Period spacings of gravity modes in rapidly rotating magnetic stars

Context. Stellar magnetic fields are often invoked to explain the missing transport of angular momentum observed in models of stellar interiors. However, the properties of an internal magnetic field and the consequences of its presence on stellar evolution are largely unknown. Aims. We study the effect of an axisymmetric internal magnetic field on the frequency of gravity modes in rapidly rotating stars to check whether gravity modes can be used to detect and probe such a field. Methods. Rotation is taken into account using the traditional approximation of rotation and the effect of the magnetic field is computed using a perturbative approach. As a proof of concept, we compute frequency shifts due to a mixed (i.e. with both poloidal and toroidal components) fossil magnetic field for a representative model of a known magnetic, rapidly rotating, slowly pulsating B-type star: HD 43317. Results. We find that frequency shifts induced by the magnetic field scale with the square of its amplitude. A magnetic field with a near-core strength of the order of 150 kG (which is consistent with the observed surface field strength of the order of 1 kG) leads to signatures that are detectable in period spacings for high-radial-order gravity modes. Conclusions. The predicted frequency shifts can be used to constrain internal magnetic fields and offer the potential for a significant step forward in our interpretation of the observed structure of gravity-mode period spacing patterns in rapidly rotating stars.

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Period spacings of gravity modes in rapidly rotating magnetic stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937398 ◽

2020 ◽

Vol 636 ◽

pp. A100 ◽

Cited By ~ 4

Author(s):

V. Prat ◽

S. Mathis ◽

C. Neiner ◽

J. Van Beeck ◽

D. M. Bowman ◽

...

Keyword(s):

Magnetic Field ◽

Rotation Axis ◽

Equatorial Region ◽

Rotating Stars ◽

Frequency Shifts ◽

New Formalism ◽

Magnetic Stars ◽

Internal Oblique ◽

Rapidly Rotating Stars ◽

The Magnetic Field

Context. Stellar internal magnetic fields have recently been shown to leave a detectable signature on period spacing patterns of gravity modes. Aims. We aim to investigate the effect of the obliquity of a mixed (poloidal and toroidal) dipolar internal fossil magnetic field with respect to the rotation axis on the frequency of gravity modes in rapidly rotating stars. Methods. We used the traditional approximation of rotation to compute non-magnetic modes, and a perturbative treatment of the magnetic field to compute the corresponding frequency shifts. We applied the new formalism to HD 43317, a magnetic, rapidly rotating, slowly pulsating B-type star, whose field has an obliquity angle of about 80°. Results. We find that frequency shifts induced by the magnetic field on high-radial-order gravity modes are larger with increasing obliquity angle, when the magnetic axis is closer to the equatorial region, where these modes are trapped. The maximum value is reached for an obliquity angle of 90°. This trend is observed for all mode geometries. Conclusions. Our results predict that the signature of an internal oblique dipolar magnetic field is detectable using asteroseismology of gravity modes.

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First magnetic stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030853 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 399-400

Author(s):

Iosif I. Romanyuk ◽

Dimitry O. Kudryavtsev

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Field Strength ◽

Main Sequence ◽

Published Data ◽

Rotating Stars ◽

Exponential Law ◽

Chemically Peculiar ◽

Magnetic Stars ◽

The Magnetic Field

AbstractThis contribution dedicated to the analysis of the magnetism of chemically peculiar (CP) stars of the upper Main Sequence. We use our own measurements and published data to compile a catalog of magnetic CP stars containing a total of 326 objects with confidently detected magnetic fields and 29 stars which are very likely to possess magnetic field. Our analysis shows that the number of magnetic CP stars decreases with increasing field strength in accordance with exponential law, hotter and faster rotating stars have stronger fields. Intensity of depressions in the continua correlates with the magnetic field strength.

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Detecting axisymmetric magnetic fields using gravity modes in intermediate-mass stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201937363 ◽

2020 ◽

Vol 638 ◽

pp. A149 ◽

Cited By ~ 2

Author(s):

J. Van Beeck ◽

V. Prat ◽

T. Van Reeth ◽

S. Mathis ◽

D. M. Bowman ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Stellar Evolution ◽

Main Sequence ◽

Mode Frequency ◽

Internal Magnetic Field ◽

Main Sequence Stars ◽

Frequency Shifts ◽

Perturbative Approach ◽

Gravity Mode

Context. Angular momentum (AM) transport models of stellar interiors require improvements to explain the strong extraction of AM from stellar cores that is observed with asteroseismology. One of the frequently invoked mediators of AM transport are internal magnetic fields, even though their properties, observational signatures, and influence on stellar evolution are largely unknown. Aims. We study how a fossil, axisymmetric internal magnetic field affects period spacing patterns of dipolar gravity mode oscillations in main sequence stars with masses of 1.3, 2.0, and 3.0 M⊙. We assess the influence of fundamental stellar parameters on the magnitude of pulsation mode frequency shifts. Methods. We computed dipolar gravity mode frequency shifts due to a fossil, axisymmetric poloidal–toroidal internal magnetic field for a grid of stellar evolution models, varying stellar fundamental parameters. Rigid rotation was taken into account using the traditional approximation of rotation, and the influence of the magnetic field was computed using a perturbative approach. Results. We find magnetic signatures for dipolar gravity mode oscillations in terminal-age main sequence stars that are measurable for a near-core field strength larger than 105 G. The predicted signatures differ appreciably from those due to rotation. Conclusions. Our formalism demonstrates the potential for the future detection and characterization of strong fossil, axisymmetric internal magnetic fields in gravity-mode pulsators near the end of core-hydrogen burning from Kepler photometry, if such fields exist.

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Investigations of space magnetism at the Crimean Astrophysical Observatory. II. Direct spectropolarimetric measurements of stellar magnetic fields

Acta Astrophysica Taurica ◽

10.31059/aat.vol1.iss2.pp26-36 ◽

2020 ◽

Vol 1 (2) ◽

pp. 26-36

Author(s):

Sergei Plachinda ◽

Varvara Butkovskaya

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Spectral Lines ◽

Crimean Astrophysical Observatory ◽

Scientific School ◽

Pulsation Period ◽

Echelle Spectrograph ◽

Direct Measurements ◽

Stellar Magnetic Fields ◽

The Magnetic Field

A research on stellar magnetism in Crimea was initiated by pioneer works of A.B. Severny, V.E. Stepanov, and D.N. Rachkovsky. Today, the study of stellar magnetic fields is a key field of research at the Crimean Astrophysical Observatory (CrAO). The 2.6 m Shajn telescope equipped with the echelle spectrograph ESPL, CCD, and Stokesmeter (a circular polarization analyzer) allows us to study the magnetic field of bright stars up to 5m–6m. The Single Line (SL) technique is developed for measuring magnetic fields at CrAO. This technique is based on the calculation of the Zeeman effect in individual spectral lines. A key advantage of the SL technique is its ability to detect local magnetic fields on the surface of stars. Many results in the field of direct measurements of stellar magnetic fields were obtained at CrAO for the first time. In particular, the magnetic field on supergiants (ǫ Gem), as well as on a number of subgiants, giants, and bright giants was first detected. This, and investigations of other authors, confirmed the hypothesis that a magnetic field is generated at all the stages of evolution of late-type stars, including the stage of star formation. The emergence of large magnetic flux tubes at the surface of stars of V-IV-III luminosity classes (61 Cyg A, β Gem, β Aql) was first registered. In subgiants, the magnetic field behavior with the activity cycle was first established for β Aql. Using the long-term Crimean spectroscopic and spectropolarimetric observations of α Lyr, the 22-year variability cycle of the star, supposedly associated with meridional flows, is confirmed. Magnetic field variability with the pulsation period was first detected for different types of pulsating variables: the classical Cepheid β Aql, the low-amplitude β Cep-type variable γ Peg, and others. In this review we cover more than a half-century history of the formation of the Crimean scientific school for high-precision direct measurements of stellar magnetic fields.

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Pulsation of Rotating Magnetic Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100117117 ◽

1993 ◽

Vol 139 ◽

pp. 134-134

Author(s):

H. Shibahashi ◽

M. Takata

Keyword(s):

Magnetic Field ◽

Fine Structure ◽

Magnetic Fields ◽

Rotation Axis ◽

Dipole Field ◽

Dipole Mode ◽

Magnetic Stars ◽

Ap Stars ◽

The Magnetic Field ◽

Split Frequency

Recently, one of the rapidly oscillating Ap stars, HR 3831, has been found to have an equally split frequency septuplet, though its oscillation seems to be essentially an axisymmetric dipole mode with respect to the magnetic axis which is oblique to the rotation axis (Kurtz et al. 1992; Kurtz 1992). In order to explain this fine structure, we investigate oscillations of obliquely rotating magnetic stars by taking account of the perturbations due to the magnetic fields and the rotation. We suppose that the star is rigidly rotating and that the magnetic field is a dipole field and its axis is oblique to the rotation axis. We treat the effects of the rotation and of the magnetic field as perturbations. In doing so, we suppose that the rotation of the star is slow enough so that the effect of the rotation on oscillations is smaller than that of the magnetic field.

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Solar and Stellar Magnetic Fields and Atmospheric Structures: Theory

International Astronomical Union Colloquium ◽

10.1017/s0252921100031936 ◽

1989 ◽

Vol 104 (1) ◽

pp. 271-288

Author(s):

E. N. Parker

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Dynamical Behavior ◽

Convective Zone ◽

Coronal Heating ◽

The Sun ◽

Current Sheets ◽

Spontaneous Formation ◽

Stellar Magnetic Fields ◽

The Magnetic Field

AbstractThis presentation reviews selected ideas on the origin of the magnetic field of the Sun, the dynamical behavior of the azimuthal field in the convective zone, the fibril state of the field at the photosphere, the formation of sunspots, prominences, the spontaneous formation of current sheets in the bipolar field above the surface of the Sun, coronal heating, and flares.

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The basic role of magnetic fields in stellar evolution

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030671 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 311-322 ◽

Cited By ~ 3

Author(s):

André Maeder ◽

Georges Meynet ◽

Cyril Georgy ◽

Sylvia Ekström

Keyword(s):

Magnetic Field ◽

Internal Rotation ◽

Rotating Stars ◽

Critical Question ◽

Large N ◽

Internal Mixing ◽

Magnetic Stars ◽

Main Effects ◽

The Magnetic Field

AbstractMagnetic field is playing an important role at all stages of star evolution from star formation to the endpoints. The main effects are briefly reviewed. We also show that O–type stars have large convective envelopes, where convective dynamo could work. There, fields in magnetostatic balance have intensities of the order of 100 G.A few OB stars with strong polar fields (Henrichs et al. 2003a) show large N–enhancements indicating a strong internal mixing. We suggest that the meridional circulation enhanced by an internal rotation law close to uniform in these magnetic stars is responsible for the observed mixing. Thus, it is not the magnetic field itself which makes the mixing, but the strong thermal instability associated to solid body rotation.A critical question for evolution is whether a dynamo is at work in radiative zones of rotating stars. The Tayler-Spruit (TS) dynamo is the best candidate. We derive some basic relations for dynamos in radiative layers. Evolutionary models with TS dynamo show important effects: internal rotation coupling and enhanced mixing, all model outputs being affected.

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Erklärung stellarer und planetarer Magnetfelder durch einen turbulenzbedingten Dynamomechanismus

Zeitschrift für Naturforschung A ◽

10.1515/zna-1966-0813 ◽

1966 ◽

Vol 21 (8) ◽

pp. 1285-1296 ◽

Cited By ~ 37

Author(s):

M. Steenbeck ◽

F. Krause

Keyword(s):

Magnetic Field ◽

Angular Velocity ◽

Magnetic Fields ◽

Cross Product ◽

Skin Depth ◽

Electrically Conducting ◽

Magnetic Stars ◽

The Magnetic Field ◽

Component Parallel ◽

Good Agreement

In a foregoing paper 1 the effects of a turbulent motion on magnetic fields were investigated. Especially turbulence was treated under the influence of CORIOLiS-forces and gradients of density and/or turbulence intensity. It was shown that on these conditions the average cross-product of velocity and magnetic field has a non-vanishing component parallel to the average magnetic field. Here we give the consequences of this effect for rotating, electrically conducting spheres.At first a sphere rotating with constant angular velocity is investigated. The quadratic effect provides for dynamo maintainance of the magnetic fields. A field of dipol-type has the weakest condition for maintainance. Applications to the magnetic field of the earth show a good agreement with the conceptions of the physical state of the earth’s core.For a second model differential rotation is included. We have also dynamo maintainance. Since we have to assume that generally the angular velocity is a function decreasing with the distance from the centre of the sphere, the calculations show that we have a preferred self-excited build-up of a quadrupol-type field. This model may be applicable to magnetic stars.Finally we look for dynamo maintainance of alternating fields. We consider the skin-depth to be small compared with the radius of the sphere, then we have plane geometry. The existence of periodical solutions is proved. Applications to the general magnetic field of the sun, which has a period of 22 years, are discussed.

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Masers and Stellar Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900189429 ◽

1990 ◽

Vol 140 ◽

pp. 21-25

Author(s):

Mark J. Reid

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Late Type ◽

Circumstellar Envelopes ◽

Average Surface ◽

Stellar Photosphere ◽

Type Giant ◽

Stellar Magnetic Fields ◽

The Magnetic Field ◽

Field Strengths

Observations of circular polarization of molecular masers associated with late type giant and supergiant stars can be used to estimate the magnetic field strength in the masing region. Magnetic field strengths of ~ 5 mG are deduced for OH masers in circumstellar envelopes at distances of ~ 1016 cm from the star, and magnetic field strengths of ~ 50 G are deduced for SiO masers that reside above the photosphere. Extrapolation to the stellar photosphere suggests that average surface magnetic fields are on the order of 103 G.

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Magnetic field evolution in solar-type stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921320001404 ◽

2019 ◽

Vol 15 (S354) ◽

pp. 169-180

Author(s):

Axel Brandenburg

Keyword(s):

Magnetic Field ◽

Linear Polarization ◽

Large Scale ◽

Rossby Number ◽

Rotating Stars ◽

Field Evolution ◽

Solar Type ◽

Activity Cycles ◽

Rapidly Rotating Stars ◽

The Magnetic Field

AbstractWe discuss selected aspects regarding the magnetic field evolution of solar-type stars. Most of the stars with activity cycles are in the range where the normalized chromospheric Calcium emission increases linearly with the inverse Rossby number. For Rossby numbers below about a quarter of the solar value, the activity saturates and no cycles have been found. For Rossby numbers above the solar value, again no activity cycles have been found, but now the activity goes up again for a major fraction of the stars. Rapidly rotating stars show nonaxisymmetric large-scale magnetic fields, but there is disagreement between models and observations regarding the actual value of the Rossby number where this happens. We also discuss the prospects of detecting the sign of magnetic helicity using various linear polarization techniques both at the stellar surface using the parity-odd contribution to linear polarization and above the surface using Faraday rotation.

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