The pulsation of Delta Scuti stars
Abstract
A theoretical study of the pulsation of Delta Scuti
stars is presented. A modified Henyey code is used to
produce a grid of evolutionary models of population I
composition, spanning the area of the Instability Strip on
and near the Main Sequence (mass range 1.4 to 3.0 Mₒ ; log T
> 3.8). Linear pulsation analysis of these models is
carried out, using both adiabatic (Sturm Sequence) and
non-adiabatic techniques. The results are found to agree
closely, and to be consistent with the observed properties
of real stars.
A number of the models are subjected to non-linear
analysis, using a modified Christy hydrodynamic code and the
Carson opacities. A series of models is given an initial
velocity kick of surface amplitude 10 km/s and profile
varying as the fifth power of the fractional radius. The
light curves of these models are found to be dominated by
the fundamental mode and first harmonic. The first five
eigenfunctions produced by the Sturm Sequence method are
used as initial velocity profiles for another series of
models, again with an initial kick of 10 km/s. The light
curves of these models are found to be dominated by the
frequency corresponding to the initially imposed mode of
pulsation. For both sets of models, the light amplitudes
are found to be larger than those of real stars. However,
the indication of a number of criteria is that the models
have not settled down to a final (smaller) amplitude.
The first three Sturm Sequence eigenfunctions are used
as initial velocity profiles for a smaller set of models
with an initial kick of surface amplitude 3 km/s. Again, a
final, settled amplitude is not achieved, but amplitudes are
closer to those of real stars. The shapes of the light and
velocity curves correspond closely to those of the same
models with the larger kick. It is suggested that the form
of the light curve may depend on the mode of pulsation. An
attempt to use the shape of the light curves of real
stars as a diagnostic for the mode of pulsation is
unsuccessful, due to lack of data in the literature.
A number of models depleted of helium are studied. One
set has (X,Y,Z) = (0.88,0.10,0.02). The opacity table for
this composition is obtained by interpolating between the
Carson tables C312 and C402. The second set has (X,Y,Z) =
(0.98,0.00,0.02), corresponding to table C402. Both sets of
models continue to pulsate, and it is concluded that the
region of hydrogen ionisation is an important source of
driving in these stars.
Type
Thesis, PhD Doctor of Philosophy
Rights
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http://creativecommons.org/licenses/by-nc-nd/4.0/
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