scholarly journals Magnetic Fields and Stellar Evolution

1981 ◽  
Vol 93 ◽  
pp. 257-272 ◽  
Author(s):  
L. Mestel
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Late Type ◽  
Main Sequence Stars ◽  
Stellar Objects ◽  
Cool Stars ◽  
Solar Type ◽  
Early Type Stars ◽  
M Stars

Magnetic fields are now observed or inferred in a wide variety of stellar objects. The class of early-type stars with strong large-scale fields extends from types B to F, with effective fields from 300 gauss up to several x 104 gauss (Borra and Landstreet 1980). Fields between 4 × 106 and 108 gauss have been inferred in a small percentage of white dwarfs, and of over 1012 gauss in neutron stars. Some Cepheids show measurable fields. Evidence has built up of solar-type activity in late-type stars. The pioneering work by Wilson (1978) on Ca activity has shown convincingly the occurrence of periodicity reminiscent of the solar cycle in a number of G, K and M stars. Ca II emission appears to be a good predictor of simultaneous X-ray emission from hot coronae around cool stars (Vaiana 1979, Mewe and Zwaan 1980). Fields of some 2 × 103 gauss have been reported in two late-type main sequence stars (Robinson, Worden and Harvey 1980).

scholarly journals Magnetic main sequence stars as progenitors of blue supergiants

2014 ◽  
Vol 9 (S307) ◽  
pp. 391-392
Author(s):  
I. Petermann ◽  
N. Castro ◽  
N. Langer
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Mass Range ◽  
Main Sequence Stars ◽  
Core Mass ◽  
Hydrogen Burning ◽  
Sequence Band ◽  
Convective Core ◽  
The Right

AbstractBlue supergiants (BSGs) to the right the main sequence band in the HR diagram can not be reproduced by standard stellar evolution calculations. We investigate whether a reduced convective core mass due to strong internal magnetic fields during the main sequence might be able to recover this population of stars. We perform calculations with a reduced mass of the hydrogen burning convective core of stars in the mass range 3–30 M⊙ in a parametric way, which indeed lead to BSGs. It is expected that these BSGs would still show large scale magnetic fields in the order of 10 G.

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.

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).

Observational constraints for solar-type Stellar winds

2019 ◽  
Vol 15 (S354) ◽  
pp. 313-332
Author(s):  
Manuel Güdel
Keyword(s):  
Solar Wind ◽  
Cosmic Rays ◽  
Main Sequence ◽  
Galactic Cosmic Rays ◽  
Observational Constraints ◽  
Stellar Winds ◽  
Late Type ◽  
Main Sequence Stars ◽  
Planetary Environments ◽  
Solar Type

AbstractIonized winds from late-type main-sequence stars are important for stellar spin-down and therefore the evolution of stellar activity; winds blow an “astrosphere” into the interstellar medium that absorbs a large part of galactic cosmic rays; and the winds play a key role in shaping planetary environments, in particular their upper atmospheres. These issues have been well studied for the solar wind but little is known about winds escaping from other solar-type stars. Several methods have been devised to either detect winds directly or to infer the presence of such winds from features that are shaped by the winds. This paper summarizes these methods and discusses exemplary findings. There is need for more studies using multiple methods for the same stars.

Magnetic Star Models with Toroidal Component of Field

1976 ◽  
Vol 32 ◽  
pp. 25-27
Author(s):  
D.L. Moss
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Field Component ◽  
Main Sequence ◽  
Toroidal Field ◽  
Magnetic Star ◽  
Main Sequence Stars ◽  
Star Models ◽  
Rotation Axes ◽  
Poloidal Magnetic Fields

A variety of models of main sequence stars of a few solar masses containing large scale magnetic fields have been published in the last few years. Most of them have contained purely poloidal magnetic fields (with magnetic and rotation axes aligned). Those containing a toroidal field component were either polytropic models, or, if more realistic, the toroidal field was not well behaved at infinity.

scholarly journals Magnetic Fields and Filling Factors in Late-Type Stars: Predictions from Dynamo Theory

1991 ◽  
Vol 130 ◽  
pp. 430-434
Author(s):  
B. Montesinos ◽  
J.L. Fernández-Villacañas ◽  
C. Jordan
Keyword(s):  
Angular Velocity ◽  
Magnetic Fields ◽  
Filling Factor ◽  
Main Sequence ◽  
Dynamo Model ◽  
Late Type ◽  
Main Sequence Stars ◽  
Dynamo Theory ◽  
Theoretical Approaches

AbstractIn this paper we examine the link between observed magnetic fields and filling factors in main-sequence stars of types G and K, and the results obtained for these parameters from a simple dynamo model. We explore how the predicted magnetic fluxes, fB, for a selected sample of stars, vary with rotation, adopting two theoretical approaches to estimate the filling factor, and considering different expressions for the variation of the stellar angular velocity, ω, with depth.

scholarly journals The search for magnetic fields in mercury-manganese stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 202-203
Author(s):  
Vitalii Makaganiuk ◽  
Oleg Kochukhov ◽  
Nikolai Piskunov ◽  
Sandra V. Jeffers ◽  
Christopher M. Johns-Krull ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Chemical Elements ◽  
Field Measurements ◽  
Atomic Diffusion ◽  
Main Sequence Stars ◽  
Weak Magnetic Fields ◽  
Ap Stars ◽  
Spot Formation

AbstractMercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars.

scholarly journals Observations of Magnetic Fields on Late-Type Stars

1983 ◽  
Vol 102 ◽  
pp. 1-22
Author(s):  
Geoffrey W. Marcy
Keyword(s):  
Magnetic Fields ◽  
Flux Tube ◽  
Spectral Type ◽  
Transport Mechanism ◽  
Main Sequence ◽  
Energy Transport ◽  
Slow Mode ◽  
Late Type ◽  
Main Sequence Stars ◽  
X Ray

The “Robinson” method for measuring magnetic fields on solar- and late-type stars is reviewed. The results of such measurements for a sample of 29 G and K main-sequence stars are presented. The area covering-factors of magnetic regions are greater in the K dwarfs than in the G dwarfs, but no spectral-type dependence is found for the field strengths, contrary to expectations of some flux-tube models. The dependence of Ca II H and K emission on magnetic fields and Teff is consistent with theoretical expectations for “slow-mode” mhd wave-generation rates, but inconsistent with those of other mhd modes. Coronal soft X-ray fluxes correlate well with the magnetic fields, and it is argued that Alfvén waves are the likely energy-transport mechanism. Surface magnetic fluxes vary with rotation as , depending on spectral type.

scholarly journals The Boundary between the Magnetic Fields of Ap Stars and the Fields of Solar-type Stars

1991 ◽  
Vol 130 ◽  
pp. 342-346
Author(s):  
John D. Landstreet
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Convection Zone ◽  
Frequency Of Occurrence ◽  
Main Sequence Stars ◽  
Chemically Peculiar ◽  
Solar Type ◽  
F Stars ◽  
Ap Stars

AbstractThe boundary between Ap-type magnetic fields and the magnetic fields of solar-type stars occurs near Te ~ 7000K, about where deep envelope convection develops in main sequence stars. This seems natural for solar-type stars, in which the field is generated by the convection zone. However, among magnetic Ap stars the frequency of occurrence declines from about 10% of all A stars near A0 to about 1% near F0. It is not clear what produces this decline in frequency, but the convection zone is probably not responsible. In fact, it seems likely that if global fossil fields occur in main sequence F stars, such fields should be detectable even if the stars having them are not chemically peculiar.

scholarly journals The role of magnetic fields in pre-main sequence stars

2013 ◽  
Vol 9 (S302) ◽  
pp. 25-37 ◽  
Author(s):  
Gaitee A. J. Hussain ◽  
Evelyne Alecian
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Circumstellar Disks ◽  
T Tauri Stars ◽  
Main Sequence Stars ◽  
T Tauri ◽  
First Results ◽  
Pms Stars

AbstractStrong, kilo-Gauss, magnetic fields are required to explain a range of observational properties in young, accreting pre-main sequence (PMS) systems. We review the techniques used to detect magnetic fields in PMS stars. Key results from a long running campaign aimed at characterising the large scale magnetic fields in accreting T Tauri stars are presented. Maps of surface magnetic flux in these systems can be used to build 3-D models exploring the role of magnetic fields and the efficiency with which magnetic fields can channel accretion from circumstellar disks on to young stars. Long-term variability in T Tauri star magnetic fields strongly point to a dynamo origin of the magnetic fields. Studies are underway to quantify how changes in magnetic fields affect their accretion properties. We also present the first results from a new programme that investigates the evolution of magnetic fields in intermediate mass (1.5–3M⊙) pre-main sequence stars as they evolve from being convective T Tauri stars to fully radiative Herbig AeBe stars.

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