scholarly journals Convective cells in Betelgeuse: imaging through spectropolarimetry

2018 ◽  
Vol 620 ◽  
pp. A199 ◽  
Author(s):  
A. López Ariste ◽  
P. Mathias ◽  
B. Tessore ◽  
A. Lèbre ◽  
M. Aurière ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Linear Polarization ◽  
Rayleigh Scattering ◽  
Characteristic Size ◽  
Spectral Lines ◽  
Stellar Disk ◽  
Convective Structures ◽  
Weak Magnetic Fields ◽  
Red Supergiants ◽  
Convective Cells

Aims. We assess the ability to image the photosphere of red supergiants and, in particular Betelgeuse, through the modelling of the observed linear polarization in atomic spectral lines. We also aim to analyse the resulting images over time, to measure the size and dynamics of the convective structures in these stars. Methods. Rayleigh scattering polarizes the continuum and spectral lines depolarize it. This depolarization is seen as a linear polarization signal parallel to the radial direction on the stellar disk. Integrated over the disk, it would result in a null signal, except if brightness asymmetries/inhomogeneities are present. This is the basic concept behind our imaging technique. Through several tests and comparisons, we have tried to assess and extend its validity, and to determine what can be learnt unambiguously through it. Results. The several tests and comparisons performed prove that our technique reliably retrieves the salient brightness structures in the photosphere of Betelgeuse, and should be relevant to other red supergiants. For Betelgeuse, we demonstrate that these structures we infer are convective cells, with a characteristic size of more than 60% of the stellar radius. We also derive the characteristic upflow and downflow speeds, 22 and 10 km s−1, respectively. We find weak magnetic fields concentrated in the downflow lanes in between granules, similar to the quiet sun magnetism. We follow those convective structures in time. Changes happen on timescales of 1 week, but individual structures can be tracked over 4 yr of observations. Conclusions. The measured characteristics of the convection in Betelgeuse confirm the predictions of numerical simulations in both the strong, supersonic upflows and the size of the convective cells. They also concur in the presence of weak magnetic fields that are completely dominated by the convective flows and constrained to the dark lanes of down-flowing plasma.

scholarly journals Ultra-weak magnetic fields in Am stars: β UMa and θ Leo

2014 ◽  
Vol 10 (S305) ◽  
pp. 67-72 ◽  
Author(s):  
A. Blazère ◽  
P. Petit ◽  
F. Lignières ◽  
M. Aurière ◽  
J. Ballot ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Large Fraction ◽  
Spectral Lines ◽  
Line Profiles ◽  
Circularly Polarized ◽  
Structure And Dynamics ◽  
Weak Magnetic Fields ◽  
Magnetic Origin ◽  
The One ◽  
Ap Stars

AbstractAn extremely weak circularly-polarized signature was recently detected in the spectral lines of the Am star Sirius A. With a prominent positive lobe, the shape of the phase-averaged Stokes V line profile is atypical of stellar Zeeman signatures, casting doubts on its magnetic origin. We report here on ultra-deep spectropolarimetric observations of two more bright Am stars: β Uma and θ Leo. Stokes V line signatures are detected in both objects, with a shape and amplitude similar to the one observed on Sirius A. We demonstrate that the amplitude of the Stokes V line profiles depend on various line parameters (Landé factor, wavelength, depth) as expected from a Zeeman signature, confirming that extremely weak magnetic fields are likely present in a large fraction of Am stars. We suggest that the strong asymmetry of the polarized signatures, systematically observed so far in Am stars and never reported in strongly magnetic Ap stars, bears unique information about the structure and dynamics of the thin surface convective shell of Am stars.

An Operator Perturbation Method of Polarized Line Transfer V. Diagnosis of Solar Weak Magnetic Fields

2000 ◽  
Vol 179 ◽  
pp. 255-258
Author(s):  
K. N. Nagendra ◽  
H. Frisch ◽  
M. Faurobert-Scholl ◽  
F. Paletou
Keyword(s):  
Magnetic Fields ◽  
Perturbation Method ◽  
Solar Atmosphere ◽  
Iteration Method ◽  
Resonance Scattering ◽  
Spectral Lines ◽  
Perturbation Approach ◽  
Weak Magnetic Fields ◽  
Line Transfer

AbstractWe present an application of the PALI (Polarized Approximate Lambda Iteration) method to the resonance scattering in spectral lines formed in the presence of weak magnetic fields. The method is based on an operator perturbation approach, and can efficiently give solutions for oriented vector magnetic fields in the solar atmosphere.

The Hanle Effect and the Determination of Magnetic Fields

1979 ◽  
Vol 44 ◽  
pp. 87-91 ◽  
Author(s):  
V. Bommier ◽  
S. Sahal-Bréchot
Keyword(s):  
Magnetic Fields ◽  
Linear Polarization ◽  
Optical Pumping ◽  
Polarization Vector ◽  
Resonance Scattering ◽  
Incident Radiation ◽  
Research Area ◽  
Spectral Lines ◽  
Hanle Effect

A new research area has recently been developed in solar physics, i.e. the determination of magnetic fields by means of the interpretation of the two measured parameters describing the linear polarization of spectral lines (Leroy et al., 1977, Sahal-Bréchot et al., 1977, Leroy 1977a, b, 1978, Stenflo 1977, 1978, Sahal-Bréchot and Bommier 1977, Bommier and Sahal-Bréchot 1978). In solar prominences, this linear polarization is due to resonance scattering of the incident radiation field; the effect of the magnetic field B (Hanle effect) leads to a rotation of the polarization vector and a decrease of the degree of linear polarization. Both these parameters do depend on the strength and the direction of B and the dependence may be calculated using the theory of quantum optics and optical pumping.

scholarly journals Coherence Properties of Polarized Radiation in Weak Magnetic Fields

1971 ◽  
Vol 43 ◽  
pp. 130-140 ◽  
Author(s):  
Lewis L. House
Keyword(s):  
Magnetic Fields ◽  
Linear Polarization ◽  
Scattered Radiation ◽  
Scattering Function ◽  
Polarization Measurements ◽  
Weak Magnetic Fields ◽  
Frequency Independent ◽  
Initial Work ◽  
Redistribution Function ◽  
Polarized Radiation

The scattering of radiation in the presence of weak magnetic fields can give rise to coherence or interference phenomena that will profoundly affect the frequency, geometric, and polarization properties of the scattering event. In this paper we discuss and illustrate some of the features of the coherence phenomena associated with the scattering redistribution for the normal Zeeman triplet. The frequency dependent as well as the frequency independent scattering function is considered in a linear polarization basis. In addition we illustrate some properties of this redistribution function in the Stokes representation. Since the primary purpose of this paper is to demonstrate the nature of some of the properties of the coherence problems, that might be important in the interpretation of magnetic fields from polarization measurements of scattered radiation, it has been necessary in this initial work to neglect several features of the problem which are noted in the paper and are currently under investigation.

The Interpretation of Line Profiles

1977 ◽  
Vol 36 ◽  
pp. 191-215
Author(s):  
G.B. Rybicki
Keyword(s):  
Magnetic Fields ◽  
Atomic Physics ◽  
Velocity Fields ◽  
Spectral Lines ◽  
Inhomogeneous Structure ◽  
The Sun ◽  
Line Profiles ◽  
Physical Phenomena

Observations of the shapes and intensities of spectral lines provide a bounty of information about the outer layers of the sun. In order to utilize this information, however, one is faced with a seemingly monumental task. The sun’s chromosphere and corona are extremely complex, and the underlying physical phenomena are far from being understood. Velocity fields, magnetic fields, Inhomogeneous structure, hydromagnetic phenomena – these are some of the complications that must be faced. Other uncertainties involve the atomic physics upon which all of the deductions depend.

Tuning the Magnetic Alignment of Cellulose Nanocrystals from Perpendicular to Parallel Using Lepidocrocite Nanoparticles

2019 ◽  
Author(s):  
Valentina Guccini ◽  
Sugam Kumar ◽  
Yulia Trushkina ◽  
Gergely Nagy ◽  
Christina Schütz ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Cellulose Nanocrystals ◽  
Small Angle Neutron Scattering ◽  
Lower Amount ◽  
Magnetic Alignment ◽  
Polarized Optical Microscopy ◽  
Parallel Orientation ◽  
Weak Magnetic Fields

The magnetic alignment of cellulose nanocrystals (CNC) and lepidocrocite nanorods (LpN), pristine and in hybrid suspensions has been investigated using contrast-matched small-angle neutron scattering (SANS) under in situ magnetic fields (0 – 6.8 T) and polarized optical microscopy. The pristine CNC (diamagnetic) and pristine LpN (paramagnetic) align perpendicular and parallel to the direction of field, respectively. The alignment of both the nanoparticles in their hybrid suspensions depends on the relative amount of the two components (CNC and LpN) and strength of the applied magnetic field. In the presence of 10 wt% LpN and fields < 1.0 T, the CNC align parallel to the field. In the hybrid containing lower amount of LpN (1 wt%), the ordering of CNC is partially frustrated in all range of magnetic field. At the same time, the LpN shows both perpendicular and parallel orientation, in the presence of CNC. This study highlights that the natural perpendicular ordering of CNC can be switched to parallel by weak magnetic fields and the incorporation of paramagnetic nanoparticle as LpN, as well it gives a method to influence the orientation of LpN.<br>

scholarly journals Sensitivity of PS/CoPd Janus particles to an external magnetic field

RSC Advances ◽  
2021 ◽  
Vol 11 (28) ◽  
pp. 17051-17057
Author(s):  
Anna Eichler-Volf ◽  
Yara Alsaadawi ◽  
Fernando Vazquez Luna ◽  
Qaiser Ali Khan ◽  
Simon Stierle ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Magnetic Properties ◽  
External Magnetic Field ◽  
Magnetic Fields ◽  
Janus Particles ◽  
Weak Magnetic Fields

PS/CoPd Janus particles respond very sensitively to application of low external magnetic fields. Owing to the magnetic properties, the PS/CoPd particles may be used, for example, to sense the presence of weak magnetic fields as micro-magnetometers.

Off-diagonal magnetoimpedance in amorphous wires and its application in miniature sensors of weak magnetic fields

2014 ◽  
Vol 78 (11) ◽  
pp. 1169-1173 ◽  
Author(s):  
N. A. Yudanov ◽  
A. A. Rudyonok ◽  
L. V. Panina ◽  
A. T. Morchenko ◽  
A. V. Kolesnikov ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Amorphous Wires ◽  
Weak Magnetic Fields

scholarly journals Broadband Linear Polarization in Ap Stars

1993 ◽  
Vol 138 ◽  
pp. 305-309
Author(s):  
Marco Landolfi ◽  
Egidio Landi Degl’Innocenti ◽  
Maurizio Landi Degl’Innocenti ◽  
Jean-Louis Leroy ◽  
Stefano Bagnulo
Keyword(s):  
Magnetic Field ◽  
Circular Polarization ◽  
Linear Polarization ◽  
Rotation Axis ◽  
Spectral Lines ◽  
Line Of Sight ◽  
Long Time ◽  
Rotating Star ◽  
Ap Stars ◽  
The Magnetic Field

AbstractBroadband linear polarization in the spectra of Ap stars is believed to be due to differential saturation between σ and π Zeeman components in spectral lines. This mechanism has been known for a long time to be the main agent of a similar phenomenon observed in sunspots. Since this phenomenon has been carefully calibrated in the solar case, it can be confidently used to deduce the magnetic field of Ap stars.Given the magnetic configuration of a rotating star, it is possible to deduce the broadband polarization at any phase. Calculations performed for the oblique dipole model show that the resulting polarization diagrams are very sensitive to the values of i (the angle between the rotation axis and the line of sight) and β (the angle between the rotation and magnetic axes). The dependence on i and β is such that the four-fold ambiguity typical of the circular polarization observations ((i,β), (β,i), (π-i,π-β), (π-β,π-i)) can be removed.

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