CE31 - Physique Subatomique, Sciences de l'Univers, Structure et Histoire de la Terre

Better understanding the evolution of angular momentum in red giants – BEAMING

BEAMING: Understanding the transport of angular momentum in red giant stars

Measurements of internal rotation in stars at various evolutionary stages have shown that angular momentum is redistributed much more efficicently than predicted by current models. The BEAMING project explores the hypothesis that internal magnetic fields are causing this additional transport of angular momentum in red giant stars.

General objectives

Understanding the effects of rotation on stars is one of the key open questions in modern stellar physics. Rotation induces a mixing of chemical elements and thus modifies the evolution of stars. Despite their importance, the internal rotation of stars and the processes that transport angular momentum remain poorly known. Asteroseismology, which consists in the study of waves that propagate in stellar interiors, is currently the only tool that can probe the internal rotation of stars. Subgiants and red giant stars show oscillation modes that have a « mixed » behavior. They probe both the envelope (where they behave as pressure modes) and the core (where they act as gravity modes). The internal rotation of subgiants and red giants, as measured with asteroseismology, has shown the need to invoke an additional mechanism that efficiently transports angular momentum in these stars. The goal of the BEAMING project is to test whether internal magnetic fields (which are one of the most serious candidates) are responsible for this additional transport. For this purpose, we tackle the following questions :<br />- How does the internal rotation of stars evolve during the subgiant and red-giant phases ? What is the characteristic timescale over which angular momentum is transported in these stars ?<br />- Can we detect signatures of the presence of internal magnetic fields in red giants ? If yes, what is their impact on the internal rotation ?<br />- From a theoretical point of view, how do magnetic fields interact with (differential) rotation ? If magnetic instabilities arise, how efficient are they at transporting angular momentum ?

To tackle these questions, several steps are necessary. First, we use seismology to monitor the internal rotation of subgiant and red giant stars along their evolution, that is, during the phase of « young » subgiant, as stars ascend the red giant branch, and during the helium-core burning phase. Using mixed modes, we compare the average rotation of the core to that of the envelope. We can thus estimate the efficiency of the transport of angular momentum in all these phases. We improve seismic inversion techniques in order to more systematically measure envelope rotation rates and to probe the rotation profiles in the very core.

We also search for observational signatures of internal magnetic fields in red giants. For this purpose, we use spectropolarimetry to detect strong surface fields that would indicate the presence of strong core fields. We also search for internal magnetic fields through their influence on the oscillation frequencies of mixed modes. Measuring the internal rotation of magnetic red giants can shed light on the impact of magnetic fields on rotation profiles.

Finally, we use numerical simulations to model the interactions between magentic fields and rotation in stellar interiors. We investigate the conditions required for the development of magnetohydrodynamic instabilities. When such instabilities arise, we estimate their efficiency at transporting angular momentum. The ultimate goal is to establish prescriptions for this transport, which can then be implemented in current stellar evolution codes.

Search for internal magnetic fields:

- Li et al. (2022), published in Nature : We obtained the first detection of magnetic fields in the interior of a star. Such a detection had been long-awaited. We could thus measure the strength of the magnetic field in the core of 3 red giants and we placed constraints on their topology. This opens a number of perspectives to understand the origin and evolution of magnetic fields in stars, as well as their role in angular momentum transport.
- Deheuvels et al. (2023) : We detected magnetic fields (stronger than those detected in Li et al. 2022) in the cores of 11 red giants observed with Kepler using a different methodology.
- Li et al. (2023) : We applied the same methodology as in Li et al. (2022) to systematically search for red giants with core magnetic fields in Kepler data. We identified 10 additional magnetic red giants and we could explore the variations of the field strength with the evolution.

Monitoring internal rotation of red giants:

- Eggenberger et al. (2019) : We have shown that the efficiency of angular moment transport decreases with evolution along the subgiant branch.
- Deheuvels et al. (2020) : Seismic measurement of the internal rotation of two « young » subgiants observed with the Kepler satellite. We showed that these two stars rotate nearly rigidly despite the severe core contraction during this phase. Our results suggest that the process that transport angular momentum during the main sequence might persist at the beginning of the subgiant phase.

Modeling the interaction between magnetic fields and rotation:

- Gouhier, Lignières & Jouve (2021) : 2D hydrodynamical simulations (MagIC code) where stationary states were obtained. A main result is that rotation profile may present gradients in latitude as well as in radius, contrary to what is often assumed in 1D stellar evolution models.
- Gouhier, Jouve & Lignières (2022) : By studying magnetohydrodynamical stationary states in 2D simulations (MagIC), we found that the differential rotation profile is modified by the large-scale field and can become unstable with respect to a magneto-rotational instability (MRI). We proposed a scenario potentially accounting for the rotation in young red giants.
- Meduri, Jouve & Lignières (2023) : We obtained MHD instabilities (either MRI or Tayler-like) in 3D simulations. This is the first time dynamo action in radiative zones relying on a MRI is obtained for parameters realistic for red giant stars. A scaling of the efficiency of angular momentum transport with respect to the stable stratification was obtained, opening the possibility to implement this prescription in 1D stellar evolution models.

Search for internal magnetic fields:

- We are currently leading a theoretical study to determine the influence of non-axisymmetric magnetic fields on the frequencies mixed modes. This will allow us to search for other magnetic red giants and to eventually assess the prevalence of such stars.

Monitoring internal rotation of red giants:

- Li et al. (to be submitted soon) : We obtained measurements of envelope rotation for ~ 300 red giants (such measurements so far existed for only a dozen of stars). This allows us to study how the core-envelope rotation contrast evolves in time along the red giant branch.
- Mosser et al. (to be submitted soon) : We studied through seismology the evolution of the core rotation in helium-core burning giants. We showed that in these stars, the core rotation seems locked to the envelope rotation (which demonstrates an efficient transport of angular momentum) while preserving a moderate, non-vanishing, level of differential rotation.

Modeling the interaction between magnetic fields and rotation:

- Introduction in 1D stellar evolution models (work with A. Palacios) of prescriptions coming from the 3D simulations of Meduri et al. (2023).
- The regime of Tayler instability in the setup of Meduri et al. (2023) is now investigated in more details. A new prescription for the angular momentum transport in such a regime will be obtained.

- Li, Deheuvels, Ballot, Lignières (2022) Nature 610, 43 : Magnetic fields of 30 to 100 kG in the cores of red giant stars
- Deheuvels, Li, Ballot, Lignières (2023) A&A 670L, 16 : Strong magnetic fields detected in the cores of 11 red giant stars using gravity-mode period spacings
- Li, Deheuvels, Li, Ballot, Lignières (2023) accepted in A&A : Internal magnetic fields in 13 red giants detected by
asteroseismology
- Deheuvels, Ballot, Eggenberger et al. (2020) A&A 641A, 117 : Seismic evidence for near solid-body rotation in two Kepler subgiants and implications for angular momentum transport
- Eggenberger, Deheuvels, Miglio et al. (2019) A&A 621A, 66 : Asteroseismology of evolved stars to constrain the internal transport of angular momentum. I. Efficiency of transport during the subgiant phase
- den Hartogh, Eggenberger, Deheuvels (2020) A&A 634L, 16: Asteroseismology of evolved stars to constrain the internal transport of angular momentum. III. Using the rotation rates of intermediate-mass stars to test the Fuller-formalism
- Deheuvels, Ballot, Gehan (2022) A&A 659A, 106 : Seismic signature of electron degeneracy in the core of red giants: Hints for mass transfer between close red-giant companions
- Li, Deheuvels, Ballot to be submitted to A&A : Measurements of internal rotation in ~ 2500 red-giant-branch stars
- Mosser, Dréau, Pinçon, Deheuvels et al. to be submitted to A&A : Frozen differential rotation in core-helium burning red giants
- Gouhier, Lignières & Jouve (2021), A&A, 648, 109: Axisymmetric investigation of differential rotation in contracting stellar radiative zones
- Gouhier, Jouve & Lignières 2022, A&A, 661, 119 :Angular momentum transport in a contracting stellar radiative zone embedded in a large-scale magnetic field
- Meduri, Jouve & Lignières 2023, submitted to A&A

This project tackles the problem of the transport of angular momentum (AM) in stellar interiors, which is one of the most challenging issues faced today by stellar physics. Our poor understanding of the AM redistribution inside stars is a barrier to the modeling of stellar formation and evolution. The seismology of red giants has recently demonstrated that it can spectacularly contribute to making progress on this issue. The detection in the oscillation spectra of red giants of so-called mixed modes, which probe both the stellar core and the envelope, has made it possible to seismically measure their internal rotation. This has brought clear evidence that an additional mechanism of AM transport takes place in subgiants and red giants, the origin of which is unknown. In this project, we will thoroughly investigate whether this AM redistribution has a magnetic origin, which is one of the main scenarios that have been proposed. For this purpose, we will combine asteroseismology, spectropolarimetric observations, and state-of-the-art multidimensional MHD simulations.

We will exploit the exquisite seismic data from the Kepler satellite to bring tight constraints on the time during the evolution and the timescale over which the episodes of AM redistribution occur. The cases of low-mass and intermediate-mass stars, which have qualitatively different evolution in the post-main-sequence, will both be addressed. By improving seismic inversion methods, we will also obtain constraints as localized as possible on the shape of the internal rotation profiles that result from this transport. These observations will yield key information to discriminate between the potential transport mechanisms.

To test more particularly the hypothesis of a magnetically-induced transport of AM, we will seismically measure the rotation profiles of Kepler red giants for which an internal magnetic field can be detected and characterized (strength and topology). Using spectropolarimetric observations, we will identify among Kepler targets the descendants of so-called Ap stars, which harbor strong internal magnetic fields during the main sequence. We will also search for the seismic signature of internal magnetic fields in red giants, by attempting to measure the magnetic splitting of oscillation mode.

Through numerical simulations, we will investigate the interaction between differential rotation and magnetic fields in red giants. We will first study the differential rotation produced by the core contraction and envelope expansion alone. By introducing magnetic fields, we will then determine the conditions under which different types of MHD instabilities could occur in the radiative interior of red giants and we will precisely estimate their efficiency at transporting AM. This will enable us to provide prescriptions for magnetically-induced AM transport with strong physical basis to be used in a new generation of 1D stellar evolution models. Direct comparisons will be performed between the predictions of these models and seismic inferences on the rotation of red giants. The Kepler targets for which we will have measured the topology and amplitude of the internal magnetic field will yield the most critical tests of the magnetic origin of AM transport in red giants.

Project coordinator

Monsieur Sébastien Deheuvels (Institut de recherche en astrophysique et planétologie)

The author of this summary is the project coordinator, who is responsible for the content of this summary. The ANR declines any responsibility as for its contents.

Partner

IRAP Institut de recherche en astrophysique et planétologie

Help of the ANR 320,727 euros
Beginning and duration of the scientific project: March 2019 - 48 Months

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