Magnetism in High-Mass Stars

Keszthelyi, Z.

doi:10.3390/galaxies11020040

Cited by 6 publications

(2 citation statements)

References 247 publications

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“…Nevertheless, the inferred Ω/Ω c ratios are large enough to favor the appearance of more or less extended regions in the stellar envelope and in the sub-photospheric regions that become unstable to convection (Clement 1979;Maeder et al 2008;Cantiello et al 2009). Since convection favors the setting out of differential rotation with a concomitant production of internal magnetic fields, magnetic dynamos, and cyclic activities (Zorec et al 2011;Zorec 2023;Keszthelyi 2023), we may ask whether instabilities related with these magnetic fields could be the cause of significant upheavals in the outer stellar structure, where the correlated non-radial pulsations and huge sporadic mass ejections could both be phenomena triggered by these disturbances. In the framework of this phenomenology, the study of non-radial pulsations in Be stars may acquire a new major area of interest centered on the study of the internal structure of rapidly rotating stars.…”

Section: Discussionmentioning

confidence: 99%

Study of a sample of faint Be stars in the exofield of CoRoT

Zorec

Hubert

Martayan

et al. 2023

A&A

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Context. The search and interpretation of non-radial pulsations from Be star light curves observed with the CoRoT satellite requires high-quality stellar astrophysical parameters. Aims. The present work is devoted to the spectroscopic study of a sample of faint Be stars observed by CoRoT in the fourth long run (LRA02). Methods. The astrophysical parameters were determined from the spectra in the λλ4000–4500 Å wavelength domain observed with the VLT/FLAMES instruments at ESO. Spectra were fitted with models of stellar atmospheres using our GIRFIT package. Spectra obtained in the λλ6400–7200 Å wavelength domain enabled the confirmation or, otherwise, a first identification of Be star candidates. Results. The apparent parameters (Teff, log g, Vsin i) for a set of 19 B and Be stars were corrected for the effects induced by the rapid rotation. These allowed us to determine: (1) stellar masses that are in agreement with those measured for detached binary systems; (2) spectroscopic distances that agree with the Gaia parallaxes; and (3) centrifugal/gravity equatorial force ratios of ~0.6–0.7, which indicate that our Be stars are subcritical rotators. A study of the Balmer Hα, Hγ and Hδ emission lines produced: (1) extents of the circumstellar disk (CD) emitting regions that agree with the interferometric inferences in other Be stars; (2) R– dependent exponents n(R) = ln[ρ(R)/ρo]/ln(Ro/R) of the CD radial density distributions; and (3) CD base densities ρo similar to those inferred in other recent works. Conclusions. The Hγ and Hδ emission lines are formed in CD layers close to the central star. These lines produced a different value of the exponent n(R) than assumed for Hα. Further detailed studies of Hγ and Hδ emission lines could reveal the physical properties of regions where the viscous transport of angular momentum to the remaining CD regions is likely to originate from. The subcritical rotation of Be stars suggests that their huge discrete mass-ejections and concomitant non-radial pulsations might have a common origin in stellar envelope regions that become unstable to convection due to rotation. If it is proven that the studied Be stars are products of binary mass transfer phases, the errors induced on the estimated Teff by the presence of stripped sub-dwarf O/B companions are not likely to exceed their present uncertainties.

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Section: Discussionmentioning

confidence: 99%

Study of a sample of faint Be stars in the exofield of CoRoT

Zorec

Hubert

Martayan

et al. 2023

A&A

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“…In the current era of large-scale time-series photometric surveys, such as Kepler [14,15], TESS [86] and soon also PLATO [87], delivering years-long high-precision light curves and pulsation frequencies for tens of thousands of stars, the break-through application of magneto-asteroseismology to many more massive stars has just begun. The recent advances in theoretical, numerical and observational work focusing on magnetic fields in massive stars is described in Chapter 3 of this Special Issue by Keszthelyi (2023) [88].…”

Section: Chapter 3: Magnetism In High-mass Starsmentioning

confidence: 99%

The Structure and Evolution of Stars: Introductory Remarks

Bowman,

van Saders,

Vink

2023

Galaxies

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In this introductory chapter of the Special Issue entitled ‘The Structure and Evolution of Stars’, we highlight the recent major progress made in our understanding of the physics that governs stellar interiors. In so doing, we combine insight from observations, 1D evolutionary modelling and 2D + 3D rotating (magneto)hydrodynamical simulations. Therefore, a complete and compelling picture of the necessary ingredients in state-of-the-art stellar structure theory and areas in which improvements still need to be made are contextualised. Additionally, the over-arching perspective linking all the themes of subsequent chapters is presented.

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Turbulent dynamo action and its effects on the mixing at the convective boundary of an idealized oxygen-burning shell

Leidi,

Andrassy,

Higl

et al. 2023

A&A

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Convection is one of the most important mixing processes in stellar interiors. Hydrodynamic mass entrainment can bring fresh fuel from neighboring stable layers into a convection zone, modifying the structure and evolution of the star. Because flows in stellar convection zones are highly turbulent, multidimensional hydrodynamic simulations are fundamental to accurately capture the physics of mixing processes. Under some conditions, strong magnetic fields can be sustained by the action of a turbulent dynamo, adding another layer of complexity and possibly altering the dynamics in the convection zone and at its boundaries. In this study, we used our fully compressible SEVEN-LEAGUE HYDRO code to run detailed and highly resolved three-dimensional magnetohydrodynamic simulations of turbulent convection, dynamo amplification, and convective boundary mixing in a simplified setup whose stratification is similar to that of an oxygen-burning shell in a star with an initial mass of 25 M⊙. We find that the random stretching of magnetic field lines by fluid motions in the inertial range of the turbulent spectrum (i.e., a small-scale dynamo) naturally amplifies the seed field by several orders of magnitude in a few convective turnover timescales. During the subsequent saturated regime, the magnetic-to-kinetic energy ratio inside the convective shell reaches values as high as 0.33, and the average magnetic field strength is ∼1010 G. Such strong fields efficiently suppress shear instabilities, which feed the turbulent cascade of kinetic energy, on a wide range of spatial scales. The resulting convective flows are characterized by thread-like structures that extend over a large fraction of the convective shell. The reduced flow speeds and the presence of magnetic fields with strengths up to 60% of the equipartition value at the upper convective boundary diminish the rate of mass entrainment from the stable layer by ≈20% as compared to the purely hydrodynamic case.

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Magnetism in High-Mass Stars

Cited by 6 publications

References 247 publications

Study of a sample of faint Be stars in the exofield of CoRoT

Study of a sample of faint Be stars in the exofield of CoRoT

The Structure and Evolution of Stars: Introductory Remarks

Turbulent dynamo action and its effects on the mixing at the convective boundary of an idealized oxygen-burning shell

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