Observations of Strong Magnetic Fields in Nondegenerate Stars

Chapter
Part of the Space Sciences Series of ISSI book series (SSSI, volume 54)

Abstract

We review magnetic-field measurements of nondegenerate stars across the Hertzprung–Russell diagram for main sequence, premain sequence, and postmain sequence stars. For stars with complex magnetic-field morphologies, which includes all G–M main sequence stars, the analysis of spectra obtained in polarized vs unpolarized light provides very different magnetic measurements because of the presence or absence of cancellation by oppositely directed magnetic fields within the instrument’s spatial resolution. This cancellation can be severe, as indicated by the spatially averaged magnetic field of the Sun viewed as a star. These averaged fields are smaller by a factor of 1000 or more compared to spatially resolved magnetic-field strengths. We explain magnetic-field terms that characterize the fields obtained with different measurement techniques. Magnetic fields typically control the structure of stellar atmospheres in and above the photosphere, the heating rates of stellar chromospheres and coronae, mass and angular momentum loss through stellar winds, chemical peculiarity, and the emission of high energy photons, which is critically important for the evolution of protoplanetary disks and the habitability of exoplanets. Since these effects are governed by the star’s magnetic energy, which is proportional to the magnetic-field strength squared and its fractional surface coverage, it is important to measure or reliably infer the true magnetic-field strength and filling factor across a stellar disk. We summarize magnetic-field measurements obtained with the different observing techniques for different types of stars and estimate the highest magnetic-field strengths. We also comment on the different field morphologies observed for stars across the H–R diagram, typically inferred from Zeeman-Doppler imaging and rotational modulation observations.

Keywords

Magnetic fields Pre-main sequence stars Main sequence stars Post-main sequence stars 

List of Acronyms and Abbreviations

BBLP

Broad-band linear polarization

BOB

B fields in OB stars consortium

CFHT

Canada-France-Hawaii Telescope

CP

Chemically preculiar A- and B-type stars

CRIRES

CRyogenic high-resolution InfraRed Echelle Spectrograph

DI

Doppler imaging

ESO

European Southern Observatory

\(\mathrm{ESP}_{a}\mathrm{DO}_{n}\mathrm{S}\)

a high-resolution spectrograph on the CFHT telescope

FEROS

Fiber-fed Extended Range Optical Spectrograph

FORS1

FOcal Reducer and low dispersion Spectrograph

FORS2

New version of FORS1

FUV

Far ultraviolet (912–1700 Å)

GTO

Guaranteed time observer

HARPS

High Accuracy Radial Velocity Planet Searcher

HARPSpol

HARPS with polarizing optics

HINODE

Solar-B satellite

IVQU

Components of the Stokes polarimetry vector

LTE

Local thermodynamic equilibrium

LSD

Least-squares deconvolution

MDI

Michelson Doppler imager

MHD

Magnetohydrodynamics

MiMeS

Magnetism in Massive Stars collaboration

MOST

Microvariability and Oscillations of STars satellite

NIM

Near Infrared Magnetograph

NSO

National Solar Observatory

RGB

Red giant branch stars

SB

Spectroscopic binary stars

SOFIN

An optical high-resolution spectrograph on the Nordic Optical Telescope

SOHO

Solar and Heliospheric Observatory

SOT

Solar Optical Telescope

SOT/SP

SOT Spectropolarimeter

SPB

Slowly pulsating B-type stars

SVD

Singular value decomposition method for Stokes profile reconstruction

UV

Ultraviolet (912–3000 Å)

VLT

Very Large Telescope

ZAMS

Zero-age main sequence

ZDI

Zeeman Doppler imaging

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Copyright information

© Springer Science+Business Media New York 2016

Authors and Affiliations

  1. 1.JILA/University of Colorado and NISTBoulderUSA
  2. 2.European Southern ObservatoryGarchingGermany

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