What
is Asteroseismology?
The search for solar-like
oscillations in selected stars was motivated by the discovery of a multitude
of nonradial (low-degree pressure modes) in our Sun. The analysis of the
frequencies of those oscillations allows the probing of the physical parameters
of inner layers of the Sun. The success Of Helioseismology suggests similar
applications to other stars (asteroseismology).
Promising candidates for asteroseismology
on the main sequence are the Delta Scuti pulsators, which are core-hydrogen-burning
stars with surface temperatures near 7500 K. The amplitudes of pulsation
(1% or less) are larger than those of the Sun, which makes their detection
easier. The matching of observed pulsation frequencies and those computed
from pulsation models (see Breger & Pamyatnykh 1998) enables us to
probe the deep interior of these stars.
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The excitation of dual-character pulsation modes
(gravity-mode behavior in the interior and pressure-mode behaviour near the
surface) permits the probing of the stellar interior to determine its properties
as a function of advancing hydrogen burning. The progess in the last few years
in both the observational and theoretical studies of multiple nonradial pulsation
modes has reached the stage that realistic comparisons of the results and examination
of predictions can be carried out.
On the observational side, the properties of
the small oscillations need to be measured: the frequencies of the multitude
of simultaneously excited nodes and their properties (such as the quantum numbers
of the nonradial pulsation). This requires worldwide telescope campaigns and/or
measurements from space. Probalbly the largest campaign so far is that for the
star FG Vir (Breger at al. 2005). 80 frequencies of pulsation were detected
from the 2002 - 2004 photometry. The properties of many individual pulsation
modes were determined through new spectroscopic techniques (dissertation Zima
2005) and photometric details. Stellar models to include those findings are
presently calculated.
References:
Breger M., Pamyatnykh A.A. 1998, Astronomy and
Astrophysics, 332, 958.
Breger M., Lenz P., Antoci V., et al. 2005, Astronomy and Astrophysics, 435,
955.
Zima W. 2005, Ph. D. dissertation, University of Vienna