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.

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 Seeking the progenitors of the magnetic Ap stars: A search for magnetic fields in Herbig Ae/Be stars with FORS1 at the ESO VLT

2004 ◽  
Vol 2004 (IAUS224) ◽  
pp. 595-598
Author(s):  
D. Drouine ◽  
G.A. Wade ◽  
S. Bagnulo ◽  
J.D. Landstreet ◽  
E. Mason ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Be Stars ◽  
Ap Stars

scholarly journals An MHD Model of Be Stars with Disks

2000 ◽  
Vol 175 ◽  
pp. 611-616
Author(s):  
A. E. Dudorov ◽  
R. E. Pudritz
Keyword(s):  
Magnetic Fields ◽  
Accretion Disks ◽  
Large Scale ◽  
Be Stars ◽  
Magnetic Disk ◽  
X Ray ◽  
History Of ◽  
Formation And Evolution ◽  
Dynamical Features ◽  
A Current

AbstractAn investigation of the formation and evolution of Be stars show that these stars could have dipolar fossil magnetic fields. This magnetic field should constrain the circumstellar magnetic disk and influence its dynamical features. For studying these effects we refine the alphamodel of accretion disks of Shakura and Sunyaev by incorporating into this model the evolution of large-scale magnetic fields. In the frame of our model we can investigate the rotational history of the star with the disk and the ionization and thermal properties of disks. We show in particular that in the magnetopause region a current sheet can form that can generate X-ray radiation.

3D simulations of accretion onto a star: Fast funnel-wall accretion

2018 ◽  
Vol 14 (A30) ◽  
pp. 138-138
Author(s):  
Shinsuke Takasao ◽  
Kengo Tomida ◽  
Kazunari Iwasaki ◽  
Takeru K. Suzuki
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Accretion Disk ◽  
Large Scale ◽  
Free Fall ◽  
Central Star ◽  
Be Stars ◽  
Momentum Exchange ◽  
Fall Velocity ◽  
Poloidal Magnetic Field

AbstractWe show the results of global 3D magnetohydrodynamics simulations of an accretion disk with a rotating, weakly magnetized central star (Takasao et al. 2018). The disk is threaded by a weak large-scale poloidal magnetic field. The central star has no strong stellar magnetosphere initially and is only weakly magnetized. We investigate the structure of the accretion flows from a turbulent accretion disk onto the star. Our simulations reveal that fast accretion onto the star at high latitudes is established even without a stellar magnetosphere. We find that the failed disk wind becomes the fast, high-latitude accretion as a result of angular momentum exchange mediated by magnetic fields. The rapid angular momentum exchange occurs well above the disk, where the Lorentz force that decelerates the rotational motion of gas can be comparable to the centrifugal force. Unlike the classical magnetospheric accretion model, fast accretion streams are not guided by magnetic fields of the stellar magnetosphere. Nevertheless, the accretion velocity reaches the free-fall velocity at the stellar surface owing to the efficient angular momentum loss at a distant place from the star. Our model can be applied to Herbig Ae/Be stars whose magnetic fields are generally not strong enough to form magnetospheres, and also provides a possible explanation why Herbig Ae/Be stars show indications of fast accretion.

scholarly journals Spectroscopic analysis of accretion/ejection signatures in the Herbig Ae/Be stars HD 261941 and V590 Mon

2020 ◽  
Vol 494 (3) ◽  
pp. 3512-3535 ◽  
Author(s):  
T Moura ◽  
S H P Alencar ◽  
A P Sousa ◽  
E Alecian ◽  
Y Lebreton
Keyword(s):  
Magnetic Fields ◽  
T Tauri Stars ◽  
Be Stars ◽  
Intermediate Mass ◽  
Stellar Cluster ◽  
Circumstellar Material ◽  
T Tauri ◽  
Low Mass ◽  
Circumstellar Environment ◽  
Magnetospheric Accretion

ABSTRACT Herbig Ae/Be (HAeBe) stars are the intermediate-mass analogues of low-mass T Tauri stars. Both groups may present signs of accretion, outflow, and IR excess related to the presence of circumstellar discs. Magnetospheric accretion models are generally used to describe accreting T Tauri stars, which are known to have magnetic fields strong enough to truncate their inner discs and form accretion funnels. Since few HAeBe stars have had magnetic fields detected, they may accrete through a different mechanism. Our goal is to analyse the morphology and variability of emission lines that are formed in the circumstellar environment of HAeBe stars and use them as tools to understand the physics of the accretion/ejection processes in these systems. We analyse high-resolution (R ∼ 47 000) UVES/ESO spectra of two HAeBe stars – HD 261941 (HAe) and V590 Mon (HBe) that are members of the young (∼3 Myr) NGC 2264 stellar cluster and present indications of sufficient circumstellar material for accretion and ejection processes to occur. We determine stellar parameters with synthetic spectra, and also analyse and classify circumstellar lines such as H α, H β, and He i λ5875.7, according to their morphologies. We model the H α mean line profile, using a hybrid Magnetohydrodynamics (MHD) model that includes a stellar magnetosphere and a disc wind, and find signatures of magnetically driven outflow and accretion in HD 261941, while the H α line of V590Mon seems to originate predominantly in a disc wind.

scholarly journals The Link between Radiation-Driven Winds and Pulsation in Massive Stars

2002 ◽  
Vol 185 ◽  
pp. 512-519
Author(s):  
S.P. Owocki ◽  
S.R. Cranmer
Keyword(s):  
Large Scale ◽  
Massive Stars ◽  
Optical Emission ◽  
Stellar Winds ◽  
Radial Pulsation ◽  
Be Stars ◽  
Line Profiles ◽  
Wind Variability ◽  
Dynamical Simulation ◽  
Radial Pulsations

AbstractHot, luminous, massive stars have strong stellar winds driven by line-scattering of the star’s continuum radiation. They are also often observed to exhibit radial or non-radial pulsations. Such pulsations are possible candidates for providing the base perturbations that induce large-scale structure in the overlying wind, and as such they could help explain various observational manifestions of wind variability, e.g., absorption enhancemens or modulations in UV P-Cygni lines of OB stars, and perhaps even moving bumps in optical emission lines of Wolf-Rayet (WR) stars. Here we review the physics of line driving, with emphasis on how perturbations induce variations in a wind outflow. In particular, we present results of a time-dependent dynamical simulation of wind variations induced by the radial pulsation of the β Cep variable BW Vulpeculae, and show that observed variability in UV wind lines can be quite well reproduced by synthetic line profiles for this model. We conclude with a discussion of recent evidence that resonances among multiple modes of non-radial pulsation in Be stars play a role in inducing mass ejections that contribute to formation of a circumstellar disk.

scholarly journals The search for magnetic fields in mercury-manganese stars

2010 ◽  
Vol 6 (S272) ◽  
pp. 202-203
Author(s):  
Vitalii Makaganiuk ◽  
Oleg Kochukhov ◽  
Nikolai Piskunov ◽  
Sandra V. Jeffers ◽  
Christopher M. Johns-Krull ◽  
...  
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Main Sequence ◽  
Chemical Elements ◽  
Field Measurements ◽  
Atomic Diffusion ◽  
Main Sequence Stars ◽  
Weak Magnetic Fields ◽  
Ap Stars ◽  
Spot Formation

AbstractMercury-manganese (HgMn) stars were considered to be non-magnetic, showing no evidence of surface spots. However, recent investigations revealed that some stars in this class possess an inhomogeneous distribution of chemical elements on their surfaces. According to our current understanding, the most probable mechanism of spot formation involves magnetic fields. Taking the advantage of a newly-built polarimeter attached to the HARPS spectrometer at the ESO 3.6m-telescope, we performed a high-precision spectropolarimetric survey of a large group of HgMn stars. The main purpose of this study was to find out how typical it is for HgMn stars to have weak magnetic fields. We report no magnetic field detection for any of the studied objects, with a typical precision of the longitudinal field measurements of 10 G and down to 1 Gauss for some of the stars. We conclude that HgMn stars lack large-scale magnetic fields typical of spotted magnetic Ap stars and probably lack any fields capable of creating and sustaining chemical spots. Our study confirms that alongside the magnetically altered atomic diffusion, there must be other structure formation mechanism operating in the atmospheres of late-B main sequence stars.

Ap-Star Models

1976 ◽  
Vol 32 ◽  
pp. 1-24
Author(s):  
L. Mestel
Keyword(s):  
Magnetic Fields ◽  
Spectral Line ◽  
Small Group ◽  
Large Scale ◽  
Zeeman Effect ◽  
Circularly Polarized ◽  
Star Models ◽  
The Cross ◽  
The Difference ◽  
Ap Stars

PhenomenologyThe outstanding observational facts that impress the theorist studying the magnetic Ap stars are as follows:1) The very strong, large-scale magnetic fields, inferred from the integrated Zeeman effect. Typical polar fields are 103- 104gauss, the strongest known being =3.5 x 104gauss.2) The variability of the fields, spectra and luminosities, with the fields often reversing in sign. Typical periods are 5 - 9 days, but periods shorter and very much longer are found, with a small group of stars having periods of several years.3) The low rotations of most Ap and apparently all known Am stars, compared with normal A stars. The possibility that we are seeing normal A stars pole-on seems statistically untenable.The "cross-over effect" (Babcock 1956). This describes how the difference in the widths of two circularly polarized Zeeman components of a spectral line becomes particularly large at the phase when the net field becomes small.

scholarly journals Revealing the Location of the Mixing Layer in a Hot Bubble

2016 ◽  
Vol 12 (S323) ◽  
pp. 114-118
Author(s):  
M. A. Guerrero ◽  
X. Fang ◽  
Y.-H. Chu ◽  
J. A. Toalá ◽  
R. A. Gruendl
Keyword(s):  
Stellar Evolution ◽  
Large Scale ◽  
Massive Star ◽  
Thermal Structure ◽  
Mixing Layer ◽  
Stellar Winds ◽  
X Ray ◽  
Circumstellar Material ◽  
Spatially Resolved ◽  
Mass Evaporation

AbstractThe fast stellar winds can blow bubbles in the circumstellar material ejected from previous phases of stellar evolution. These are found at different scales, from planetary nebulae (PNe) around stars evolving to the white dwarf stage, to Wolf-Rayet (WR) bubbles and up to large-scale bubbles around massive star clusters. In all cases, the fast stellar wind is shock-heated and a hot bubble is produced. Processes of mass evaporation and mixing of nebular material and heat conduction occurring at the mixing layer between the hot bubble and the optical nebula are key to determine the thermal structure of these bubbles and their evolution. In this contribution we review our current understanding of the X-ray observations of hot bubbles in PNe and present the first spatially-resolved study of a mixing layer in a PN.

scholarly journals Destruction of Be star disk by large scale magnetic fields

2016 ◽  
Vol 12 (S329) ◽  
pp. 453-453
Author(s):  
Asif ud-Doula ◽  
Stanley Owocki ◽  
Nathaniel (Dylan) Kee ◽  
Michael Vanyo
Keyword(s):  
Magnetic Fields ◽  
Large Scale ◽  
Circumstellar Disks ◽  
Be Stars ◽  
Rotating Stars ◽  
Apparent Lack ◽  
B Stars ◽  
Large Scale Magnetic Field ◽  
Be Star ◽  
Rapidly Rotating Stars

AbstractClassical Be stars are rapidly rotating stars with circumstellar disks that come and go on time scale of years. Recent observational data strongly suggests that these stars lack the 10% incidence of global magnetic fields observed in other main-sequence B stars. Such an apparent lack of magnetic fields may indicate that Be disks are fundamentally incompatible with a significant large scale magnetic field. In this work, using numerical magnetohydrodynamics (MHD) simulations, we show that a dipole field of only 100G can lead to the quick disruption of a Be disk. Such a limit is in line with the observational upper limits for these objects.

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