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Helioseismology
Helioseismology
uses observed frequencies of solar oscillation
to infer detailed properties of the solar interior.
In this way it has been possible to determine the sound speed
throughout the Sun with a precision much better than 0.1 %;
this provides a test of the computations of stellar evolution and hence
detailed information about the physical properties of matter in the Sun.
Also,
the rotation rate has been measured in most of the solar interior.
The Aarhus group has contributed to both these areas.
The
group collaborates with the principal observational efforts
aimed at measuring the oscillation frequencies;
these include the GONG project and the SOI/MDI instrument
and GOLF instrument on the SOHO satellite.
A great deal of emphasis has been on the development of
techniques for inversion of the observations, to determine structure
and rotation of the solar interior. These techniques have been
used to make detailed inferences about solar internal rotation,
and to test computations of solar models.
The
latter aspect is closely related to extensive work in the group
on computations of solar models, utilizing up-to-date descriptions
of the physics of the solar interior. This has allowed the use
of the helioseismic data to test the equation of state and opacity
of matter in the Sun, providing a basis for more accurate computations
of other types of stars, and yielding insight into the properties
of matter under extreme conditions.
Observational Asteroseismology
The
research is based on the measurement of standing acoustic
waves through the temperature or velocity effects on the emitted
radiation.
We attempt
to develop techniques that will make seismic studies
of stars the main tool for testing stellar evolution.
The work divides according to objects: for solar-type stars
the difficulty is simply to detect the oscillations.
For more massive stars the amplitudes are larger and the
problems lie in the identification of the mode belonging to
each of the observed oscillation frequencies.
Photometric observation with CCD cameras is one of the
favoured observing methods, but more recently spectral
methods have led to important discoveries, such as
1. Detection of oscillations in eta Boo
2. Mode identification in the delta Scuti star FG Vir
Emphasis is placed on the analysis and observations of stars
in open clusters. Among the clusters studied are Praesepe,
NGC 6134 and NGC 1817.
Observations
from the ground of oscillations in solar-type stars are
almost impossible due to the influence of the Earth's atmosphere.
To address this difficulty the group is involved in space programs
(MONS, COROT, SPEX).
Theoretical asteroseismology and stellar evolution
Models
of stellar evolution have been used to compute frequencies
for comparison with the observations mentioned above.
For eta Boo the results provided an estimate of the distance
to the star, subsequently confirmed by measurements from
the HIPPARCOS satellite, and showed tantalizing
hints that the observations might be sensitive to the structure
of the core of the star.
Investigations
are also under way to
study the effect of rapid rotation on the stellar structure and
oscillation frequencies, of importance for many of the delta Scuti
stars being observed. Furthermore, detailed investigations are being
made into the separation of elements within stars under the influence
of gravity and radiative forces.
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