scholarly journals Magnetic fields in non-convective regions of stars

2017 ◽  
Vol 4 (2) ◽  
pp. 160271 ◽  
Author(s):  
Jonathan Braithwaite ◽  
Henk C. Spruit
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Field Model ◽  
The Sun ◽  
Early Type ◽  
Main Sequence Stars ◽  
Current State ◽  
Recent Developments ◽  
Early Type Stars

We review the current state of knowledge of magnetic fields inside stars, concentrating on recent developments concerning magnetic fields in stably stratified (zones of) stars, leaving out convective dynamo theories and observations of convective envelopes. We include the observational properties of A, B and O-type main-sequence stars, which have radiative envelopes, and the fossil field model which is normally invoked to explain the strong fields sometimes seen in these stars. Observations seem to show that Ap-type stable fields are excluded in stars with convective envelopes. Most stars contain both radiative and convective zones, and there are potentially important effects arising from the interaction of magnetic fields at the boundaries between them; the solar cycle being one of the better known examples. Related to this, we discuss whether the Sun could harbour a magnetic field in its core. Recent developments regarding the various convective and radiative layers near the surfaces of early-type stars and their observational effects are examined. We look at possible dynamo mechanisms that run on differential rotation rather than convection. Finally, we turn to neutron stars with a discussion of the possible origins for their magnetic fields.

Fossil magnetic fields and rotation of early-type stars

1994 ◽  
Vol 162 ◽  
pp. 184-185
Author(s):  
A.E. Dudorov
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Main Sequence ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars ◽  
Quadrupole Magnetic Field ◽  
Interstellar Molecular Clouds

Observational data of the last 10 years allow two main conclusions:a) Main sequence stars can be separated in two classes: - magnetic (Bp) stars with surface strengths of a dipole or quadrupole magnetic field of Bs ≈ n · (102 − 103) G, n = 2,3,4…7, and - normal main sequence stars (F-O) with magnetic fields Bs ≈ 1 − 100 G (< 300 G);b) Typical star formation takes place in interstellar molecular clouds with magnetic field strengths B ≈ 10-5 G (See Dudorov 1990).

scholarly journals Photospheric and “Chromospheric” CIV Lines in B-Main-Sequence Stars

1986 ◽  
Vol 116 ◽  
pp. 113-116
Author(s):  
Fiorella Castelli ◽  
Carlo Morossi ◽  
Roberto Stalio
Keyword(s):  
High Temperature ◽  
Stellar Wind ◽  
High Velocity ◽  
Main Sequence ◽  
Stellar Atmosphere ◽  
Uv Spectra ◽  
Spectral Lines ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars

The presence in the far-UV spectra of early-type stars of spectral lines of superionized atoms is argument of controversial debate among astronomers. Presently there is agreement on the non-radiative origin of these ions but not on the proposed mechanisms for their production nor on the proposed locations in the stellar atmosphere where they are abundant. Cassinelli et al. (1978) suggest that the Auger mechanism is operative in a cool wind blowing above a narrow corona to produce these ions; Lucy and White (1980) introduce radiative instabilities growing into hot blobs distributed across the stellar wind; Doazan and Thomas (1982) make these ions to be formed in both pre- and post-coronal, high temperature regions at low and high velocity respectively.

scholarly journals Magnetic Archaeology of Early-type Stellar Dynamos

2021 ◽  
Vol 923 (1) ◽  
pp. 104
Author(s):  
Adam S. Jermyn ◽  
Matteo Cantiello
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Mass Loss ◽  
Bimodal Distribution ◽  
Early Type ◽  
Surface Magnetism ◽  
Near Surface ◽  
Early Type Stars ◽  
Slow Evolution ◽  
Weak Fields

Abstract Early-type stars show a bimodal distribution of magnetic field strengths, with some showing very strong fields (≳1 kG) and others very weak fields (≲10 G). Recently, we proposed that this reflects the processing or lack thereof of fossil fields by subsurface convection zones. Stars with weak fossil fields process these at the surface into even weaker dynamo-generated fields, while in stars with stronger fossil fields magnetism inhibits convection, allowing the fossil field to remain as is. We now expand on this theory and explore the timescales involved in the evolution of near-surface magnetic fields. We find that mass loss strips near-surface regions faster than magnetic fields can diffuse through them. As a result, observations of surface magnetism directly probe the frozen-in remains of the convective dynamo. This explains the slow evolution of magnetism in stars with very weak fields: these dynamo-generated magnetic fields evolve on the timescale of the mass loss, not that of the dynamo.

scholarly journals Observations of stellar winds in early type stars

1981 ◽  
Vol 59 ◽  
pp. 1-18
Author(s):  
Peter S. Conti
Keyword(s):  
Resonance Line ◽  
Optical Spectrum ◽  
Main Sequence ◽  
Emission Lines ◽  
Stellar Winds ◽  
Be Stars ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars ◽  
Line P

I have been asked to review the “observations” of winds in “early-type” stars. This normally means stars of spectral type OB and those of the Wolf-Rayet (WR) class. In this paper I will concentrate on the massive population I stars of these types, and primarily the O and WR classes on which most of the recent work has been done. The early B type supergiants share many of the wind properties of the O stars but the later supergiant types, Be stars, and main sequence stars may not. Stellar winds are a ubiquitous phenomenon among these early type stars (Snow and Morton 1976). We see evidence of their winds in the resonance line P Cygni profiles in the UV region, in the emission lines of Hα and λ4686 He II in the optical spectrum, and in the free-free emission from the ionized plasma as observed in the IR and radio regions of the spectrum.

scholarly journals Magnetic Doppler imaging of early-type stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 166-171
Author(s):  
Oleg Kochukhov ◽  
Thomas Rivinius ◽  
Mary E. Oksala ◽  
Iosif Romanyuk
Keyword(s):  
Magnetic Field ◽  
Doppler Imaging ◽  
Early Type ◽  
Main Sequence Stars ◽  
Surface Mapping ◽  
B Stars ◽  
Magnetic Stars ◽  
Early Type Stars ◽  
Complex Magnetic Field ◽  
Fast Rotating

AbstractDoppler imaging of early-type magnetic stars is the most advanced method to interpret their line profile variations. DI allows us to study directly a complex interplay between chemical spots, magnetic fields, and the mass loss. Here we outline the general principles of the surface mapping of stars, discuss adaption of this technique to early-type stars and present several recent examples of the abundance and magnetic mapping performed for rapidly rotating early-B stars. In particular, we present the first Doppler images for the very fast rotating He-rich star HR 7355 and a reconstruction of magnetic field for the well-known Bp star σ Ori E. We also present new magnetic maps for the He-strong star HD 37776, which possesses one of the most complex magnetic field topologies among the upper main sequence stars.

scholarly journals Dynamo and Fossil Magnetic Field in Young Stars

1993 ◽  
Vol 157 ◽  
pp. 171-175
Author(s):  
A.E. Dudorov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Young Stars ◽  
Small Scale ◽  
Scaling Relations ◽  
Main Sequence Stars ◽  
Nonlinear Stabilization ◽  
Turbulent Dynamo ◽  
Solar Type

The theory of fossil magnetic fields shows that new born stars may have internal magnetic fields of more than 1 million gauss. Convection inside young solar type stars will tangle any strong fossil magnetic field. The small scale magnetic field rises to the surface and determines the young stars activity attenuating with their age. When a fossil field is diminished a turbulent dynamo may begin to work in the condition of nonlinear stabilization. The scaling relations for the turbulent αω dynamo show that the strength of the generated “fossil” magnetic field inside the main sequence stars is stabilized on the level one tenth — 10 millions gauss, depending on the mass of the stars.

scholarly journals Study of the effects of magnetic braking on the lithium abundances of the Sun and solar-type stars

2020 ◽  
Vol 496 (2) ◽  
pp. 1343-1354
Author(s):  
R Caballero Navarro ◽  
A García Hernández ◽  
A Ayala ◽  
J C Suárez
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Globular Clusters ◽  
Main Sequence ◽  
The Sun ◽  
Depletion Rate ◽  
Solar Type ◽  
Magnetic Braking ◽  
Physical Phenomena ◽  
The Impact

ABSTRACT The study of lithium (Li) surface abundance in the Sun and young stellar globular clusters which are seemingly anomalous in present-day scenarios, as well as the influence of rotation and magnetic braking (MB) on its depletion during pre-main sequence (PMS) and main sequence (MS). In this work, the effects of rotational mixing and of the rotational hydrostatic effects on Li abundances are studied by simulating several grids of PMS and MS rotating and non-rotating models. Those effects are combined with the additional impact of the MB (with magnetic field intensities ranging between 3.0 and 5.0 G). The data obtained from simulations are confronted by comparing different stellar parameters. The results show that the surface Li abundance for the Sun-like models at the end of the PMS and throughout the MS decreases when rotational effects are included, that is the Li depletion rate for rotating models is higher than for non-rotating ones. This effect is attenuated when the MB produced by a magnetic field is present. This physical phenomena impacts also the star effective temperature (Teff) and its location in the HR diagram. The impact of MB in Li depletion is sensitive to the magnetic field intensity: the higher it is, the lower the Li destruction. A direct link between the magnetic fields and the convective zone (CZ) size is observed: stronger magnetic fields produce shallower CZ’s. This result suggests that MB effect must be taken into consideration during PMS if we aim to reproduce Li abundances in young clusters.

scholarly journals Emission of Gravitational Waves from a Magnetohydrodynamic Dynamo in the Center of the Sun

2015 ◽  
Vol 70 (7) ◽  
pp. 545-551
Author(s):  
Friedwardt Winterberg
Keyword(s):  
Magnetic Field ◽  
Gravitational Waves ◽  
Main Sequence ◽  
Fusion Reaction ◽  
Large Mass ◽  
The Sun ◽  
The Moon ◽  
Convection Pattern ◽  
Main Sequence Stars ◽  
The Waves

AbstractBy Birkhoff’s theorem a spherical symmetric convection pattern, as it is assumed to exist for main sequence stars as our sun, cannot lead to the emission of gravitational waves, but all stars that have a magnetic field generated by a magnetohydrodynamic dynamo must by a theorem of Cowling have a non-spherical symmetric convections pattern and for this reason have to emit gravitational waves. The intensity of the thusly emitted gravitational waves depends on the efficiency of this dynamo, expressed by the departure from a spherical convection pattern. The magnitude of the asymmetry is determined by a solution of Elsaesser’s dynamo equations which only recently has become possible with supercomputers. The waves are emitted through large mass motions in the center of the sun by a thermonuclear fusion reaction-driven magnetohydrodynamic dynamo, with thermomagnetic currents in the tachocline shielding the strong magnetic field in the solar core. Using the moon as a large Weber bar, the gravitational waves are focused into the lunar shadow by Poisson diffraction where their effect might become observable during a total solar eclipse.

scholarly journals Abundance and Magnetic Field Geometries of Helium-Strong and Helium-Weak Stars

1988 ◽  
Vol 132 ◽  
pp. 309-312
Author(s):  
David A. Bohlender ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Field ◽  
High Temperature ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Hot Stars ◽  
Surface Magnetic Field ◽  
Peculiar Stars ◽  
Ap Stars

The helium-weak and helium-strong stars are main sequence stars with anomalously weak and strong helium lines for their spectral types respectively. Many members of the two classes have strong, globally ordered magnetic fields (Thompson and Landstreet 1985; Bohlender et al. 1987) and are currently thought to represent high temperature extensions of the Ap stars. In collaboration with C. T. Bolton (U. of Toronto), we have obtained high S/N phase resolved spectra of several stars using the coudé reticon detector at CFHT. One of the principle goals of this work is to determine abundance and surface magnetic field geometries of several helium peculiar stars with large, well-determined effective fields. We employ a line synthesis program (Landstreet 1987) that incorporates the effects of surface magnetic fields and non-uniform abundances on the observed line profiles of a star. Since these stars are rapid rotators the surface magnetic field strength must be inferred from differential magnetic intensification of lines with different magnetic sensitivities. Of the few lines with suitable strengths in these hot stars we have decided that the Si III multiplet 2 lines are best suited for this aspect of our investigation. We have also modelled the unblended He I line λ4437, ignoring magnetic effects for the time being. Individual results are discussed below.

scholarly journals On the stellar gravity and effective temperature dependence of the ratio of terminal to escape velocities in stellar winds

1981 ◽  
Vol 59 ◽  
pp. 75-78
Author(s):  
Luis Carrasco
Keyword(s):  
Temperature Dependence ◽  
Effective Temperature ◽  
Main Sequence ◽  
Stellar Winds ◽  
Early Type ◽  
Main Sequence Stars ◽  
Velocity Information ◽  
Early Type Stars ◽  
Fitting Parameters

Contrary to the results of some investigators, the ratio of terminal to escape velocities (V∞/Vesc) observed for the winds in early-type stars is found to be linearly correlated with log Γ - Γ being the ratio of stellar to Eddington's luminosities.Although the determination of terminal velocities for O-type main sequence stars from edge velocity information may be somewhat questionable (Lamers, 1980). The determined values for the β and γ fitting parameters for the observed profiles by Conti and Garmany (1980) are tipically on the order of one for these stars. Hence, from the grid of profiles by Castor and Lamers (1979), we estimate that at most a 10% error is introduced by adopting the edge velocities as representative values of the terminal velocities of main sequence O-type stars.

Sign in / Sign up

Export Citation Format

Share Document