Sun to solar-type stars by quantifying flare timing relative to spot evolution in a scale-independent manner. The second part focuses on the magnitude. The fact that even slowly rotating solar-type stars can exhibit much larger activity than the modern Sun motivates the expectation that similarly extreme events may have occurred in the Sun’s past. Such rare extremes are being explored using “non-telescopic” solar proxies. I review representative examples of extreme solar-flare events inferred from these records, and introduce my recent work on an extreme event recorded by solar-origin particles implanted in extraterrestrial material returned by sample-return missions.

Stellar Astrophysics and Galactic Archaeology with Asteroseismology

Christopher Lindsay, Yale University

The analysis of stellar oscillations has enabled detailed asteroseismic studies of the internal structures of stars, particularly subgiants and red giants. Because many areas of astrophysics depend on accurate stellar modeling, asteroseismology provides a vital tool for improving our understanding of how astrophysical systems evolve over time. In this talk, I will describe my research using data from the Kepler and TESS missions to model stars in detail using their individual oscillation frequencies, thereby probing their interiors. I will discuss how detailed asteroseismic modeling enables precise age determinations for notable stars. As an example, I will present the asteroseismic modeling of a sample of evolved metal-poor, α-enhanced stars in the Milky Way’s stellar halo, comparing a full treatment of α-enhancement against ad hoc metallicity corrections. We find that both approaches yield consistent stellar parameters with comparable uncertainties when individual oscillation mode frequencies are used for stellar modeling, in turn also indicating a breakdown of widely used asteroseismic scaling relations in this regime. I will also show how asteroseismic measurements can inform the treatment of convective boundaries in stellar models, particularly for red giants and stars with convective cores.

Non-adiabatic study of γ Doradus stars, application to low metallicity stars and binary interactions

Loïc Fellay, University of Liège

γ Doradus stars, which span spectral types A7 to F5, exhibit g-mode instabilities driven by convective flux blocking at the base of their convective envelopes. Accurately modelling their instability strips requires non-adiabatic stellar oscillation codes that incorporate a time-dependent treatment of convection–oscillation interactions, such as MAD.

In this seminar, I will present our current understanding of their instability strips and the excitation of oscillation modes by convective flux blocking, including the effects of rotation and metallicity. I will then discuss two main applications of this knowledge in the context of non-adiabatic asteroseismology.

The first application focuses on low-metallicity γ-Doradus stars observed by Kepler in the open field. For these stars, we find significant discrepancies between the modelled and observed instability strips. I will explore possible explanations for these differences and discuss what physical constraints we can infer for such stars.

The second application concerns the interactions within binary systems through tidally excited oscillations, which drive exchanges of energy and angular momentum between the orbit and the stellar components. In this context, non-adiabatic oscillation modelling is essential. I will show how the rotational and orbital evolution of binary systems hosting γ Doradus stars is influenced by the growth and damping of tidally excited oscillations across the entire γ-Doradus main sequence.

Why is it so hard to model red clump stars?

Lea Schimak (University of Sydney, Australia)

Asteroseismology has long been proven to be a powerful tool for probing the interior of stars and testing stellar models. For red clump (RC) stars, which experienced a He-flash in their past, these tests show limited success compared to main-sequence or red giant branch (RGB) stars of a similar mass range. The models are confronted with various discrepancies and numerical issues. For example, RC stars in clusters often provide inconsistent ages compared to their RGB counterparts; the observed period spacing suggests enhanced core overshooting; and at the convective core boundary, the models often predict a split convection zone due to the sharp changes in the composition gradient.

In this talk, I will present how standard models fail, what has been done to improve them and discuss different asteroseismic constraints to further investigate the discrepancies. I will explore standard models using the asteroseismic binary system KIC 1081730, consisting of an RC and RGB star and measure the frequency-dependent coupling strength of mixed modes as a surface-independent parameter to probe the inside of red giants and discuss how this parameter is an excellent probe, especially for the structure of RC stars.

CAPASS: Towards a Comprehensive Asteroseismic Catalogue of Kepler Red-giant stars.

Francisca Espinoza Rojas (Heidelberg Institute for Theoretical Studies, Germany)

Precise asteroseismic parameters derived from reliably identified oscillation modes are essential
for constraining stellar structure, testing stellar evolution theory, and deriving accurate stellar
parameters. In this seminar, I present the ongoing development of CAPASS (the Catalogue of
Asteroseismic Parameters for Stellar Structure), a catalogue of ~25,000 Kepler red giants that
provides global asteroseismic parameters (e.g., νmax and Δν), properties of individual modes
(frequencies, amplitudes, linewiths) and mode identification. Its construction required the
development of robust, fully automated unsupervised methods.

The inherent complexity of red-giant oscillation spectra presents a significant challenge for
automation and complicates reliable mode identification. In this context, I focus on two aspects
that limit large-scale analyses. First, I address power density spectra exhibiting multiple
oscillation power excesses and present strategies to characterise these systems. Second, I
introduce a novel method for identifying dipole (ℓ = 1) mixed modes based on spectral template
similarity. This robust approach can handle missing modes or spurious peaks and avoids strong
assumptions about parametric distributions.

These developments are intended to be integrated into the fully automated, data-driven pipeline,
TACO (Tools for the Automated Characterisation of Oscillations), and, together with it, form the
methodological spine of CAPASS. This framework will enable the reliable extraction of
asteroseismic information at unprecedented scale, opening the way to detailed population-level
studies of red-giant interiors.