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ASTEROSEISMOLOGY
CoRoT session, January 13, 2007
Jadwiga Daszyńska-Daszkiewicz
Instytut Astronomiczny, Uniwersytet Wrocławski
European Helio- and Asteroseismology
Network
Participants
HELAS Activities:
Global Helioseismology
Local Helioseismology
Asteroseismology
Public Outreach
CoRoT Mission
Sir Arthur Eddington (1882 – 1944)
„At first sight it would seem that the
deep interior of the sun and stars is less
accessible to scientific investigation than
any other region of the universe.”
Asteroseismology
Investigation of stellar interiors by
means of the oscillation frequencies
aster – from Greek means star
seismos – Gr. quake, tremor
logos – Gr. word, reason
Helioseismology
helios – Gr. Sun
Pulsating star - star in which variability is due to
pulsations, i.e. acoustic and/or gravity waves
propagating in its envelope and interior.
Changes of the brightness and/or the radial velocity
are the observed evidences of pulsations.
WHY STARS PULSATE ?
1. self-excitation
2. excitation by an external force
Ad. 1. there are regions in a star which work like a heat engine,
e.g. pulsation of classical Cepheids
Ad. 2. stochastic excitation by turbulent convection in the nearsurface regions, e.g. solar-like oscillations
When a Cepheid envelope begins to shrink (red
arrows), it is almost transparent for the outgoing
radiation (brown arrows). This phase corresponds
to the onset of the compression stroke in an
internal combustion engine.
In the phase of maximum compression the
envelope absorbs outgoing radiation and
begins to expand. This phase corresponds to
the ignition at the beginning of the
combustion stroke.
The driving zone has to be located at an optimal
geometrical depth in the stellar envelope.
The driving region located too shallow  the amount of
the energy absorbed by thin matter will be insufficient
to maintain pulsations
The driving region located too deep  the amplitude of
the temperature variations is very small and the layer
will absorb too small amount of energy to be efficient
In a star cooler than
Teff~5500K convection
prevents the accumulation
of heat and pressure.
log (L/L)
A star hotter than Teff~7500K
has regions of partial ionization
too close to the surface.
Blue edge of the
classical instability
strip
Red edge of the
log Teffclassical instability
strip
Various types of pulsating stars in the HR diagram
J. Christensen-Dalsgaard
The sound waves are generated by a stochastic velocity
field in the near-surface convection, where turbulent
motions have speeds close to the speed of sound.
These waves propagate into the interior
and produce the standing waves.
Solar oscillations are damped oscillations
excited stochasticaly by near-surface convection.
The main effect of excitation takes place in a thin
subphotospheric layer, where the speeds are close
to the sound speed, cs.
The Sun as pulsating star
5-minute oscillations of the Sun were discovered in 1962.
amplitudes of the brightness variations: ~2 mag
amplitudes of the radial velocity variations: ~20 cm/s
oscillations periods: 3-25 min
lifetimes of modes: of the order of days, weeks
number of modes: ~ 107
HOW STARS PULSATE ?
1-dimensional oscillations
Fundamental
First overtone
Second overtone
nodes
D. Kurtz
2-dimensional radial oscillations
Fundamental
First overtone
Second overtone
3-dimensional radial pulsations with n = 2
2-dimensional non-radial oscillations
dipole =1
quadrupole =2
3-dimensional non-radial oscillations  = 3
W. Zima
 = 1, m=0
 = 1, m=1
T. Bedding
 = 2, m=1
 = 2, m=2
 = 3, m=0
 = 3, m=2
 = 3, m=1
 = 3, m=3
 = 4, m=1
 = 4, m=2
 = 4, m=4
 = 5, m=0
 = 5, m=2
 = 5, m=3
 = 8, m=1
 = 8, m=2
 = 8, m=3
CAN WE HEAR
STELLAR PULSATIONS ?
NO !
BUT WE CAN OBSERVE
THE EFFECTS OF PULSATIONS
Mira (  Cet ) – the first pulsating star
discovered in 1596 by David Fabricius.
Visual magnitude: from 3.5 to 9, period equal to 332 days
Doppler shift can be used to derive
radial velocity
Line profile variations
Amplitude
Asteroseismology
Pulsation frequency [c/d]
=2
 = 20
 = 25
 = 75
http://astro.phys.au.dk/helio_outreach
SEISMIC MODEL OF THE STAR
theoretical frequencies = observed frequencies
Which constraints can be obtained from asteroseismology ?
Mass
Age
Chemical abundance
Efficiency of convection
Test of atomic data (opacities)
Internal rotation
Helioseismology
Oscillation frequencies can be used to yield information
on the structure and dynamics inside the Sun.
Periodogram from the radial velocity
measurements on the Sun (BiSON experiment)
What have we learnt from helioseismology ?
Age of the Sun
Depth of convection zone
Test of opacities, equation of state
Helium abundance
Internal rotation rate of the Sun
Inferred rotation rate of the Sun as a function of radius for
indicated heliographic latitudes; from MDI data.
J. Christensen-Dalsgaard
Rotation of the Sun
J. Christensen-Dalsgaard
Local helioseismology
L. Gizon
ASTEROSEISMOLOGY:
THE MUSIC OF THE SPHERES
The audible range
from 20 to 20.000 Hz
1 cycle per second = 1 Hz
5 min
0.003 Hz
„SOUNDS” OF PULSATIONS
The Sun
 Centauri
 Hydrae
Zoltan Kollath