Multiple, short-lived “stellar prominences” on O stars: the supergiant λ Cephei

AbstractMany OB stars show unexplained cyclical variability in their winds and in many optical lines, which are formed at the base of the wind. For these stars no dipolar magnetic fields have been detected. We propose that these cyclical variations are caused by the presence of multiple, transient, short-lived, corotating magnetic loops, which we call “stellar prominences”. We present a simplified model representing these prominences as corotating spherical blobs and fit the rapid variability in the Heiiλ4686 line of the O supergiant λ Cep for time-resolved spectra obtained in 1989. Our conclusions are: (1) From model fits we find that the life time of the prominences varies, and is between 2–7 h. (2) The adopted inclination angle is 68° with a rotation period of ≈ 4.1 d (but not well constrained). (3) The contribution of non-radial pulsations is negligible (4) Similar behavior is observed in at least 4 other O stars. We propose that prominences are a common phenomenon among O stars.

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Investigating the origin of cyclical spectral variations in hot, massive stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002439 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 334-337

Author(s):

Alexandre David-Uraz ◽

Gregg A. Wade ◽

Véronique Petit ◽

Asif ud-Doula

Keyword(s):

Magnetic Fields ◽

Large Scale ◽

Massive Stars ◽

Physical Processes ◽

Corotating Interaction Regions ◽

Ob Stars ◽

Wind Variability ◽

Competing Hypotheses ◽

Interaction Regions ◽

Radial Pulsations

AbstractOB stars are known to exhibit various types of wind variability, as detected in their ultraviolet spectra, amongst which are the ubiquitous discrete absorption components (DACs). These features have been associated with large-scale azimuthal structures extending from the base of the wind to its outer regions: corotating interaction regions (CIRs). There are several competing hypotheses as to which physical processes may perturb the star's surface and generate CIRs, including magnetic fields and non radial pulsations (NRPs), the subjects of this paper with a particular emphasis on the former. Although large-scale magnetic fields are ruled out, magnetic spots deserve further investigation, both on the observational and theoretical fronts.

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Frequency Analysis of the Rapidly Oscillating Ap Star HD 60435

International Astronomical Union Colloquium ◽

10.1017/s0252921100091600 ◽

1986 ◽

Vol 90 ◽

pp. 239-242

Author(s):

Jaymie M. Matthews ◽

Donald W. Kurtz ◽

William H. Wehlau

Keyword(s):

South African ◽

Frequency Spectrum ◽

Rotation Period ◽

Frequency Spectra ◽

Rapid Variability ◽

University Of Toronto ◽

The University ◽

Ap Stars ◽

Radial Pulsations ◽

Equal Spacing

AbstractThe cool Ap star HD 60435 was monitored in a programme of rapid B photometry during 18 nights in January/February 1984, from two stations widely spaced in longitude (the University of Toronto 0.6-m telescope at the Carnegie Southern Observatory (CARSO) on Las Campanas, Chile, and the 0.5-m telescope of the South African Astronomical Observatory ( SAAO)). On six of those nights, contiguous light curves from both sites were obtained.Fourier analysis of these data confirms the rapid variability first reported by Kurtz (1984) and reveals several additional transient oscillations. HD 60435 exhibits persistent - but modulated - oscillations at a frequency near 1.4 IRHZ (period = 11.9 minutes), and short-lived oscillations at frequencies near 1.1 and 4.2 mHz (periods of 15.2 and 4.0 minutes, respectively). These latter two periods represent the longest and shortest yet observed in the class of rapidly oscillating Ap stars.We have applied the oblique pulsator model (Kurtz 1982) to the fine-scale splittings detected in the frequency spectra of the 1.4 and 1.1 mHz oscillations. Also, the series of frequencies close to 1.4 mHz which fall into a pattern of roughly equal spacing is compared to such spacings predicted for overtones in pulsating main-sequence A stars (Shibahashi and Saio 1984). Both approaches suggest that HD 60435 is undergoing non-radial pulsations of odd and even degree (probably with ℓ ≲ 3).The oblique pulsator interpretation of the splittings in the frequency spectrum and the amplitude modulation of the 1.4 mHz oscillations also predict: a rotation period of approximately eight days for this star. Mean photometry of HD 60435, collected by the authors, supports a similar value of 7.7 days for the period.Analysis of the oscillations is hampered by ambiguities due to daily aliases present in the data, and by the complicated structure and time-dependence of the frequency spectrum. Further observations of HD 60435 are essential if we are to fully understand its rapid variations.

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“Stellar Prominences” on OB starsto explain wind-line variability

Proceedings of the International Astronomical Union ◽

10.1017/s1743921314002282 ◽

2013 ◽

Vol 9 (S302) ◽

pp. 280-283 ◽

Cited By ~ 2

Author(s):

H. F. Henrichs ◽

N. P. Sudnik

Keyword(s):

Magnetic Fields ◽

Simplified Model ◽

Supporting Evidence ◽

Photometric Variability ◽

B Stars ◽

Bright Spots ◽

Upper Limits ◽

Cyclical Variability ◽

Magnetic Origin ◽

Absorption Features

AbstractMany O and B stars show unexplained cyclical variability in their winds, i.e. modulation of absorption features on the rotational timescale, but not strictly periodic over longer timescales. For these stars no dipolar magnetic fields have been detected, with upper limits below 300 G. Similar cyclical variability is also found in many optical lines, which are formed at the base of the wind. We propose that these cyclical variations are caused by the presence of multiple, transient, short-lived, corotating magnetic loops, which we call “stellar prominences”. We present a simplified model representing these prominences to explain the cyclical optical wind-line variability in the O supergiant λ Cephei. Other supporting evidence for such prominences comes from the recent discovery of photometric variability in a comparable O star, which was explained by the presence of multiple transient bright spots, presumably of magnetic origin as well.

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Optical Imaging of Magnetic Particle Cluster Oscillation and Rotation in Glycerol

Journal of Imaging ◽

10.3390/jimaging7050082 ◽

2021 ◽

Vol 7 (5) ◽

pp. 82

Author(s):

River Gassen ◽

Dennis Thompkins ◽

Austin Routt ◽

Philippe Jones ◽

Meghan Smith ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Magnetic Particles ◽

Magnetic Particle ◽

Barium Hexaferrite ◽

Optical Microscope ◽

Simplified Model ◽

Particle Cluster ◽

Average Deviation ◽

Cross Sectional

Magnetic particles have been evaluated for their biomedical applications as a drug delivery system to treat asthma and other lung diseases. In this study, ferromagnetic barium hexaferrite (BaFe12O19) and iron oxide (Fe3O4) particles were suspended in water or glycerol, as glycerol can be 1000 times more viscous than water. The particle concentration was 2.50 mg/mL for BaFe12O19 particle clusters and 1.00 mg/mL for Fe3O4 particle clusters. The magnetic particle cluster cross-sectional area ranged from 15 to 1000 μμm2, and the particle cluster diameter ranged from 5 to 45 μμm. The magnetic particle clusters were exposed to oscillating or rotating magnetic fields and imaged with an optical microscope. The oscillation frequency of the applied magnetic fields, which was created by homemade wire spools inserted into an optical microscope, ranged from 10 to 180 Hz. The magnetic field magnitudes varied from 0.25 to 9 mT. The minimum magnetic field required for particle cluster rotation or oscillation in glycerol was experimentally measured at different frequencies. The results are in qualitative agreement with a simplified model for single-domain magnetic particles, with an average deviation from the model of 1.7 ± 1.3. The observed difference may be accounted for by the fact that our simplified model does not include effects on particle cluster motion caused by randomly oriented domains in multi-domain magnetic particle clusters, irregular particle cluster size, or magnetic anisotropy, among other effects.

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Implementation of a Faraday rotation diagnostic at the OMEGA laser facility

High Power Laser Science and Engineering ◽

10.1017/hpl.2018.42 ◽

2018 ◽

Vol 6 ◽

Author(s):

A. Rigby ◽

J. Katz ◽

A. F. A. Bott ◽

T. G. White ◽

P. Tzeferacos ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Faraday Rotation ◽

Field Measurements ◽

Thomson Scattering ◽

Proton Radiography ◽

Time Resolved ◽

Rotation Measurement ◽

Laser Facility ◽

Omega Laser

Magnetic field measurements in turbulent plasmas are often difficult to perform. Here we show that for ${\geqslant}$kG magnetic fields, a time-resolved Faraday rotation measurement can be made at the OMEGA laser facility. This diagnostic has been implemented using the Thomson scattering probe beam and the resultant path-integrated magnetic field has been compared with that of proton radiography. Accurate measurement of magnetic fields is essential for satisfying the scientific goals of many current laser–plasma experiments.

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The search for magnetic fields in two Wolf–Rayet stars and the discovery of a variable magnetic field in WR 55

Monthly Notices of the Royal Astronomical Society Letters ◽

10.1093/mnrasl/slaa170 ◽

2020 ◽

Vol 499 (1) ◽

pp. L116-L120

Author(s):

S Hubrig ◽

M Schöller ◽

A Cikota ◽

S P Järvinen

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Rotation Period ◽

Indirect Evidence ◽

Variable Magnetic Field ◽

Spectral Variability ◽

Significance Level ◽

Field Detection ◽

Ring Nebula ◽

Far Ultraviolet

ABSTRACT Magnetic fields in Wolf–Rayet (WR) stars are not well explored, although there is indirect evidence, e.g. from spectral variability and X-ray emission, that magnetic fields should be present in these stars. Being in an advanced stage of their evolution, WR stars have lost their hydrogen envelope, but their dense winds make the stellar core almost unobservable. To substantiate the expectations on the presence of magnetic fields in the most-evolved massive stars, we selected two WR stars, WR 46 and WR 55, for the search of the presence of magnetic fields using FORS 2 spectropolarimetric observations. We achieve a formally definite detection of a variable mean longitudinal magnetic field of the order of a few hundred gauss in WR 55. The field detection in this star, which is associated with the ring nebula RCW 78 and the molecular environment, is of exceptional importance for our understanding of star formation. No field detection at a significance level of 3σ was achieved for WR 46, but the variability of the measured field strengths can be rather well phased with the rotation period of 15.5 h previously suggested by FUSE(Far Ultraviolet Spectroscopic Explorer) observations.

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Pulsations of Ob-Stars: New Observations

Symposium - International Astronomical Union ◽

10.1017/s0074180900238928 ◽

1998 ◽

Vol 185 ◽

pp. 347-354 ◽

Cited By ~ 1

Author(s):

Dietrich Baade

Keyword(s):

Main Sequence ◽

Physical Parameters ◽

Be Stars ◽

Instability Strip ◽

Ob Stars ◽

Analysis Strategies ◽

The Galaxy ◽

Order Line ◽

Nonradial Pulsation ◽

Radial Pulsations

Improved observing and data analysis strategies have initiated a considerable expansion of the empirical knowledge about the pulsations of OB stars. Possible correlations between physical parameters and associated pulsation characteristics are becoming more clearly perceivable. This starts to include the asteroseismologically fundamental areas of g-modes and rapid rotation. The β Cephei instability strip continues to be the only locus where radial pulsations occur (but apparently not in all stars located in that strip). Except for spectral types B8/B9 near the main sequence, where pulsations are hardly detected even at low amplitudes, any major group of stars in the Galaxy that are obviously not candidate pulsators still remains to be identified. However, the incidence and amplitudes of OB star pulsations decrease steeply with metallicity. The behaviour of high-luminosity stars is less often dominated by very few modes. In broad-lined stars the moving-bump phenomenon is more common than low-order line-profile variability. But its relation to nonradial pulsation is not clear. The beating of low-ℓ nonradial pulsation modes that have identical angular mode indices may be the clockwork of the outbursts of at least some Be stars. The physics of this episodic mass loss process remains to be identified.

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Small-Scale Solar Magnetic Fields

Symposium - International Astronomical Union ◽

10.1017/s0074180900008081 ◽

1976 ◽

Vol 71 ◽

pp. 69-99 ◽

Cited By ~ 7

Author(s):

J. O. Stenflo

Keyword(s):

Magnetic Fields ◽

Magnetic Flux ◽

Solar Surface ◽

Life Time ◽

Wave Number ◽

Small Scale ◽

Solar Magnetic Fields ◽

Field Amplification ◽

Wave Numbers ◽

Diffuse Form

The observed properties of small-scale solar magnetic fields are reviewed. Most of the magnetic flux in the photosphere is in the form of strong fields of about 100–200 mT (1–2 kG), which have remarkably similar properties regardless of whether they occur in active or quiet regions. These fields are associated with strong atmospheric heating. Flux concentrations decay at a rate of about 107 Wb s-1, independent of the amount of flux in the decaying structure. The decay occurs by smaller flux fragments breaking loose from the larger ones, i.e. a transfer of magnetic flux from smaller to larger Fourier wave numbers, into the wave-number regime where ohmic diffusion becomes significant. This takes place in a time-scale much shorter than the length of the solar cycle.The field amplification occurs mainly below the solar surface, since very little magnetic flux appears in diffuse form in the photosphere, and the life-time of the smallest flux elements is very short. The observations further suggest that most of the magnetic flux in quiet regions is supplied directly from below the solar surface rather than being the result of turbulent diffusion of active-region magnetic fields.

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Magnetic fields, winds and X-rays of massive stars in the Orion nebula cluster

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311010374 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 208-209 ◽

Cited By ~ 1

Author(s):

Véronique Petit ◽

Gregg A. Wade ◽

Evelyne Alecian ◽

Laurent Drissen ◽

Thierry Montmerle ◽

...

Keyword(s):

Magnetic Fields ◽

Massive Stars ◽

Theoretical Models ◽

Magnetic Star ◽

X Rays ◽

Orion Nebula ◽

X Ray ◽

Ob Stars ◽

The Impact ◽

The Relationship

AbstractIn some massive stars, magnetic fields are thought to confine the outflowing radiatively-driven wind. Although theoretical models and MHD simulations are able to illustrate the dynamics of such a magnetized wind, the impact of this wind-field interaction on the observable properties of a magnetic star - X-ray emission, photometric and spectral variability - is still unclear. The aim of this study is to examine the relationship between magnetism, stellar winds and X-ray emission of OB stars, by providing empirical observations and confronting theory. In conjunction with the COUP survey of the Orion Nebula Cluster, we carried out spectropolarimatric ESPaDOnS observations to determine the magnetic properties of massive OB stars of this cluster.

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