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

2013 ◽  
Vol 9 (S302) ◽  
pp. 280-283 ◽  
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.

scholarly journals Observations of Magnetic Fields in B Stars

1994 ◽  
Vol 162 ◽  
pp. 155-166
Author(s):  
David A. Bohlender
Keyword(s):  
Magnetic Fields ◽  
Radio Emission ◽  
Effective Temperature ◽  
Be Stars ◽  
Hot Stars ◽  
Photometric Variability ◽  
B Stars ◽  
Peculiar Stars ◽  
Thermal Radio Emission ◽  
Circumstellar Environments

Globally ordered magnetic fields are known to exist in non-degenerate stars with spectral types between approximately F0 and B2. Among the B stars, and in order of increasing effective temperature, these include the Bp Si stars, helium-weak stars, and the helium-strong stars. These rather remarkable objects present us with an excellent opportunity to quantitatively examine the possible effects of magnetic fields on the photospheres, winds, and circumstellar environments of hot stars. In this paper we review some of the observations of the magnetic fields and field geometries of magnetic B stars, and also briefly discuss the success of attempts to measure magnetic fields in hotter OB and Be stars. We point out some of the interesting observational similarities of the helium-weak and helium-strong stars to Be and other hot stars, including their spectroscopic and photometric variability, variable winds as demonstrated by the UV resonance lines of C IV and Si IV, and their non-thermal radio emission. Continuing work also suggests that a considerable fraction of the rapidly rotating magnetic helium-peculiar stars are in fact variable Be and Be shell stars.

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

2014 ◽  
Vol 9 (S307) ◽  
pp. 379-380
Author(s):  
H. F. Henrichs ◽  
N. Sudnik
Keyword(s):  
Magnetic Fields ◽  
Rotation Period ◽  
Life Time ◽  
Simplified Model ◽  
Common Phenomenon ◽  
Rapid Variability ◽  
Time Resolved ◽  
Ob Stars ◽  
Cyclical Variability ◽  
Radial Pulsations

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.

scholarly journals Optical Imaging of Magnetic Particle Cluster Oscillation and Rotation in Glycerol

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.

scholarly journals Short-Term Spectral Variability in the Herbig Ae Star Ab Aur: Preliminary Analysis of Chromospheric Lines

1993 ◽  
Vol 137 ◽  
pp. 665-668
Author(s):  
Torsten Böhm ◽  
Claude Catala
Keyword(s):  
Magnetic Fields ◽  
Acoustic Waves ◽  
Main Sequence ◽  
Main Question ◽  
Spectral Variability ◽  
Surface Magnetic Field ◽  
External Agent ◽  
Static Contraction ◽  
Magnetic Origin ◽  
Herbig Ae Stars

The Herbig Ae stars are PMS objects of intermediate mass. Their location in the H-R diagram indicates that they are in the radiative phase of their quasi-static contraction toward the main sequence, i.e. that they do not possess outer convective zones, according to the standard stellar evolution theory (Iben, 1965; Gilliland, 1986). In spite of the expected absence of subphotospheric convective envelopes, these stars show remarkable signs of activity: emission in the Mg II h and k lines, presence of the CIV resonance lines at 1550 A and He I 5875.7 A line, Ca II IR triplet in emission, etc... Considering that stellar activity, witnessed by the same type of indicators in other parts of the H-R diagram, is generally attributed to dynamo magnetic fields and/or acoustic waves generated in the convection zone, these active phenomena are quite paradoxical in the Herbig Ae stars.The main question concerns the origin of their activity: is this activity linked to phenomena occurring within the stars, like e.g. dynamo-generated magnetic fields, or to an external agent, like e.g. a boundary layer between an accretion disk and the stellar surface? We already have some indirect clues that the activity of the Herbig Ae stars might be of magnetic origin (Praderie et al., 1986; Catala et al. 1986) observed a rotational modulation of lines formed in the wind of AB Aur, prototype of the Herbig Ae stars. By analogy with the solar wind, they proposed that the modulation might be due to the corotation of azimuthal structures in the wind, controlled by a surface magnetic field.

Upper limits to magnetic fields in C II regions

1984 ◽  
Vol 279 ◽  
pp. 694 ◽  
Author(s):  
P. R. Silverglate
Keyword(s):  
Magnetic Fields ◽  
Upper Limits

scholarly journals Detection of Heavy Elements in the EUVE Spectrum of a Hot White Dwarf

1996 ◽  
Vol 152 ◽  
pp. 235-240
Author(s):  
S. Jordan ◽  
D. Koester ◽  
D. Finley
Keyword(s):  
White Dwarf ◽  
Theoretical Calculations ◽  
Strong Absorption ◽  
Heavy Elements ◽  
Level Of Confidence ◽  
Upper Limits ◽  
Absorption Features

Observations with the ROSAT satellite have already indicated that metal absorbers must be present in the atmosphere of the hot DA white dwarf PG 1234+482. This is now confirmed by strong absorption features found in the short and medium wavelength EUVE spectrum of the star. With fully blanked model atmospheres, taking into account several million lines of heavy elements, we could attribute the strongest features to absorption by FeVI and FeVII. Since the spectrum has not been dithered during the observation other elements could not be identified with the same level of confidence, but upper limits could be determined. These are in general lower than predicted by models, which attempt to explain the presence of the metals by theoretical calculations of radiative forces in hot DA white dwarf atmospheres.

scholarly journals Effect of a Dipole Magnetic Field on Stellar Mass-Loss

2016 ◽  
Vol 12 (S329) ◽  
pp. 242-245
Author(s):  
Chris Bard ◽  
Richard Townsend
Keyword(s):  
Magnetic Fields ◽  
Mass Loss ◽  
Large Scale ◽  
Massive Star ◽  
Loss Rate ◽  
Mass Loss Rate ◽  
Local Environment ◽  
Surface Mass ◽  
B Stars ◽  
Dipole Magnetic Field

AbstractMassive star winds greatly influence the evolution of both their host star and local environment though their mass-loss rates, but current radiative line-driven wind models do not incorporate any magnetic effects. Recent surveys of O and B stars have found that about ten percent have large-scale, organized magnetic fields. These massive-star magnetic fields, which are thousands of times stronger than the Sun’s, affect the inherent properties of their own winds by changing the mass-loss rate. To quantify this, we present a simple surface mass-flux scaling over the stellar surface which can be easily integrated to get an estimate of the mass-loss rate for a magnetic massive star. The overall mass-loss rate is found to decrease by factors of 2-5 relative to the non-magnetic CAK mass-loss rate.

scholarly journals Magnetic Fields in Be Stars? (Review Paper)

1987 ◽  
Vol 92 ◽  
pp. 38-48
Author(s):  
Paul K. Barker
Keyword(s):  
Magnetic Fields ◽  
Field Strength ◽  
Review Paper ◽  
Stellar Winds ◽  
Be Stars ◽  
Circumstellar Envelopes ◽  
Star Models ◽  
Upper Limits ◽  
Be Star

AbstractNo mean longitudinal or toroidal magnetic fields have yet been detected on any classical Be star. Models of stellar winds and circumstellar envelopes around magnetic Be stars are not appreciably constrained by present observed upper limits on field strength. A few magnetic Be stars do exist among the helium strong stars, but these objects show spectral phenomenology which is unmistakably distinct from that shown by every other object known as a Be star.

scholarly journals On the Ultraviolet Photometric Variability of the Helium-Weak B Stars (From ANS Data)

1986 ◽  
Vol 90 ◽  
pp. 271-274
Author(s):  
E.I. Želwanowa ◽  
W. Sohöneich
Keyword(s):  
Open Cluster ◽  
Ultraviolet Region ◽  
Rotational Period ◽  
Slight Improvement ◽  
Photometric Variability ◽  
B Stars ◽  
Photometric Observations

AbstractPhotometric observations in the ultraviolet region obtained by the ANS satellite enabled us: to confirm with a slight improvement the rotational period of HD 22470 (UV lightcurves of HD 22470 compared with u and magnetic curves are shown in fig. 1); to confirm the variability with very small amplitudes for HD 74196, a member of the open cluster IC 2391 (Fig. 2)to compare the amplitude-wavelength relations of some other He-weak stars with known periods (HD 35298, HD 142884, HD 144334, HD 175362, and HD 109026). A preliminary rotational period for the star HD 109026 using the 11 ANS observations was found to be about 1.5 days.

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