206 research outputs found
Rotational Splitting of Pulsational Modes
Mode splittings produced by uniform rotation and a particular form of
differential rotation are computed for two-dimensional rotating 10 Mo ZAMS
stellar models. The change in the character of the mode splitting is traced as
a function of uniform rotation rate, and it is found that only relatively slow
rotation rates are required before the mode splitting becomes asymmetric about
the azimuthally symmetric (m=0) mode. Increased rotation produces a
progressively altered pattern of the individual modes with respect to each
other. Large mode splittings begin to overlap with the mode splittings produced
by different radial and latitudinal modes at relatively low rotation rates. The
mode splitting pattern for the differentially rotating stars we model is
different than that for uniformly rotating stars, making the mode splitting a
possible discriminant of the internal angular momentum distribution if one
assumes the formidable challenge of mode identification can be overcome.Comment: 6 journal pages, 7 Figures, accepted by Ap
Pulsations as a Driver for LBV Variability
Among the most spectacular variable stars are the Luminous Blue Variables
(LBVs), which can show three types of variability. The LBV phase of evolution
is poorly understood, and the driving mechanisms for the variability are not
known. The most common type of variability, the S Dor instability, occurs on
timescales of tens of years. During an S Dor outburst, the visual magnitude of
the star increases, while the bolometric magnitude stays approximately
constant. In this work, we investigate pulsation as a possible trigger for the
S Dor type outbursts. We calculate the pulsations of envelope models using a
nonlinear hydrodynamics code including a time-dependent convection treatment.
We initialize the pulsation in the hydrodynamic model based on linear
non-adiabatic calculations. Pulsation properties for a full grid of models from
20 to 85 M were calculated, and in this paper we focus on the few
models that show either long-period pulsations or outburst-like behaviour, with
photospheric radial velocities reaching 70-80 km/s. At the present time, our
models cannot follow mass loss, so once the outburst event begins, our
simulations are terminated. Our results show that pulsations alone are not able
to drive enough surface expansion to eject the outer layers. However, the
outbursts and long-period pulsations discussed here produce large variations in
effective temperature and luminosity, which are expected to produce large
variations in the radiatively driven mass-loss rates.Comment: 9 pages, 10 figures, accepted for publication in MNRA
Radial and Nonradial Oscillation Modes in Rapidly Rotating Stars
Radial and nonradial oscillations offer the opportunity to investigate the
interior properties of stars. We use 2D stellar models and a 2D finite
difference integration of the linearized pulsation equations to calculate
non-radial oscillations. This approach allows us to directly calculate the
pulsation modes for a distorted rotating star without treating the rotation as
a perturbation. We are also able to express the finite difference solution in
the horizontal direction as a sum of multiple spherical harmonics for any given
mode. Using these methods, we have investigated the effects of increasing
rotation and the number of spherical harmonics on the calculated
eigenfrequencies and eigenfunctions and compared the results to perturbation
theory. In slowly rotating stars, current methods work well, and we show that
the eigenfunction can be accurately modelled using 2nd order perturbation
theory and a single spherical harmonic. We use 10 Msun models with velocities
ranging from 0 to 420 km/s (0.89 Omega_c) and examine low order p modes. We
find that one spherical harmonic remains reasonable up to a rotation rate
around 300km s^{-1} (0.69 Omega_c) for the radial fundamental mode, but can
fail at rotation rates as low as 90 km/s (0.23 Omega_c) for the 2H mode or l =
2 p_2 mode, based on the eigenfrequencies alone. Depending on the mode in
question, a single spherical harmonic may fail at lower rotation rates if the
shape of the eigenfunction is taken into consideration. Perturbation theory, in
contrast, remains valid up to relatively high rotation rates for most modes. We
find the lowest failure surface equatorial velocity is 120 km/s (0.30 Omega_c)
for the l = 2 p_2 mode, but failure velocities between 240 and 300 km/s
(0.58-0.69 Omega_c)are more typical.Comment: accepted for publication in Ap
Rotation and Convective Core Overshoot in theta Ophiuchi
(abridged) Recent work on several beta Cephei stars has succeeded in
constraining both their interior rotation profile and their convective core
overshoot. In particular, a recent study focusing on theta$ Oph has shown that
a convective core overshoot parameter of alpha = 0.44 is required to model the
observed pulsation frequencies, significantly higher than for other stars of
this type. We investigate the effects of rotation and overshoot in early type
main sequence pulsators, and attempt to use the low order pulsation frequencies
to constrain these parameters. This will be applied to a few test models and
theta Oph. We use a 2D stellar evolution code and a 2D linear adiabatic
pulsation code to calculate pulsation frequencies for 9.5 Msun models. We
calculate low order p-modes for models with a range of rotation rates and
convective core overshoot parameters. Using these models, we find that the
convective core overshoot has a larger effect on the pulsation frequencies than
the rotation, except in the most rapidly rotating models considered. When the
differences in radii are accounted for by scaling the frequencies, the effects
of rotation diminish, but are not entirely accounted for. We find that
increasing the convective core overshoot decreases the large separation, while
producing a slight increase in the small separations. We created a model
frequency grid which spanned several rotation rates and convective core
overshoot values. Using a modified chi^2 statistic, we are able to recover the
rotation velocity and core overshoot for a few test models. Finally, we discuss
the case of the beta Cephei star theta Oph. Using the observed frequencies and
a fixed mass and metallicity, we find a lower overshoot than previously
determined, with alpha = 0.28 +/- 0.05. Our determination of the rotation rate
agrees well with both previous work and observations, around 30 km/s.Comment: 10 pages, 14 figures. Accepted for publication in A&A
Effects of Uniform and Differential Rotation on Stellar Pulsations
We have investigated the effects of uniform rotation and a specific model for
differential rotation on the pulsation frequencies of 10 \Msun\ stellar models.
Uniform rotation decreases the frequencies for all modes. Differential rotation
does not appear to have a significant effect on the frequencies, except for the
most extreme differentially rotating models. In all cases, the large and small
separations show the effects of rotation at lower velocities than do the
individual frequencies. Unfortunately, to a certain extent, differential
rotation mimics the effects o f more rapid rotation, and only the presence of
some specific observed frequencies with well identified modes will be able to
uniquely constrain the internal rotation of pulsating stars.Comment: 33 pages, 16 figures. Accepted for publication in Ap
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