8,132 research outputs found
Surface trapping and leakage of low-frequency g-modes in rotating early-type stars -- I. Qualitative analysis
A qualitative study of the surface trapping of low-frequency non-radial
g-modes in rotating early-type stars is undertaken within the Cowling,
adiabatic and traditional approximations. It is demonstrated that, at
frequencies below a cut-off, waves cannot be fully trapped within the star, and
will leak through the surface. Expressions for the cut-off frequency are
derived in both the non-rotating and rotating cases; it is found from these
expressions that the cut-off frequency increases with the rotation rate for all
but prograde sectoral modes.
The results are of possible relevance to the 53 Per and SPB classes of
variable star, which exhibit pulsation frequencies of the same order of
magnitude as the cut-off frequencies found for the stellar model. It is
suggested that observations either of an upper limit on variability periods
(corresponding to the cut-off), or of line-profile variations due to leaking
modes, may permit asteroseismological studies of the outer layers of these
systems.Comment: 8 pages, 2 figures, to be published in MNRA
Excitation of g modes in Wolf-Rayet stars by a deep opacity bump
We examine the stability of l=1 and l=2 g modes in a pair of nitrogen-rich
Wolf-Rayet stellar models characterized by differing hydrogen abundances. We
find that modes with intermediate radial orders are destabilized by a kappa
mechanism operating on an opacity bump at an envelope temperature log T ~ 6.25.
This `deep opacity bump' is due primarily to L-shell bound-free transitions of
iron. Periods of the unstable modes span ~ 11-21 hr in the model containing
some hydrogen, and ~ 3-12 hr in the hydrogen-depleted model. Based on the
latter finding, we suggest that self-excited g modes may be the source of the
9.8 hr-periodic variation of WR 123 recently reported by Lefevre et al. (2005).Comment: 5 pages, 3 figures, accepted by MNRAS letter
Influence of the Coriolis force on the instability of slowly pulsating B stars
This paper explores the effect of rotation on the kappa-mechanism instability
of slowly pulsating B stars. A new nonadiabatic code, that adopts the so-called
`traditional approximation' to treat the Coriolis force, is used to investigate
the influence exerted by rotation over the stability of stellar models covering
the mass range 2.5 M_sun <= M_* <= 13.0 M_sun. The principal finding is that,
for all modes considered apart from the prograde sectoral class, rotation
shifts the kappa-mechanism instability toward higher luminosities and effective
temperatures; these shifts are accompanied by broadenings in the extent of
instability strips. Such behaviour is traced to the shortening of mode periods
under the action of the Coriolis force. Instability strips associated with
prograde sectoral modes behave rather differently, being shifted to marginally
lower luminosities and effective temperatures under the influence of rotation.
The implications of these results are discussed in the context of the
observational scarcity of pulsation in B-type stars having significant
rotation; various scenarios are explored to explain the apparent dichotomy
between theory and observations. Furthermore, the possible significance of the
findings to Be stars is briefly examined.Comment: 14 pages, 4 figures, to be published by MNRA
Centrifugal Breakout of Magnetically Confined Line-Driven Stellar Winds
We present 2D MHD simulations of the radiatively driven outflow from a
rotating hot star with a dipole magnetic field aligned with the star's rotation
axis. We focus primarily on a model with moderately rapid rotation (half the
critical value), and also a large magnetic confinement parameter, . The magnetic field
channels and torques the wind outflow into an equatorial, rigidly rotating disk
extending from near the Kepler corotation radius outwards. Even with
fine-tuning at lower magnetic confinement, none of the MHD models produce a
stable Keplerian disk. Instead, material below the Kepler radius falls back on
to the stellar surface, while the strong centrifugal force on material beyond
the corotation escape radius stretches the magnetic loops outwards, leading to
episodic breakout of mass when the field reconnects. The associated dissipation
of magnetic energy heats material to temperatures of nearly K, high
enough to emit hard (several keV) X-rays. Such \emph{centrifugal mass ejection}
represents a novel mechanism for driving magnetic reconnection, and seems a
very promising basis for modeling X-ray flares recently observed in rotating
magnetic Bp stars like Ori E.Comment: 5 pages, 3 figures, accepted by ApJ
The Rigidly Rotating Magnetosphere of Sigma Ori E
We attempt to characterize the observed variability of the magnetic
helium-strong star sigma Ori E in terms of a recently developed rigidly
rotating magnetosphere model. This model predicts the accumulation of
circumstellar plasma in two co-rotating clouds, situated in magnetohydrostatic
equilibrium at the intersection between magnetic and rotational equators. We
find that the model can reproduce well the periodic modulations observed in the
star's light curve, H alpha emission-line profile, and longitudinal field
strength, confirming that it furnishes an essentially correct, quantitative
description of the star's magnetically controlled circumstellar environment.Comment: 4 pages, 3 figures, accepted by Ap
A Rigid-Field Hydrodynamics approach to modeling the magnetospheres of massive stars
We introduce a new Rigid-Field Hydrodynamics approach to modeling the
magnetospheres of massive stars in the limit of very-strong magnetic fields.
Treating the field lines as effectively rigid, we develop hydrodynamical
equations describing the 1-dimensional flow along each, subject to pressure,
radiative, gravitational, and centrifugal forces. We solve these equations
numerically for a large ensemble of field lines, to build up a 3-dimensional
time-dependent simulation of a model star with parameters similar to the
archetypal Bp star sigma Ori E. Since the flow along each field line can be
solved for independently of other field lines, the computational cost of this
approach is a fraction of an equivalent magnetohydrodynamical treatment.
The simulations confirm many of the predictions of previous analytical and
numerical studies. Collisions between wind streams from opposing magnetic
hemispheres lead to strong shock heating. The post-shock plasma cools initially
via X-ray emission, and eventually accumulates into a warped, rigidly rotating
disk defined by the locus of minima of the effective (gravitational plus
centrifugal) potential. But a number of novel results also emerge. For field
lines extending far from the star, the rapid area divergence enhances the
radiative acceleration of the wind, resulting in high shock velocities (up to
~3,000 km/s) and hard X-rays. Moreover, the release of centrifugal potential
energy continues to heat the wind plasma after the shocks, up to temperatures
around twice those achieved at the shocks themselves. Finally, in some
circumstances the cool plasma in the accumulating disk can oscillate about its
equilibrium position, possibly due to radiative cooling instabilities in the
adjacent post-shock regions.Comment: 21 pages, 12 figures w/ color, accepted by MNRA
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