scholarly journals Magnetic field observations of low-mass stars

2008 ◽  
Vol 4 (S259) ◽  
pp. 339-344
Author(s):  
Ansgar Reiners
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Magnetic Measurements ◽  
Polarized Light ◽  
M Dwarfs ◽  
Magnetic Field Generation ◽  
Low Mass Stars ◽  
Strong Magnetic Fields ◽  
Direct Measurements ◽  
Low Mass

AbstractDirect measurements of magnetic fields in low-mass stars of spectral class M have become available during the last years. This contribution summarizes the data available on direct magnetic measurements in M dwarfs from Zeeman analysis in integrated and polarized light. Strong magnetic fields at kilo-Gauss strength are found throughout the whole M spectral range, and so far all field M dwarfs of spectral type M6 and later show strong magnetic fields. Zeeman Doppler images from polarized light find weaker fields, which may carry important information on magnetic field generation in partially and fully convective stars.

scholarly journals Non-ideal magnetohydrodynamics versus turbulence – I. Which is the dominant process in protostellar disc formation?

2020 ◽  
Vol 495 (4) ◽  
pp. 3795-3806 ◽  
Author(s):  
James Wurster ◽  
Benjamin T Lewis
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Low Mass Stars ◽  
Rotation Rates ◽  
Ideal Mhd ◽  
Low Mass ◽  
Dominant Process ◽  
Ideal Magnetohydrodynamics ◽  
Disc Formation

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M⊙ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.

scholarly journals Non-ideal magnetohydrodynamics versus turbulence II: Which is the dominant process in stellar core formation?

2020 ◽  
Vol 495 (4) ◽  
pp. 3807-3818 ◽  
Author(s):  
James Wurster ◽  
Benjamin T Lewis
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Rotation Axis ◽  
Core Formation ◽  
Low Mass Stars ◽  
Stellar Core ◽  
Ideal Mhd ◽  
Low Mass ◽  
Dominant Process ◽  
Ideal Magnetohydrodynamics

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of protostars by following the gravitational collapse of 1 M⊙ gas clouds through the first hydrostatic core to stellar densities. The clouds are imposed with both rotational and turbulent velocities, and are threaded with a magnetic field that is parallel/antiparallel or perpendicular to the rotation axis; we investigate two rotation rates and four Mach numbers. The initial radius and mass of the stellar core are only weakly dependent on the initial parameters. In the models that include ideal MHD, the magnetic field strength implanted in the protostar at birth is much higher than observed, independent of the initial level of turbulence; only non-ideal MHD can reduce this strength to near or below the observed levels. This suggests that not only is ideal MHD an incomplete picture of star formation, but that the magnetic fields in low mass stars are implanted later in life by a dynamo process. Non-ideal MHD suppresses magnetically launched stellar core outflows, but turbulence permits thermally launched outflows to form a few years after stellar core formation.

scholarly journals Magnetic fields in M-dwarfs from high-resolution infrared spectroscopy

2013 ◽  
Vol 9 (S302) ◽  
pp. 170-173
Author(s):  
D. Shulyak ◽  
A. Reiners ◽  
U. Seemann ◽  
O. Kochukhov ◽  
N. Piskunov
Keyword(s):  
High Resolution ◽  
Magnetic Fields ◽  
Spectral Synthesis ◽  
Strong Line ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Depth Analysis ◽  
Low Mass ◽  
Atomic Lines

AbstractAccurate spectroscopic measurements of magnetic fields in low mass stars remain challenging because of their cool temperatures, strong line blending, and often fast rotation. This is why previous estimates were based either on the analysis of only a few lines or made use of some indirect techniques. This frequently led to noticeable scatter in obtained results. In this talk I will present and discuss new results on the determination of the intensity and geometry of the magnetic fields in M-dwarfs using IR observations obtained with CRIRES@VLT. The instrument provides unprecedented data of high resolution (R = 100 000) which is crucial for resolving individual magnetically broadened molecular and atomic lines. Such an in-depth analysis based on direct magnetic spectral synthesis opens a possibility to deduce both field intensity and geometry avoiding most of the limitation and assumptions made in previous studies.

scholarly journals Magnetic Fields in Low-Mass Stars: An Overview of Observational Biases

2013 ◽  
Vol 9 (S302) ◽  
pp. 156-163 ◽  
Author(s):  
Ansgar Reiners
Keyword(s):  
Magnetic Fields ◽  
Zeeman Effect ◽  
Field Measurements ◽  
Rotating Stars ◽  
Magnetic Field Generation ◽  
Low Mass Stars ◽  
Important Indicator ◽  
Stellar Radius ◽  
Low Mass ◽  
Rapidly Rotating Stars

AbstractStellar magnetic dynamos are driven by rotation, rapidly rotating stars produce stronger magnetic fields than slowly rotating stars do. The Zeeman effect is the most important indicator of magnetic fields, but Zeeman broadening must be disentangled from other broadening mechanisms, mainly rotation. The relations between rotation and magnetic field generation, between Doppler and Zeeman line broadening, and between rotation, stellar radius, and angular momentum evolution introduce several observational biases that affect our picture of stellar magnetism. In this overview, a few of these relations are explicitly shown, and the currently known distribution of field measurements is presented.

scholarly journals MAGNETIC FIELD TOPOLOGY IN LOW-MASS STARS: SPECTROPOLARIMETRIC OBSERVATIONS OF M DWARFS

2009 ◽  
Vol 704 (2) ◽  
pp. 1721-1729 ◽  
Author(s):  
Ngoc Phan-Bao ◽  
Jeremy Lim ◽  
Jean-François Donati ◽  
Christopher M. Johns-Krull ◽  
Eduardo L. Martín
Keyword(s):  
Magnetic Field ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Magnetic Field Topology ◽  
Low Mass

Electromagnetic induction heating of planets orbiting late M dwarfs

2018 ◽  
Vol 14 (S345) ◽  
pp. 232-233
Author(s):  
Kristina G. Kislyakova
Keyword(s):  
Magnetic Field ◽  
Energy Source ◽  
Magnetic Fields ◽  
Induction Heating ◽  
Electromagnetic Induction ◽  
Skin Depth ◽  
M Dwarfs ◽  
Planetary Interiors ◽  
Low Mass ◽  
Electromagnetic Induction Heating

AbstractWe propose induction heating of planetary interiors as an energy source in the planetary mantles. Induction heating arises when a changing magnetic field induces currents in a conducting planetary mantle which then dissipate to heat the planet, mostly within an upper layer called the skin depth. This physical process can play a role in planetary interiors around strongly magnetized stars such as low mass M dwarfs with kG magnetic fields, which are common among these stars.

scholarly journals An 84-μG Magnetic Field in a Galaxy at Z=0.692?

2008 ◽  
Vol 4 (S254) ◽  
pp. 95-96
Author(s):  
Arthur M. Wolfe ◽  
Regina A. Jorgenson ◽  
Timothy Robishaw ◽  
Carl Heiles ◽  
Jason X. Prochaska
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Faraday Rotation ◽  
Large Scale ◽  
Dynamo Model ◽  
Magnetic Field Generation ◽  
Interstellar Gas ◽  
Splitting Technique ◽  
Average Value ◽  
The Magnetic Field

AbstractThe magnetic field pervading our Galaxy is a crucial constituent of the interstellar medium: it mediates the dynamics of interstellar clouds, the energy density of cosmic rays, and the formation of stars (Beck 2005). The field associated with ionized interstellar gas has been determined through observations of pulsars in our Galaxy. Radio-frequency measurements of pulse dispersion and the rotation of the plane of linear polarization, i.e., Faraday rotation, yield an average value B ≈ 3 μG (Han et al. 2006). The possible detection of Faraday rotation of linearly polarized photons emitted by high-redshift quasars (Kronberg et al. 2008) suggests similar magnetic fields are present in foreground galaxies with redshifts z > 1. As Faraday rotation alone, however, determines neither the magnitude nor the redshift of the magnetic field, the strength of galactic magnetic fields at redshifts z > 0 remains uncertain.Here we report a measurement of a magnetic field of B ≈ 84 μG in a galaxy at z =0.692, using the same Zeeman-splitting technique that revealed an average value of B = 6 μG in the neutral interstellar gas of our Galaxy (Heiles et al. 2004). This is unexpected, as the leading theory of magnetic field generation, the mean-field dynamo model, predicts large-scale magnetic fields to be weaker in the past, rather than stronger (Parker 1970).The full text of this paper was published in Nature (Wolfe et al. 2008).

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