scholarly journals Infrared Measurements of Stellar Magnetic Fields

1994 ◽  
Vol 154 ◽  
pp. 437-447 ◽  
Author(s):  
Steven H. Saar
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Pressure Support ◽  
Field Research ◽  
Future Directions ◽  
Rotation Rates ◽  
Infrared Measurements ◽  
Cool Stars ◽  
Rs Cvn Variables ◽  
Stellar Magnetic Fields

I review the advantages, techniques, and results of measurement of magnetic fields on cool stars in the infrared (IR). These measurements have generated several important results, including the following: the first data on the magnetic parameters of dMe and RS CVn variables; evidence for field strength confinement by photospheric gas pressure; support for the correlation between magnetic flux and rotation, with possible saturation at high rotation rates; indications of horizontal and/or vertical magnetic field structure; and evidence of spatial variations in B over a stellar surface. I discuss these results in detail, and suggest future directions for IR magnetic field research.

scholarly journals New Infrared Measurements of Magnetic Fields on Cool Stars

1994 ◽  
Vol 154 ◽  
pp. 493-497 ◽  
Author(s):  
Steven H. Saar
Keyword(s):  
Magnetic Fields ◽  
Magnetic Flux ◽  
Ir Spectra ◽  
Preliminary Analysis ◽  
M Dwarfs ◽  
Pressure Balance ◽  
Infrared Measurements ◽  
Cool Stars ◽  
Rs Cvn Variables ◽  
First Time

I present a preliminary analysis of IR spectra of five K and M dwarfs and two RS CVn variables. Evidence for significant magnetic flux is found on several stars, a number of which are detected for the first time. Field strengths (B) on the RS CVn variables are lower than in the active dwarfs, consistent with the concept of pressure balance limiting B in stellar photospheres. I compare the results with previous measurements.

The long-term variation of the effective magnetic field of the active star ε Eridani

2018 ◽  
Vol 13 (S340) ◽  
pp. 39-42
Author(s):  
Cesare Scalia ◽  
Francesco Leone ◽  
Manuele Gangi
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Effective Magnetic Field ◽  
Magnetic Signal ◽  
Active Star ◽  
Cool Stars ◽  
Term Variation ◽  
Line Slope ◽  
Stellar Magnetic Fields

AbstractLong-term periodicities of magnetic fields in cool stars are usually studied from activity indicators, which are only indirectly related to the presence of the field. Direct detections are complicated issues since even a complex magnetic structure, as the solar one, has a very low disk integrated magnetic signal, which is usually hidden in the noise level. We introduce a method for the direct measurement of small integrated longitudinal stellar magnetic fields (effective magnetic fields), called multi-line slope method, based on the regression of the Stokes V signal with respect to the first derivative of Stokes I. We present the results of the application of this technique to a dataset of 9 yr of observations of the active star epsilon Eridani, obtained with the spectropolarimeters Narval, HARPSpol and CAOS, showing that the long-term variation of the effective magnetic field corresponds to the period of the cycle retrieved by the activity indicators.

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.

Science and Techniques Using Pulsed Magnetic Fields

MRS Bulletin ◽  
1993 ◽  
Vol 18 (8) ◽  
pp. 44-49 ◽  
Author(s):  
J.J.M. Franse ◽  
N. Miura
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
High Field ◽  
Materials Science ◽  
Copper Wire ◽  
Field Research ◽  
Outer Diameter ◽  
Electricity Grid ◽  
Pulsed Magnet ◽  
Time Profiles

In this article, we examine materials behavior in the magnetic field region from about 40 T to 500 T using pulsed magnet technology. Examples of materials science using two different pulsed magnet technologies are described in this article.Semicontinuous MagnetsSince the late 1960s, the University of Amsterdam has operated a semicontinuous magnetic field installation that produces magnetic fields up to 40 T with typical time constants of about one second. The magnet coil is constructed from hard-drawn copper wire with a reinforcement cylinder of maraging steel positioned at roughly one third of the outer diameter. Before operation, the coil is cooled to 30 K by cold neon gas. The power for this installation is taken directly from a 10 kV connection to the public electricity grid. By means of a thyristor-based power control system, highly flexible field-time profiles can be realized: step-wise pulses can be generated with field levels constant within 10−4 during 100 ms; linearly increasing and decreasing fields as well as exponentially ripple-free decreasing fields are other examples of standard field-time profiles. Among the measuring techniques frequently used are magnetization, magneto-transport, quantum oscillations, relaxation phenomena, etc. Temperatures at which experiments can be performed range from 400 mK to room temperature. In the Netherlands, the Amsterdam High Field Facility has recently been combined with the High Magnetic Field Laboratory in Nijmegen, where static magnetic fields up to 30 T are produced in hybrid magnet systems, to form the Amsterdam-Nijmegen Magnet Laboratory (ANML). The high field research of ANML comprises semiconductors, magnetism in transition-metal compounds, heavy-fermion physics, superconductors, organic conductors, and magnetic separation. We present here a few selected topics.

scholarly journals Clues to the origin and properties of magnetic white dwarfs

2019 ◽  
Vol 15 (S357) ◽  
pp. 60-74
Author(s):  
Adela Kawka
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
White Dwarfs ◽  
Field Structure ◽  
Complex Structures ◽  
Surface Mapping ◽  
Time Resolved ◽  
Single Star ◽  
Rotation Rates ◽  
Star Evolution

AbstractA significant fraction of white dwarfs possess a magnetic field with strengths ranging from a few kG up to about 1000 MG. However, the incidence of magnetism varies when the white dwarf population is broken down into different spectral types providing clues on the formation of magnetic fields in white dwarfs. Several scenarios for the origin of magnetic fields have been proposed from a fossil field origin to dynamo generation at various stages of evolution. Offset dipoles are often assumed sufficient to model the field structure, however time-resolved spectropolarimetric observations have revealed more complex structures such as magnetic spots or multipoles. Surface mapping of these field structures combined with measured rotation rates help distinguish scenarios involving single star evolution from other scenarios involving binary interactions. I describe key observational properties of magnetic white dwarfs such as age, mass, and field strength, and confront proposed formation scenarios with these properties.

scholarly journals Observational methods for stellar magnetism: from detection to cartography

2012 ◽  
Vol 8 (S294) ◽  
pp. 447-458 ◽  
Author(s):  
Klaus G. Strassmeier ◽  
Thorsten A. Carroll ◽  
Ilya Ilyin ◽  
Silva Järvinen
Keyword(s):  
High Resolution ◽  
Magnetic Fields ◽  
Inverse Modeling ◽  
Doppler Imaging ◽  
Observational Methods ◽  
Related Data ◽  
Cool Stars ◽  
Stellar Magnetic Fields

AbstractWe review some of the currently used techniques to detect stellar magnetic fields on cool stars. Emphasis is put on spectropolarimetry with high-resolution spectrographs and its related data de-noising techniques and multi-line inverse modeling. Detections and results from Zeeman splittings and broadenings are briefly mentioned. We discuss some of our most recent Zeeman Doppler Imaging (ZDI) results and present a comparison of ZDI maps of the K-type WTTS V410 Tauri and the planet-hosting F8 star HD 179949 with results from other groups.

scholarly journals Planetary protection in the extreme environments of low-mass stars

2013 ◽  
Vol 9 (S302) ◽  
pp. 237-238
Author(s):  
A. A. Vidotto ◽  
M. Jardine ◽  
J. Morin ◽  
J.-F. Donati ◽  
P. Lang ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Minimum Degree ◽  
Extreme Environments ◽  
Terrestrial Planet ◽  
Outer Edge ◽  
Low Mass Stars ◽  
Planetary Protection ◽  
The One ◽  
Stellar Magnetic Fields

AbstractRecent results showed that the magnetic field of M-dwarf (dM) stars, currently the main targets in searches for terrestrial planets, is very different from the solar one, both in topology as well as in intensity. In particular, the magnetised environment surrounding a planet orbiting in the habitable zone (HZ) of dM stars can differ substantially to the one encountered around the Earth. These extreme magnetic fields can compress planetary magnetospheres to such an extent that a significant fraction of the planet's atmosphere may be exposed to erosion by the stellar wind. Using observed surface magnetic maps for a sample of 15 dM stars, we investigate the minimum degree of planetary magnetospheric compression caused by the intense stellar magnetic fields. We show that hypothetical Earth-like planets with similar terrestrial magnetisation (~1 G) orbiting at the inner (outer) edge of the HZ of these stars would present magnetospheres that extend at most up to 6.1 (11.7) planetary radii. To be able to sustain an Earth-sized magnetosphere, the terrestrial planet would either need to orbit significantly farther out than the traditional limits of the HZ; or else, if it were orbiting within the life-bearing region, it would require a minimum magnetic field ranging from a few G to up to a few thousand G.

scholarly journals On the influence of magnetic fields in neutral planetary wakes

2016 ◽  
Vol 12 (S328) ◽  
pp. 192-197
Author(s):  
C. Villarreal D’Angelo ◽  
M. Schneiter ◽  
A. Esquivel
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Radiation Pressure ◽  
Stellar Wind ◽  
Magnetic Moments ◽  
Radiative Processes ◽  
Planetary Magnetic Fields ◽  
Stellar Magnetic Fields

AbstractWe present a 3D magnetohydrodynamic study of the effect that stellar and planetary magnetic fields have on the calculated Lyα absorption during the planetary transit, employing parameters that resemble the exoplanet HD209458b. We assume a dipolar magnetic field for both the star and the planet, and use the Parker solution to initialize the stellar wind. We also consider the radiative processes and the radiation pressure.We use the numerical MHD code Guacho to run several models varying the values of the planetary and stellar magnetic moments within the range reported in the literature.We found that the presence of magnetic fields influences the escaping neutral planetary material spreading the absorption Lyα line for large stellar magnetic fields.

scholarly journals Some Developments of the Weak Stellar Magnetic Field Determination Method for the Example of Cygnus X-1

2011 ◽  
Vol 7 (S282) ◽  
pp. 75-76
Author(s):  
N. G. Bochkarev ◽  
E. A. Karitskaya
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Least Square ◽  
Determination Method ◽  
Optical Counterpart ◽  
X Ray ◽  
Stellar Magnetic Fields

AbstractSome developments of measurements of the weak stellar magnetic fields by the least square technique applied to spectropolarimetric data are proposed and used for the X-ray binary Cyg X-1 = HDE 226868 (the optical counterpart is an O 9.7 supergiant).

scholarly journals Measuring T Tauri star magnetic fields

2008 ◽  
Vol 4 (S259) ◽  
pp. 345-356 ◽  
Author(s):  
Christopher M. Johns–Krull
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Scaling Laws ◽  
Main Sequence ◽  
Critical Role ◽  
X Rays ◽  
Stellar Rotation ◽  
T Tauri ◽  
Dipole Component ◽  
Stellar Magnetic Fields

AbstractStellar magnetic fields including a strong dipole component are believed to play a critical role in the early evolution of newly formed stars and their circumstellar accretion disks. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star preferentially at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked to the Keplerian velocity near the radius where the disk is truncated. This paper reviews recent efforts to measure the magnetic field properties of low mass pre-main sequence stars, focussing on how the observations compare with the theoretical expectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on these stars; however, the dipole component of the field appears to be alarmingly small. The current measurements also suggest that given their strong magnetic fields, T Tauri stars are somewhat faint in X-rays relative to what is expected from simple main sequence star scaling laws.

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