scholarly journals Period spacing of gravity modes strongly affected by rotation

2017 ◽  
Vol 598 ◽  
pp. A105 ◽  
Author(s):  
V. Prat ◽  
S. Mathis ◽  
F. Lignières ◽  
J. Ballot ◽  
P.-M. Culpin
Keyword(s):  
Internal Rotation ◽  
Low Frequency ◽  
Rotation Vector ◽  
Mathematical Form ◽  
Be Stars ◽  
Rotating Stars ◽  
Perturbative Methods ◽  
Frequency Regime ◽  
Spectral Patterns ◽  
Rapidly Rotating Stars

Context. As of today, asteroseismology mainly allows us to probe the internal rotation of stars when modes are only weakly affected by rotation using perturbative methods. Such methods cannot be applied to rapidly rotating stars, which exhibit complex oscillation spectra. In this context, the so-called traditional approximation, which neglects the terms associated with the latitudinal component of the rotation vector, describes modes that are strongly affected by rotation. This approximation is sometimes used for interpreting asteroseismic data, however, its domain of validity is not established yet. Aims. We aim at deriving analytical prescriptions for period spacings of low-frequency gravity modes strongly affected by rotation through the full Coriolis acceleration (i.e. without neglecting any component of the rotation vector), which can be used to probe stellar internal structure and rotation. Methods. We approximated the asymptotic theory of gravito-inertial waves in uniformly rotating stars using ray theory described in a previous paper in the low-frequency regime, where waves are trapped near the equatorial plane. We put the equations of ray dynamics into a separable form and used the Einstein-Brillouin-Keller (EBK) quantisation method to compute modes frequencies from rays. Results. Two spectral patterns that depend on stratification and rotation are predicted within this new approximation: one for axisymmetric modes and one for non-axisymmetric modes. Conclusions. The detection of the predicted patterns in observed oscillation spectra would give constraints on internal rotation and chemical stratification of rapidly rotating stars exhibiting gravity modes, such as γ Doradus, SPB, or Be stars. The obtained results have a mathematical form that is similar to that of the traditional approximation, but the new approximation takes the full Coriolis, which allows for propagation near the centre, and centrifugal accelerations into account.

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

2020 ◽  
Vol 644 ◽  
pp. A9 ◽  
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.

scholarly journals Effect of Instability-Generated Clumping on Wind Compressed Disk Inhibition

2000 ◽  
Vol 175 ◽  
pp. 621-625
Author(s):  
S. P. Owocki ◽  
D. H. Cohen
Keyword(s):  
Stellar Wind ◽  
Large Scale ◽  
Small Scale ◽  
Be Stars ◽  
Rotating Stars ◽  
Disk Formation ◽  
Line Force ◽  
Force Components ◽  
Hydrodynamical Simulations ◽  
Rapidly Rotating Stars

AbstractIn rapidly rotating stars with a purely radially driven stellar wind, conservation of wind angular momentum can lead to formation of an equatorial Wind Compressed Disk (WCD). However, for a line-driven stellar wind, recent 2D hydrodynamical simulations based on a localized, Sobolev treatment of the line-force indicate that nonradial components of the line-force can effectively inhibit formation of such a WCD. This presentation will describe current efforts to extend such 2D simulations to a nonlocal, smooth-source-function treatment of the line-force that can incorporate the intrinsically strong instability of line-driving to formation of small-scale, clumped structure. Key questions are how such small-scale clumped structure affects the nonradial line-force components that arise from large-scale velocity-gradient asymmetries, and in particular whether such nonradial forces can still inhibit WCD formation in a clumped flow. The results have important implications for the viability of the WCD mechanism as a paradigm for disk formation in Be stars.

scholarly journals A Comparison of the Anelastic and Subseismic Approximations for Low-Frequency Stellar Oscillations: an Application to Rapidly Rotating Stars

2002 ◽  
Vol 185 ◽  
pp. 186-189
Author(s):  
B. Dintrans ◽  
M. Rieutord
Keyword(s):  
Acoustic Waves ◽  
Low Frequency ◽  
Rotating Stars ◽  
Stellar Oscillations ◽  
Rapidly Rotating Stars ◽  
Better Than

AbstractAfter showing that the anelastic approximation is better than the subseismic one to filter out acoustic waves when studying low-frequency stellar oscillations, we compute gravito-inertial modes of a typical γ Doradus star using this approximation. We show that eigenmodes can be regular or singular, according to the possible focusing towards attractors of the underlying characteristics. Consequences on the oscillations spectrum are then discussed.

scholarly journals Line Profile and Photometric Variations of the Be Star η Cen

2002 ◽  
Vol 185 ◽  
pp. 252-253
Author(s):  
R. Levenhagen ◽  
N. Leister ◽  
E. Janot-Pacheco ◽  
J. Zorec ◽  
A. Hubert ◽  
...  
Keyword(s):  
High Resolution ◽  
Line Profile ◽  
Rotational Frequency ◽  
Current Status ◽  
Be Stars ◽  
Rotating Stars ◽  
Fundamental Parameters ◽  
Spectrophotometric Data ◽  
Be Star ◽  
Rapidly Rotating Stars

AbstractWe review the current status of our monitoring project on Be stars. Line profile variations in Helλ667.8 nm were detected in the Be star η Cen, by means of high resolution and S/N Spectroscopic observations. They were interpreted in terms of nonradial pulsations (NRP). The fundamental parameters of η Cen obtained from BCD spectrophotometric data and interpreted using models of rapidly rotating stars, have been used to estimate the stellar rotational frequency.

scholarly journals Unified NRP-Modelling of Be Stars

2002 ◽  
Vol 185 ◽  
pp. 240-243 ◽  
Author(s):  
Th. Rivinius ◽  
D. Baade ◽  
S. Štefl ◽  
M. Maintz
Keyword(s):  
Line Profile ◽  
Inclination Angle ◽  
Be Stars ◽  
Rotating Stars ◽  
Early Type ◽  
Rapidly Rotating Stars ◽  
Nonradial Pulsation ◽  
Low Order

AbstractRecently, the line profile variability (lpv) of two low-v sin i Be stars, μ Cen and ω (28) CMa was successfully modelled as nonradial pulsation (nrp) of rapidly rotating stars seen pole-on. In this work, it is shown that the lpv of low-v sin i early-type Be stars in general closely resembles these two cases, and is therefore explainable by the same mechanism. The lpv of intermediate to high-v sin i Be stars can be explained by the same model if the inclination angle of the model alone is increased. Consequently, early-type Be stars form a distinct, fairly homogeneous class of non-radial low-order g-mode pulsators.

scholarly journals Be and Bn stars: Balmer discontinuity and stellar-class relationship

2020 ◽  
Vol 634 ◽  
pp. A18
Author(s):  
Y. R. Cochetti ◽  
J. Zorec ◽  
L. S. Cidale ◽  
M. L. Arias ◽  
Y. Aidelman ◽  
...  
Keyword(s):  
High Resolution ◽  
Main Sequence ◽  
Rotational Velocity ◽  
Evolutionary Stage ◽  
Circumstellar Envelope ◽  
Be Stars ◽  
Rotating Stars ◽  
Circumstellar Material ◽  
Low Mass ◽  
Rapidly Rotating Stars

Context. A significant number of Be stars show a second Balmer discontinuity (sBD) attributed to an extended circumstellar envelope (CE). The fast rotational velocity of Be stars undoubtedly plays a significant role in the formation of the CE. However, Bn stars, which are also B-type rapidly rotating stars, do not all present clear evidence of being surrounded by circumstellar material. Aims. We aim to characterize the populations of Be and Bn stars, and discuss the appearance of the sBD as a function of the stellar parameters. We expect to find new indices characterizing the properties of CEs in Be stars and properties relating Be and Bn stars. Methods. We obtained low- and high-resolution spectra of a sample of Be and Bn stars, derived stellar parameters, characterized the sBD, and measured the emission in the Hα line. Results. Correlations of the aspect and intensity of the sBD and the emission in the Hα line with the stellar parameters and the V sin i are presented. Some Bn stars exhibit the sBD in absorption, which may indicate the presence of rather dense CEs. Six Bn stars show emission in the Hα line, so they are reclassified as Be stars. The sBD in emission appears in Be stars with V sin i ≲ 250 km s−1, and in absorption in both Be and Bn stars with V sin i ≳ 50 km s−1. Low-mass Be and Bn stars share the same region in the Hertzsprung-Russell diagram. The distributions of rotational to critical velocity ratios of Be and Bn stars corresponding to the current stellar evolutionary stage are similar, while distributions inferred for the zero-age main sequence have different skewness. Conclusions. We found emission in the Hα line and signs of a CE in some Bn stars, which motivated us to think that Bn and Be stars probably belong to the same population. It should be noted that some of the most massive Bn stars could display the Be phenomenon at any time. The similarities found among Be and Bn stars deserve to be more deeply pursued.

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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