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 Rotationally-Enhanced Stellar Winds (Review Paper)

1987 ◽  
Vol 92 ◽  
pp. 316-338
Author(s):  
J. M. Marlborough
Keyword(s):  
Mass Transfer ◽  
Stellar Wind ◽  
Review Paper ◽  
Current Status ◽  
Stellar Winds ◽  
Be Stars ◽  
Stellar Rotation ◽  
Circumstellar Envelopes ◽  
Wind Model ◽  
Be Star

The rotationally-enhanced stellar wind model for Be stars represents one attempt to understand many of the phenomena related to Be stars in terms of a stellar wind distorted and perhaps also enhanced by rapid stellar rotation. This review will concentrate exclusively on this particular approach; the current status of other attempts to model the circumstellar envelopes (CE) of Be stars are described in other reviews in this volume. It is assumed that Be stars are single stars; if a member of a multiple system, the separation of all components from the Be star is assumed to be sufficiently large that interactions due to mass transfer from any companion to the Be star are negligible.

scholarly journals Models for the Circumstellar Envelopes of Be Stars: (Review Paper)

1976 ◽  
Vol 70 ◽  
pp. 335-370 ◽  
Author(s):  
J. M. Marlborough
Keyword(s):  
Ad Hoc ◽  
Review Paper ◽  
Circumstellar Matter ◽  
Be Stars ◽  
Circumstellar Envelopes ◽  
Structure And Dynamics ◽  
Restricted Problems ◽  
Be Star ◽  
Physical Phenomena

A survey is presented of the theoretical attempts to determine the structure of the circumstellar matter around Be stars. The general equations describing the structure and dynamics of Be star envelopes are given. The complications introduced by various physical phenomena are briefly discussed and initial attempts to solve restricted problems are considered. The various ad hoc models proposed for Be stars are discussed and comparisons of the observations with predictions of these models are illustrated. The strengths and weaknesses of these models are evaluated and areas where progress is being or should be made are considered.

scholarly journals X-Ray Observations of B-Emission Stars (Review Paper)

1987 ◽  
Vol 92 ◽  
pp. 291-308 ◽  
Author(s):  
E.P.J. van den Heuvel ◽  
S. Rappaport
Keyword(s):  
Review Paper ◽  
Circumstellar Disks ◽  
Close Binaries ◽  
Be Stars ◽  
X Ray ◽  
Blue Stragglers ◽  
Galactic Clusters ◽  
Be Star ◽  
Emission Stars ◽  
X Ray Binaries

Most evidence on X-ray emission from the vicinity of Be stars concerns the Be/X-ray binaries. Presently some 20 of these systems are known, making them the most numerous class of massive X-ray binaries. Evidence for the binary nature of these systems comes from (i) Doppler modulation of X-ray pulse periods, (ii) periodic X-ray flaring behavior, and (iii) correlated optical and X-ray variability. The correlation between X-ray pulse period and orbital period found by Corbet (1984) can potentially provide important information on the densities and velocities in the circumstellar disks of Be stars.Evolutionary models indicate that the Be/X-ray binaries represent a later stage in the evolution of normal close binaries with initial primary masses predominantly in the the range 8 to 15 M⊙ . These models indicate that also a class of slightly less massive Be star binaries should exist in which the compact companions are white dwarfs. Be-type blue stragglers in galactic clusters may be such systems.

scholarly journals New evidence for weak magnetic fields in Herbig Ae/Be stars

2019 ◽  
Author(s):  
S P Järvinen ◽  
T A Carroll ◽  
S Hubrig ◽  
I Ilyin ◽  
M Schöller
Keyword(s):  
Magnetic Fields ◽  
Longitudinal Magnetic Field ◽  
Be Stars ◽  
Lower Mass ◽  
Singular Value Decomposition Method ◽  
T Tauri ◽  
Weak Magnetic Fields ◽  
Be Star ◽  
Value Decomposition ◽  
New Evidence

Abstract In recent years Herbig Ae/Be stars receive considerable attention as their disks are believed to be the sites of on-going planet formation. Confirming the presence of magnetic fields in these stars is critical for understanding the transport of angular momentum during the protostellar phase. Furthermore, magnetic fields set the conditions for strongly anisotropic accretion. In this study we present the results of our recent observing campaigns of a sample of Herbig Ae/Be stars aimed at measurements of their magnetic fields applying the Singular Value Decomposition method to high resolution spectropolarimetric observations. The strongest longitudinal magnetic field of 209 G is detected in the Herbig Be star HD 58647, whereas the weakest field of 17 G is measured in the Herbig Ae star HD 190073. A change of polarity is detected for HD 58647 and in the Herbig Be star HD 98922. The obtained results provide further evidence that Herbig Ae/Be stars possess much weaker magnetic fields than their lower mass counterpart T Tauri stars with magnetic fields of kG order.

scholarly journals Recent Results from Ultraviolet and Optical Spectropolarimetry of Hot Stars

1994 ◽  
Vol 162 ◽  
pp. 219-229
Author(s):  
K.S. Bjorkman
Keyword(s):  
Linear Polarization ◽  
Optical Data ◽  
Time Variability ◽  
Be Stars ◽  
Circumstellar Envelopes ◽  
Polarization Data ◽  
Hot Stars ◽  
New Models ◽  
Be Star ◽  
The Continuum

The first comprehensive linear polarization data on hot stars covering the spectral range from 1500 to 7600Å are presented. These results are based on recent observations made with the Wisconsin Ultraviolet Photo-Polarimeter Experiment (WUPPE), combined with ground-based observations from the Pine Bluff Observatory. Implications of the data for models of the circumstellar envelopes of hot stars are discussed, with particular emphasis on the surprising results found for the rapidly rotating Be stars. In particular, WUPPE discovered that the continuum polarization in Be stars decreases into the ultraviolet, which was not predicted by models prior to the observations. Time variability in the optical data is also discussed. Possible interpretations of these results are examined in the light of recent new models for Be star disks.

scholarly journals Radio Observations of Be Stars: (Review Paper)

1976 ◽  
Vol 70 ◽  
pp. 157-164 ◽  
Author(s):  
C. R. Purton
Keyword(s):  
Radio Emission ◽  
Radio Astronomy ◽  
Review Paper ◽  
Be Stars ◽  
Radio Observations ◽  
The Past ◽  
Be Star ◽  
Radio Stars

During the past few years the search for radio stars has been both intensive and extensive, and Be stars are among the various types of objects which have been examined. A large number of Be stars have not been detected, and to date no classical Be star has been found to emit detectable amounts of radio emission. However, following the tradition of radio astronomy to emphasise the abnormal or extreme cases, radio emission has been observed which is associated with a few peculiar Be stars.

scholarly journals Continuous spectra of circumstellar envelopes of Be stars

1994 ◽  
Vol 162 ◽  
pp. 425-426
Author(s):  
D. Rohe-Koths ◽  
J. Dachs
Keyword(s):  
Line Emission ◽  
Circumstellar Disks ◽  
Balmer Line ◽  
Be Stars ◽  
Continuous Emission ◽  
Circumstellar Envelopes ◽  
Recombination Radiation ◽  
Infrared Spectral ◽  
Be Star ◽  
Central Stars

Line emission in Be star spectra is accompanied by continuous emission both in the Balmer continuum and in the infrared spectral region, due to the same process that is responsible for Balmer line emission, i.e. to recombination radiation from ionized hydrogen in the extended circumstellar disks surrounding the hot central stars.

scholarly journals Interferometric survey of Be stars with the CHARA array

2010 ◽  
Vol 6 (S272) ◽  
pp. 634-635 ◽  
Author(s):  
Yamina N. Touhami ◽  
Douglas R. Gies ◽  
Gail H. Schaefer ◽  
Noel D. Richardson ◽  
Stephen J. Williams ◽  
...  
Keyword(s):  
Rotational Velocity ◽  
Circumstellar Disks ◽  
Geometrical Parameters ◽  
Be Stars ◽  
Circumstellar Envelopes ◽  
Disk Model ◽  
Spatially Resolved ◽  
Near Ir ◽  
Be Star ◽  
Chara Array

AbstractWe present the first spatially resolved observations of circumstellar envelopes of 25 bright northern Be stars. The survey was performed with the CHARA Array interferometer in the K-band at intermediate and long baselines. The interferometric visibilities are well fitted by a viscous disk model where the gas density steeply decreases with the radius. Physical and geometrical parameters such as the density profile, the inclination, and the position angles of the circumstellar disks are determined. We find that the density radial exponent ranges between n ≈ 2.4 − 3.2, which is consistent with previous IRAS measurements. We have also obtained simultaneous optical and near-IR spectrophotometric measurements, and found that the model reproduces well the observed disk IR-continuum excess emission. By combining the projected rotational velocity of the Be star with the disk inclination derived from interferometry, we give estimates of the equatorial rotational velocities of these Be stars.

scholarly journals A Survey of Mass Loss from Be and Shell Stars Using Ultraviolet Data from Copernicus

1976 ◽  
Vol 70 ◽  
pp. 179-189 ◽  
Author(s):  
J. M. Marlborough ◽  
Theodore P. Snow
Keyword(s):  
Mass Loss ◽  
Direct Evidence ◽  
Rotational Velocity ◽  
Stellar Winds ◽  
Be Stars ◽  
Dwarf Stars ◽  
Be Star ◽  
Effects Of Rotation ◽  
First Time

Ultraviolet spectra of intermediate resolution have been obtained with Copernicus of twelve objects classified as Be or shell stars, and an additional 19 dwarfs of spectral classes B0-B4. Some of these spectra show marked asymmetries in certain resonance lines, especially the Si iv doublet at λ 1400 Å, indicating the presence of outflowing material with maximum velocities of nearly 1000 km s−1. Direct evidence for mass loss at these velocities is seen for the first time in dwarf stars as late as B1.5. Later than B0.5, the only survey objects showing this phenomenon are Be stars. Among the stars considered there is a correlation between the presence of mass-loss effects and projected rotational velocity, suggesting that the UV flux from B1-B3 dwarfs is sufficient to drive high-velocity stellar winds only if rotation reduces the effective gravity near the equator. The role of mass-loss in producing the Be star phenomenon and the effects of rotation on mass loss are discussed.

scholarly journals Magnetic Fields and Winds of the Intermediate Helium Stars

1985 ◽  
Vol 87 ◽  
pp. 277-296
Author(s):  
Paul K. Barker
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Stellar Winds ◽  
Be Stars ◽  
Circumstellar Material ◽  
Helium Stars ◽  
Complex Variety ◽  
Hα Emission ◽  
Rapid Rotator ◽  
Ap Stars

AbstractThe intermediate helium stars are exceedingly rare hot analogs of the classical Ap stars, and are the earliest type stars to possess observable global ordered magnetic fields. A recent discovery is the existence of stellar winds which have large scale magnetospheric structure embedded within them. The nature and geometry of the detected fields are summarized, and the modulation of the circumstellar material by the field is illustrated for two examples: the rapid rotator σ Ori E, and the slow rotator HD 184927. The complex variety of stellar wind phenomenology which may be encountered is displayed by a sample of ten helium strong stars. A few of these objects show Hα emission, and thus are the only known magnetic Be stars.

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