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Solar-type stars, including the Sun, have magnetic fields that extend from their interiors to the surface and beyond, influencing both the stellar activity and interplanetary medium. Magnetic activity phenomena, such as coronal mass ejections (CMEs), significantly impacts space weather. These CMEs, composed of plasma clouds with magnetic fields ejected from the stellar corona, pose a potential threat to planets by affecting their magnetosphere and atmosphere. Despite advancements in detecting stellar CMEs, detection remains limited. We focus on understanding CME propagation by analyzing key parameters like position, velocities, and the configuration of stellar magnetic fields. Using spot transit mapping, we reconstruct magnetograms for Kepler-63 and Kepler-411, employing the ForeCAT model to simulate CME trajectories from these stars. Results indicate that CME deflections generally decrease with radial velocity and increase with ejection latitude. Additionally, stars with stronger magnetic fields, such as Kepler-63, tend to cause more significant CME deflections.

The dynamic activity of stars such as the Sun influences (exo)planetary space environments through modulation of stellar radiation, plasma wind, particle and magnetic fluxes. Energetic solar-stellar phenomena such as flares and coronal mass ejections act as transient perturbations giving rise to hazardous space weather. Magnetic fields – the primary driver of solar-stellar activity – are created via a magnetohydrodynamic dynamo mechanism within stellar convection zones. The dynamo mechanism in our host star – the Sun – is manifest in the cyclic appearance of magnetized sunspots on the solar surface. While sunspots have been directly observed for over four centuries, and theories of the origin of solar-stellar magnetism have been explored for over half a century, the inability to converge on the exact mechanism(s) governing cycle to cycle fluctuations and inconsistent predictions for the strength of future sunspot cycles have been challenging for models of the solar cycles. This review discusses observational constraints on the solar magnetic cycle with a focus on those relevant for cycle forecasting, elucidates recent physical insights which aid in understanding solar cycle variability, and presents advances in solar cycle predictions achieved via data-driven, physics-based models. The most successful prediction approaches support the Babcock-Leighton solar dynamo mechanism as the primary driver of solar cycle variability and reinforce the flux transport paradigm as a useful tool for modelling solar-stellar magnetism.

About ten per cent of ‘massive’ stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields 1 – 3 . It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields 4 , 5 , and the predicted fraction of merged massive stars is also about ten per cent 6 , 7 . The merger hypothesis is further supported by a lack of magnetic stars in close binaries 8 , 9 , which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic ‘blue straggler’ star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association 10 . Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed 11 , and their supernovae may be affected by their strong magnetic fields 12 . Simulated mergers of two massive stars provide a solution to the long-standing puzzle of the origin of strong magnetic fields in a subset of massive stars.

Several long-period radio transients have recently been discovered, with strongly polarized coherent radio pulses appearing on timescales between tens to thousands of seconds 1 , 2 . In some cases, the radio pulses have been interpreted as coming from rotating neutron stars with extremely strong magnetic fields, known as magnetars; the origin of other, occasionally periodic and less-well-sampled radio transients is still debated 3 . Coherent periodic radio emission is usually explained by rotating dipolar magnetic fields and pair-production mechanisms, but such models do not easily predict radio emission from such slowly rotating neutron stars and maintain it for extended times. On the other hand, highly magnetic isolated white dwarfs would be expected to have long spin periodicities, but periodic coherent radio emission has not yet been directly detected from these sources. Here we report observations of a long-period (21 min) radio transient, which we have labelled GPM J1839–10. The pulses vary in brightness by two orders of magnitude, last between 30 and 300 s and have quasiperiodic substructure. The observations prompted a search of radio archives and we found that the source has been repeating since at least 1988. The archival data enabled constraint of the period derivative to <3.6 × 10 −13  s s −1 , which is at the very limit of any classical theoretical model that predicts dipolar radio emission from an isolated neutron star. The discovery of a long-period radio transient, GPM J1839–10, prompted a search of radio archives, thereby finding that this source has been repeating since at least 1988.

In this work we present the results for the automatic determination of the mean longitudinal magnetic field in polarized stellar spectra through the analysis of spectropolarimetric observations. In order to determine this important parameter, we first developed a synthetic database encompassing a set of different stellar spectra, each one defined by a set of free parameters. Then, we used supervised learning for artificial neural networks, a machine learning approach, to achieve our goal.

A red giant star is an evolved low- or intermediate-mass star that has exhausted its central hydrogen content, leaving a helium core and a hydrogen-burning shell. Oscillations of stars can be observed as periodic dimmings and brightenings in the optical light curves. In red giant stars, non-radial acoustic waves couple to gravity waves and give rise to mixed modes, which behave as pressure modes in the envelope and gravity modes in the core. These modes have previously been used to measure the internal rotation of red giants 1 , 2 , leading to the conclusion that purely hydrodynamical processes of angular momentum transport from the core are too inefficient 3 . Magnetic fields could produce the additional required transport 4 – 6 . However, owing to the lack of direct measurements of magnetic fields in stellar interiors, little is currently known about their properties. Asteroseismology can provide direct detection of magnetic fields because, like rotation, the fields induce shifts in the oscillation mode frequencies 7 – 12 . Here we report the measurement of magnetic fields in the cores of three red giant stars observed with the Kepler 13 satellite. The fields induce shifts that break the symmetry of dipole mode multiplets. We thus measure field strengths ranging from about 30 kilogauss to about 100 kilogauss in the vicinity of the hydrogen-burning shell and place constraints on the field topology. Magnetic fields of 30 to 100 kilogauss are measured in the cores of three giant red stars observed with the Kepler satellite.

Magnetic fields play a fundamental role for interior and atmospheric properties of M dwarfs and greatly influence terrestrial planets orbiting in the habitable zones of these low-mass stars. Determination of the strength and topology of magnetic fields, both on stellar surfaces and throughout the extended stellar magnetospheres, is a key ingredient for advancing stellar and planetary science. Here, modern methods of magnetic field measurements applied to M-dwarf stars are reviewed, with an emphasis on direct diagnostics based on interpretation of the Zeeman effect signatures in high-resolution intensity and polarisation spectra. Results of the mean field strength measurements derived from Zeeman broadening analyses as well as information on the global magnetic geometries inferred by applying tomographic mapping methods to spectropolarimetric observations are summarised and critically evaluated. The emerging understanding of the complex, multi-scale nature of M-dwarf magnetic fields is discussed in the context of theoretical models of hydromagnetic dynamos and stellar interior structure altered by magnetic fields.

Fast radio bursts (FRBs) are millisecond-duration events detected from beyond the Milky Way. FRB emission characteristics favour highly magnetized neutron stars, or magnetars, as the sources 1 , as evidenced by FRB-like bursts from a galactic magnetar 2 , 3 , and the star-forming nature of FRB host galaxies 4 , 5 . However, the processes that produce FRB sources remain unknown 6 . Although galactic magnetars are often linked to core-collapse supernovae (CCSNe) 7 , it is uncertain what determines which supernovae result in magnetars. The galactic environments of FRB sources can be used to investigate their progenitors. Here, we present the stellar population properties of 30 FRB host galaxies discovered by the Deep Synoptic Array (DSA-110). Our analysis shows a marked deficit of low-mass FRB hosts compared with the occurrence of star formation in the Universe, implying that FRBs are a biased tracer of star formation, preferentially selecting massive star-forming galaxies. This bias may be driven by galaxy metallicity, which is positively correlated with stellar mass 8 . Metal-rich environments may favour the formation of magnetar progenitors through stellar mergers 9 , 10 , as higher-metallicity stars are less compact and more likely to fill their Roche lobes, leading to unstable mass transfer. Although massive stars do not have convective interiors to generate strong magnetic fields by dynamo 11 , merger remnants are thought to have the requisite internal magnetic-field strengths to result in magnetars 11 , 12 . The preferential occurrence of FRBs in massive star-forming galaxies suggests that a core-collapse supernova of merger remnants preferentially forms magnetars. Analysis of the stellar population properties of 30 host galaxies of fast radio bursts (FRBs) suggests an abundance of FRBs in massive star-forming galaxies, and implies that the formation of FRB sources—magnetars—is linked to core-collapse supernovae of stellar merger remnants.

The magnetic fields of solar-type stars are observed to cycle over decadal periods—11 years in the case of the Sun. The fields originate in the turbulent convective layers of stars and have a complex dependency upon stellar rotation rate. We have performed a set of turbulent global simulations that exhibit magnetic cycles varying systematically with stellar rotation and luminosity. We find that the magnetic cycle period is inversely proportional to the Rossby number, which quantifies the influence of rotation on turbulent convection. The trend relies on a fundamentally nonlinear dynamo process and is compatible with the Sun’s cycle and those of other solar-type stars.

Internal stellar magnetic fields are inaccessible to direct observations, and little is known about their amplitude, geometry, and evolution. We demonstrate that strong magnetic fields in the cores of red giant stars can be identified with asteroseismology. The fields can manifest themselves via depressed dipole stellar oscillation modes, arising from a magnetic greenhouse effect that scatters and traps oscillation-mode energy within the core of the star. The Kepler satellite has observed a few dozen red giants with depressed dipole modes, which we interpret as stars with strongly magnetized cores. We find that field strengths larger than ∼105 gauss may produce the observed depression, and in one case we infer a minimum core field strength of ≈107 gauss.