ASTEROSEISMOLOGYASTEROSEISMOLOGY
CoRoT session, January 13, 2007CoRoT session, January 13, 2007Jadwiga Daszyńska-DaszkiewiczJadwiga Daszyńska-Daszkiewicz
Instytut Astronomiczny, Uniwersytet WrocławskiInstytut Astronomiczny, Uniwersytet Wrocławski
European Helio- and Asteroseismology Network
Participants
HELAS Activities: HELAS Activities:
Global HelioseGlobal Helioseiismologsmologyy
LoLocalcal Heliose Helioseiismologsmologyy
AsteroseAsteroseiismologsmologyy
Public OutreachPublic Outreach
CoRoT Mission 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 – aster – from Greek means star
seismos – seismos – Gr. quake, tremor
logos – logos – Gr. word, reason
Helioseismology
helios – helios – Gr. Sun
Changes of the brightness and/or the radial velocity are the observed evidences of pulsations.
Pulsating star - star in which variability is due to pulsations, i.e. acoustic and/or gravity waves propagating in its envelope and interior.
WHY STARS PULSATE ?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 near-
surface 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 shallowshallow the amount of the energy absorbed by thin matter will be insufficient to maintain pulsations
The driving region located too deepdeep the amplitude of the temperature variations is very small and the layer will absorb too small amount of energy to be efficient
log Teff
log
(L
/L)
A star hotter than Teff~7500K has regions of partial ionization too close to the surface.In a star cooler than Teff~5500K convection prevents the accumulation of heat and pressure.
Blue edge of the classical
instability strip
Red edge of the classical
instability strip
Various types of pulsating stars in the HR diagram
J. Christensen-J. Christensen-DalsgaardDalsgaard
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.
The main effect of excitation takes place in a thin subphotospheric layer, where
the speeds are close to the sound speed, cs.
Solar oscillations are damped oscillationsexcited stochasticaly by near-surface
convection.
The The Sun Sun as pulsating staras pulsating star
5-minute oscillations of the Sun were 5-minute oscillations of the Sun were discovered in 1962discovered 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 HOW STARS PULSATE ?PULSATE ?
1-dimensional oscillations1-dimensional oscillations
Fundamental First overtone Second overtone
nodes
D. Kurtz
2-dimensional radial oscillations2-dimensional radial oscillations
Fundamental First overtone Second overtone
3-dimensional radial pulsations with n = 23-dimensional radial pulsations with n = 2
dipole =1 quadrupole =2
2-dimensional non-radial oscillations2-dimensional non-radial oscillations
3-dimensional non-radial oscillations 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=1
= 3, m=2 = 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 CAN WE WE HEAR HEAR STELLAR STELLAR PULSATIONS ?PULSATIONS ?
NO !NO !
BUT WE CAN OBSERVEBUT WE CAN OBSERVE
THE EFFECTTHE EFFECTSS OF OF PULSATIONSPULSATIONS
Mira 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 Doppler shift can be used to derive radial velocityradial velocity
Line profile variationsLine profile variations
AsteroseAsteroseiismolosmologygy
AAm
plit
ud
em
plit
ud
e
Pulsation frequency [c/d]Pulsation frequency [c/d]
= 2 = 20
= 25 = 75
http://astro.phys.au.dk/helio_outreach
SEISMIC MODEL OF THE STARSEISMIC MODEL OF THE STAR
theoretical frequencies = observed theoretical frequencies = observed frequenciesfrequencies
Which constraints can be obtained from Which constraints can be obtained from asteroseismology ?asteroseismology ?
MassMass
AgeAge
Chemical abundanceChemical abundance
Efficiency of convectionEfficiency of convection
Test of atomic dataTest of atomic data (opacities) (opacities)
Internal rInternal rotationotation
HelioseismologyHelioseismology
Oscillation frequencies can be used to Oscillation frequencies can be used to yield information on the structure and yield information on the structure and
dynamics inside the Sun.dynamics inside the Sun.
Periodogram from the radial velocityPeriodogram from the radial velocity measurements on the Sun (BiSON measurements on the Sun (BiSON
experiment)experiment)
What have we learnt from helioseismology ?What have we learnt from helioseismology ?
Age of the SunAge of the Sun
Depth of convection zoneDepth of convection zone
Test of opacities, equation of stateTest of opacities, equation of state
Helium abundanceHelium abundance
Internal rotation rate of the SunInternal rotation rate of the Sun
Inferred rotation rate of the Sun as a Inferred rotation rate of the Sun as a function of radius for indicated function of radius for indicated heliographic latitudes; from MDI data.heliographic latitudes; from MDI data.
J. Christensen-J. Christensen-DalsgaardDalsgaard
Rotation of the SunRotation of the Sun
J. Christensen-J. Christensen-DalsgaardDalsgaard
L. GizonL. Gizon
Local helioseismologyLocal helioseismology
ASTEROSEISMOLOGY:ASTEROSEISMOLOGY:
THE MUSIC OF THE THE MUSIC OF THE SPHERESSPHERES
The audible range
from 20 to 20.000 Hz
1 cycle per second = 1 cycle per second = 1 Hz 1 Hz
5 min 0.003 Hz
„„SOUNDSSOUNDS”” OF PULSATIONS OF PULSATIONS
Centauri
Hydrae
The Sun
Zoltan KollathZoltan Kollath
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