Dynamics of Self-Accreting Disks in Be Stars

AbstractBe star disks are formed by ejection of stellar matter from the surface of a B star rotating at almost critical velocity. In SPH simulations we find that most of the ejected particles fall back on the stellar surface but those with sufficient angular momentum are able to feed a disk-like structure. Owing to viscous interaction some particles are lifted to larger radii where they carry high angular momentum. Viscous forces also cause a thinning of the initially geometrically thick disk and the final accretion of most of the disk material. Different simulations show how the formation and the extension of the decretion disk depend on the ejection velocity, the viscous parameter α and on how long the source is active. After the outburst the disk thins out more and more, over a timescale much longer than the outburst time.The simulations are compared to Hα observations of the Be star µ Cen.

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Making a Be star: the role of rotation and pulsations

Proceedings of the International Astronomical Union ◽

10.1017/s174392131301507x ◽

2013 ◽

Vol 9 (S301) ◽

pp. 465-466

Author(s):

Coralie Neiner ◽

Stéphane Mathis

Keyword(s):

Angular Momentum ◽

Critical Value ◽

Circumstellar Disk ◽

Be Stars ◽

Stochastic Excitation ◽

Stellar Rotation ◽

Rapid Rotation ◽

Be Star

AbstractThe Be phenomenon, i.e. the ejection of matter from Be stars into a circumstellar disk, has been a long lasting mystery. In the last few years, the CoRoT satellite brought clear evidence that Be outbursts are directly correlated to pulsations and rapid rotation. In particular the stochastic excitation of gravito-inertial modes, such as those detected by CoRoT in the hot Be star HD 51452, is enhanced thanks to rapid rotation. These waves increase the transport of angular momentum and help to bring the already rapid stellar rotation to its critical value at the surface, allowing the star to eject material. Below we summarize the recent observational and theoretical findings and describe the new picture of the Be phenomenon which arose from these results.

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Periodic Variations and Mass Loss in Be Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900215118 ◽

1994 ◽

Vol 162 ◽

pp. 287-298 ◽

Cited By ~ 1

Author(s):

Hideyuki Saio

Keyword(s):

Angular Momentum ◽

Mass Loss ◽

Mass Loss Rate ◽

Rotation Period ◽

Be Stars ◽

Surface Magnetic Field ◽

Upper Limit ◽

Short Period ◽

Angular Momentum Loss ◽

Be Star

We discuss the connection between the periodic light variations and the equatorial mass loss of Be stars. The observed properties of the short period (~ day) variations seem to indicate that they arise in the photosphere. An upper limit for the surface magnetic field of Be stars is derived from the rate of angular momentum loss expected from the typical mass loss-rate in Be stars. The upper limit suggests that surface magnetic fields of Be stars are too weak to make a spot. We argue that the periodic variations of Be stars are explained by nonradial pulsations whose periods on the stellar surface are much longer than the rotation period. They transport angular momentum from the core to the envelope to accelerate the surface regions. If this mechanism works sufficiently well, the rotation speed near the surface will reach to the critical velocity and an excretion disk will be formed around the star. A simple model for a steady-state excretion disk around a Be star is found to be consistent with the density structure inferred from the IR fluxes.

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Binary Be Stars and Be Binaries

Symposium - International Astronomical Union ◽

10.1017/s0074180900122144 ◽

1992 ◽

Vol 151 ◽

pp. 147-156

Author(s):

Dietrich Baade

Keyword(s):

Angular Momentum ◽

Momentum Transfer ◽

Mass Exchange ◽

White Dwarf ◽

Be Stars ◽

Star Model ◽

Rapid Rotation ◽

Sky Survey ◽

Be Star

Two hypotheses have been put forward for the rôle of binarity in Be stars: (1) All Be stars are interacting binaries. (2) Roughly one-half of the observed Be stars are post-mass exchange binaries with compact companions. Contrary to (1), (2) does not attempt to explain also the existence of disks in Be stars. After the spin-up by mass and angular momentum transfer, the B star somehow has to succeed to form and maintain the disk. Since rapid rotation is only necessary but not sufficient for this transformation, the effect of duplicity would merely be to give more stars the opportunity to become a Be star. Model (1) is not nearly realistic as is also underlined by a new spectroscopic survey for cool companions. The verification of (2) on the basis of the ROSAT All-Sky Survey has just begun; but a serious deficiency of white dwarf companions is already apparent. Binarity currently provides no extra clue on the origin of the Be phenomenon.

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Transport of angular momentum by stochastically excited waves as an explanation for the outburst of the rapidly rotating Be star HD49330

Astronomy and Astrophysics ◽

10.1051/0004-6361/201935858 ◽

2020 ◽

Vol 644 ◽

pp. A9 ◽

Cited By ~ 1

Author(s):

C. Neiner ◽

U. Lee ◽

S. Mathis ◽

H. Saio ◽

C. C. Lovekin ◽

...

Keyword(s):

Angular Momentum ◽

Low Frequency ◽

Be Stars ◽

Rotating Stars ◽

Transient Waves ◽

Frequency Signal ◽

Unstable Phase ◽

Be Star ◽

2D Structure ◽

Rapidly Rotating Stars

Context. HD 49330 is an early Be star that underwent an outburst during its five-month observation with the CoRoT satellite. An analysis of its light curve revealed several independent p and g pulsation modes, in addition to showing that the amplitude of the modes is directly correlated with the outburst. Aims. We modelled the results obtained with CoRoT to understand the link between pulsational parameters and the outburst of this Be star. Methods. We modelled the flattening of the structure of the star due to rapid rotation in two ways: Chandrasekhar-Milne’s expansion and 2D structure computed with ROTORC. We then modelled κ-driven pulsations. We also adapted the formalism of the excitation and amplitude of stochastically excited gravito-inertial modes to rapidly rotating stars, and we modelled those pulsations as well. Results. We find that while pulsation p modes are indeed excited by the κ mechanism, the observed g modes are, rather, a result of stochastic excitation. In contrast, g and r waves are stochastically excited in the convective core and transport angular momentum to the surface, increasing its rotation rate. This destabilises the external layers of the star, which then emits transient stochastically excited g waves. These transient waves produce most of the low-frequency signal detected in the CoRoT data and ignite the outburst. During this unstable phase, p modes disappear at the surface because their cavity is broken. Following the outburst and ejection of the surface layer, relaxation occurs, making the transient g waves disappear and p modes reappear. Conclusions. This work includes the first coherent model of stochastically excited gravito-inertial pulsation modes in a rapidly rotating Be star. It provides an explanation for the correlation between the variation in the amplitude of frequencies detected in the CoRoT data and the occurrence of an outburst. This scenario could apply to other pulsating Be stars, providing an explanation to the long-standing questions surrounding Be outbursts and disks.

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Non-radial pulsations in the CoRoT Be Star 102761769

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311011203 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 507-508

Author(s):

Eduardo Janot Pacheco ◽

Laerte B.P. de Andrade ◽

Marcelo Emilio ◽

Juan Carlos Suárez ◽

Andressa Jendreieck

Keyword(s):

Critical Velocity ◽

Be Stars ◽

Stellar Rotation ◽

Alternative Scenario ◽

Be Star ◽

Radial Pulsations ◽

Nonradial Oscillations

AbstractWe investigate non-radial pulsations of the CoRoT IR1 Be Star 102761769, with a projected stellar rotation estimated to be 120±15 km/s. If the star is a typical galactic Be star it rotates near the critical velocity. We propose an alternative scenario, where the star could be seen nearly equator-on rotating at a relatively moderate velocity say, ≈ 120 km/s and therefore the nonradial oscillations could be modeled. In order to identify the pulsation modes of the observed frequencies, we computed a set of models representative of CoRoT 102761769 by means of the adiabatic pulsation package FILOU. Results indicate that the two frequencies are compatible with a high-g mode as predicted by pulsation models of Be stars.

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The Viscosity Parameter for Late-type Stable Be Stars

The Astrophysical Journal ◽

10.3847/1538-4357/ac222f ◽

2021 ◽

Vol 922 (2) ◽

pp. 148

Author(s):

A. Granada ◽

C. E. Jones ◽

T. A. A. Sigut

Keyword(s):

Angular Momentum ◽

Central Star ◽

Be Stars ◽

Late Type ◽

Viscosity Parameter ◽

Momentum Loss ◽

Self Consistent ◽

Angular Momentum Loss ◽

Be Star ◽

Loss Rates

Abstract Using hydrodynamic principles we investigate the nature of the disk viscosity following the parameterization by Shakura & Sunyaev adopted for the viscous decretion model in classical Be stars. We consider a radial viscosity distribution including a constant value, a radially variable α assuming a power-law density distribution, and isothermal disks, for a late-B central star. We also extend our analysis by determining a self-consistent temperature disk distribution to model the late-type Be star 1 Delphini, which is thought to have a nonvariable, stable disk as evidenced by Hα emission profiles that have remained relatively unchanged for decades. Using standard angular momentum loss rates given by Granada et al., we find values of α of approximately 0.3. Adopting lower values of angular momentum loss rates, i.e., smaller mass loss rates, leads to smaller values of α. The values for α vary smoothly over the Hα emitting region and exhibit the biggest variations nearest the central star within about five stellar radii for the late-type, stable Be stars.

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Radiation driven winds with rotation: the oblate finite disc correction factor

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311010015 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 83-84

Author(s):

Ignacio Araya ◽

Michel Curé ◽

Anahí Granada ◽

Lydia S. Cidale

Keyword(s):

Critical Velocity ◽

Correction Factor ◽

Rotational Velocity ◽

Momentum Equation ◽

Be Stars ◽

Stellar Rotation ◽

Polar Wind ◽

Be Star ◽

Remarkable Result ◽

Standing Problem

AbstractWe have incorporated the oblate distortion of the shape of the star due to the stellar rotation, which modifies the finite disk correction factor (fD) in the m-CAK hydrodynamical model. We implement a simplified version for the fD allowing us to solve numerically the non–linear m-CAK momentum equation. We solve this model for a classical Be star in the polar and equatorial directions. The star's oblateness modifies the polar wind, which is now much faster than the spherical one, mainly because the wind receives radiation from a larger (than the spherical) stellar surface. In the equatorial direction we obtain slow solutions, which are even slower and denser than the spherical ones. For the case when the stellar rotational velocity is about the critical velocity, the most remarkable result of our calculations is that the density contrast between the equatorial density and the polar one, is about 100. This result could explain a long-standing problem on Be stars.

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A Simple Spectropolarimetric Temperature Diagnostic for Circumstellar Disks

International Astronomical Union Colloquium ◽

10.1017/s025292110005661x ◽

2000 ◽

Vol 175 ◽

pp. 603-606 ◽

Cited By ~ 2

Author(s):

Karen S. Bjorkman ◽

Jon E. Bjorkman ◽

Kenneth Wood

Keyword(s):

Scattered Light ◽

Circumstellar Disks ◽

Spectral Lines ◽

Polarization Spectrum ◽

Be Stars ◽

Light Spectrum ◽

Temperature Estimate ◽

Disk Material ◽

Be Star ◽

Temperature Diagnostic

AbstractWe describe a technique for estimating average temperatures of Be star disks from analysis of ultraviolet spectropolarimetry. The technique utilizes the fact that the spectrum of the scattered starlight is sensitive to the circumstellar opacity, and hence temperature, since the signature of the disk material is imprinted on the scattered light spectrum. Analysis of the polarization spectrum thus allows us to disentangle the relative contributions of the star and disk, and thereby obtain an estimate of the average disk opacity as a function of wavelength. Using an LTE line-blanketed model (containing about 106 spectral lines) for the opacity, we determine a theoretical opacity as a function of temperature. By comparing this to the opacity deduced from the spectropolarimetry, we can estimate the average disk temperature. For classical Be stars, the relative strengths of the Fe II and Fe III multiplets at around 2400Å and 1900Å, respectively, are a sensitive temperature diagnostic, so that the temperature estimate can be made within ±1000K. We demonstrate our technique with analysis of UV spectropolarimetry (from WUPPE) of the classical Be star ζ Tau, for which we infer an isothermal disk temperature of14000K.

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Spectrally and spatially resolved Hα emission from Be stars: their disks rotate Keplerian

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311011008 ◽

2010 ◽

Vol 6 (S272) ◽

pp. 418-419 ◽

Cited By ~ 1

Author(s):

René D. Oudmaijer ◽

Hugh E. Wheelwright ◽

Alex C. Carciofi ◽

Jon E. Bjorkman ◽

Karen S. Bjorkman

Keyword(s):

Angular Momentum ◽

Parametric Models ◽

High Spectral Resolution ◽

Be Stars ◽

Line Profiles ◽

Disk Model ◽

Spatially Resolved ◽

Be Star ◽

Hα Emission ◽

Do So

AbstractWe test whether Be star disks rotate in a Keplerian or an Angular Momentum Conserving fashion. This is done by employing sub-milli arcsecond spectroastrometry around Hα. We spatially resolve the disks, and are the first to do so at such a high spectral resolution. We fit the emission line profiles with parametric models. The Keplerian models reproduce the spectro-astrometry, whereas the AMC models do not, thereby supporting the viscous disk model for Be stars.

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