Observational Insights into Post-Main-Sequence Rotation and Magnetism: 20 Years of Science, from the Subgiant to the White Dwarf Phase

This paper provides a brief overview of recent observational results and their implications for our understanding of stars, focusing on advances in asteroseismology and high-precision spectropolarimetry, and covering stars with low to intermediate masses and their evolution from the subgiant to the white dwarf phase.

The paper discusses the following key findings:

• Subgiants: Deheuvels et al. (2014) used Kepler data to recover the first measurements of both core rotation and envelope rotation rates in six subgiants. They found that in all six stars the cores are rotating more rapidly than the envelopes, and that core rotation rates increase as we move toward the more evolved stars while the envelope rotation rates decrease—in other words, the rotational shear increases with evolution. This was the first direct observational evidence for local angular momentum conservation in subgiants. Regarding magnetism, Metcalfe et al. (2024) studied the subgiant β Hydri, which has a similar mass and chemical composition to the Sun but is older (≈ 6.5 Gyrs) and has evolved off of the main sequence. They reported an activity cycle in the star with a period of ≈12 yrs, on par with the dominant cycle in the Sun, contrary to expectations. They found that in β Hydri the expansion of the outer layers increases the convective turnover time such that, even with decreasing surface rotation rates, the Rossby number drops below 1, resulting in the regeneration of an efficient solar-type dynamo. This phase is found to be transient.

• Red giants: Beck et al. (2012) and Mosser et al. (2012) catalogued the core rotation rates in over 300 red giants and red clump stars. They found that the measured core rotation periods are increasing as we move to more evolved stars (cores are spinning down rather than spinning up), and that the rotation rates are not varying ∝R2. This scenario is very different to what was observed in the subgiants, and none of the models for stellar rotation at the time could account for the observed trend with radius. This issue was attributed to the models missing some mechanism that transports angular momentum from the cores of stars to their surfaces—a flaw subsequently dubbed the missing angular momentum transport problem. This problem remains unsolved. Regarding magnetism, Gaulme et al. (2020) collected lightcurves for ≈ 4500 giants observed by Kepler and found a signature of rotational modulation consistent with star spots (and thus magnetism) in roughly 8% of the sample. Aurière et al. (2021) used spectropolarimetric measurements to infer the magnetic properties of Pollux, finding a weak but measurable field with a mean field strength of 0.4G and producing the first topological map of the field on that star, showing a dipolar distribution. Stello et al. (2016) searched for magnetic mode suppression in a sample of over 3600 red giants observed by Kepler and identified that about 20% of the sample showed a magnetic signature, implying minimum core field strengths from 10s kG to >1 MG. Li et al. (2022) made the first measurements of perturbations to mode frequencies in three red giants, identifying signatures consistent with average radial core field strengths of 30 kG - 100 kG. At time of writing, significant detections of core magnetic fields have been made for ≈ 70 stars, and it appears that the average field strengths decrease with increasing evolution, though this trend likely reflects the fact that for more evolved stars, modes start to drop out of the spectrum at lower field strengths.

• AGB stars: Vlemmings et al. (2018) recovered the envelope rotation rate of the AGB star R Doradus, finding vsini = 1.0 ± 0.1 kms−1, which is much more rapid than would be expected for traditional single star evolution (which would have the rotation rate on the order of 0.1 kms−1), pointing to some non-standard spin-up mechanism. Regarding magnetism, Konstantinova-Antova et al. (2014) made spectropolarimetric detections of magnetic fields in 8 AGB stars from a sample of 13, with maximum longitudinal field strengths of a few Gauss.

• White dwarfs: Hermes et al. (2017) constructed a catalogue of asteroseismic rotation rates for 27 pulsating white dwarfs, finding a peak in the distribution at ≈ 1 day, which is much slower than would be expected according to traditional models of rotational evolution. This is seen as additional evidence for the missing angular momentum transport problem. Regarding magnetism, Bagnulo & Landstreet (2022) made new spectropolarimetric measurements of magnetic fields in 85 white dwarfs and combined them with other measurements from the literature. They found the population could be split into two broad categories: the most massive white dwarfs, which have strong magnetic field strengths that emerge at the surface almost immediately as the cooling phase commences and appear frequently; and the less massive white dwarfs, where magnetism is much less common, with weaker fields that emerge at the surface slowly and appear to get stronger over time. This is potentially indicative of two formation channels for magnetism in white dwarfs—a merger event for the most massive, and a field generated earlier in the stellar lifecycle that is slowly relaxing for the second population. Einramhof et al. (2026) built competing models for magnetism in the stellar interior and isolated which was simultaneously consistent with observations on the red giant branch and in the lower mass white dwarfs. They found that to do so they have to adopt a magnetic field in the interior that extends beyond the extent of the main sequence core convection zone, which is perhaps evidence that it is not core convection but some other mechanism that produces this field.

The paper concludes that the works build a picture of post main-sequence rotation and magnetism which is complex and far from completely understood, with unanswered questions in every phase. The pressing missing angular momentum transport problem will be a very active field of research for years to come, and one of the bottlenecks is the seemingly contradictory behaviour in the subgiants versus the red giants. Putting a constraint on where on the subgiant phase/RGB we transition between the two regimes would be critical evidence aiding in understanding the physics at play. The upcoming PLATO mission will provide asteroseismic data for thousands of subgiants, allowing us to better sample the transition.

Improvements to AI Systems:

1. Physics-Informed Stellar Evolution Predictor

• Train an AI model on the observed rotational shear trends (subgiants vs. red giants) to predict the missing angular momentum transport mechanism.

• The improved system can simulate stellar interiors with a tunable extra transport coefficient and automatically fit it to match the transition point between subgiant and red giant rotation profiles, using Bayesian inference on Kepler/PLATO data.

2. Magnetic Field Inference Engine for Stellar Cores

• Use the reported core field strengths (30 kG–1 MG) and suppression signatures to build a deep learning model that maps asteroseismic mode frequency perturbations to 3D internal magnetic field configurations.

• The improved system can, given a light curve, output a probabilistic core field topology (dipole, toroidal, mixed) and its radial extent, directly testing the hypothesis that fields extend beyond the main-sequence convective core.

3. Automated Dynamo Regime Classifier

• Leverage the β Hydri finding (Rossby number drop → dynamo regeneration) to create a classifier that predicts magnetic activity cycles in subgiants from stellar parameters (mass, age, rotation, convective turnover time).

• The improved system can flag which evolved stars are likely to exhibit solar-like cycles, aiding target selection for spectropolarimetric follow-up and improving stellar activity forecasts for exoplanet habitability studies.

4. White Dwarf Magnetism Formation-Channel Discriminator

• Train a model on the bimodal field strength–mass distribution (strong/early vs. weak/late) to classify individual white dwarfs into merger-origin vs. fossil-field-origin categories.

• The improved system can, from spectropolarimetric data and asteroseismic rotation, output a posterior probability for each formation channel, and predict when a field will emerge at the surface for a given progenitor mass.

5. Missing Angular Momentum Transport Solver

• Develop a neural differential equation solver that learns the functional form of the missing transport term from the combined rotation datasets (subgiants, red giants, AGB, white dwarfs).

• The improved system can extrapolate to unobserved evolutionary phases, predict core–envelope coupling timescales, and generate synthetic rotation profiles for PLATO mission planning.

6. Cross-Phase Stellar Evolution Data Fusion Model

• Create a multi-task learning system that simultaneously predicts rotation, magnetism, and oscillation properties across all phases (subgiant → RGB → AGB → WD), using the paper’s constraints as priors.

• The improved system can fill gaps in stellar evolution models, identify where physics breaks down (e.g., the subgiant–RGB transition), and propose new observational tests to resolve contradictions.

7. Automated Literature-to-Simulation Pipeline

• Build an AI agent that reads papers like this one, extracts quantitative constraints (e.g., rotation rates, field strengths, percentages), and automatically updates stellar evolution codes (e.g., MESA) with new physics modules.

• The improved system can run thousands of simulations with varied transport/magnetic prescriptions and rank them by consistency with all reported observations, accelerating theory development.

Rotation and magnetism are important phenomena to consider when trying to understand stars. By shaping the structure and chemical mixing in the interior of stars, they can induce significant changes in both their fundamental and observable properties. Despite this, our theoretical picture of how rotation and magnetism evolve throughout the stellar lifecycle remains incomplete. Gaps in our understanding are particularly stark for evolved stars, where slow rotation rates and weak magnetic fields make observational studies difficult. In response to this issue, the last 20 years have seen instrumentation to probe magnetism and rotation progressing in leaps and bounds. Through these new tools we have found that evolved stars rotate in ways that we cannot predict and have magnetic fields with properties that we did not expect. In the following, I will provide a brief overview of some recent observational results and their implications for our understanding of stars. I will focus on advances in asteroseismology and high-precision spectropolarimetry, as these techniques have advanced significantly over the last few decades. In keeping with the theme of Cool Stars, I'll cover stars with low to intermediate masses and their evolution from the subgiant to the white dwarf phase.