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