dynamical Asteroseismology - Iowa State Universitycourse.physastro.iastate.edu/astro580/... ·...

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Asteroseismology

Steve KawalerIowa State University

observed pulsations• operate on the dynamical time scale

• accessible on convenient time scale

• probe global and local structure

• periods change on ‘evolutionary’ time scale (thermal or nuclear) - depend on global properties

• amplitudes change on ~ ‘local’ thermal time scale

dynamical stability• “stable” configuration represents a stable

mean configuration

• on short time scale, oscillations occur, but the mean value is fixed on longer time scales

• simple example: a pendulum (single mode)• most likely position - extrema• mean position is at zero displacement• with no damping would oscillate forever

• more complex example: a vibrating string• multiple modes with different frequencies• enumerated by number of nodes

a more complex example: a star

• multiple oscillation modes

• radial modes - enumerated by number of nodes between center and surface

• non-radial modes - nodes also across surface of constant radius

• modes frequencies determined by solution of the appropriate wave equation

stability, damping, and driving

• zero energy change: constant amplitude oscillation

• energy loss via pulsation: oscillation amplitude drops with time

• if net energy input: amplitude increases with time (if properly phased)

Okay, start your engines...•PG 1159: light curve

• what kind of star might this be?

• what kind of star can this not possibly be?

• what about the amplitude over the run?

•PG 1336 light curve

• huh? what time scale(s) are involved

• what kind of star (or stars)?

• tell us everything you can about this!

PG 1159-035�7

13 cycles

•13 cycles in 6900 s so P ~ 530 s

•multiperiodic!• amplitude variation cycle ~ 11,000s

• frequency interval of ~ 90 uHz

•other beats present - more periods are there...

beat period

PG 1159• primary pulsation period ~8 minutes

• but multiperiodic

• beating seen in single night’s data

• closely spaced (in frequency) pulsations

• dynamical time ~ 8 minutes?

• implies mean density of ~30 g/cm3

• or a radius of ~1/3 Rsun or log g ~ 5.4

• but spectroscopy says: log g ~ 7.5!

• so pulsations are longer than dynamical time scale by a bit...

PG 1336

• eclipse (orbital) period ~ 8790 seonds• circular orbit• pulsation period ~ 180 seconds• beat period: ~ 5500 s

frequency interval of ~ 180 uHz

8790 s

12 cycles

5500 s

PG 1336

• classic eclipsing binary light curve• period of 2.4 hours• circular orbit

• depth of primary eclipse is 0.5 mag

• partial eclipse covers ~ half of primary

• reflection effect / secondary eclipse

• secondary is invisible otherwise

• eclipse durations ~ same

• stars have similar radii (one bright, one faint)

PG 1336, cont’d• Superimposed rapid oscillations - primary

(bright) star

• period ~ 180 seconds

• dynamical time scale: mean density ~ 100 g/cm3

• radius ~ 0.2 Rsun , log g ~ 5.6 consistent with spectroscopy!

• primary = sdB star, secondary = M dwarf

Multimode pulsation

• Oscillations at “normal mode” frequencies

• mode = specific eigensolution of equations of motion within the confines of a stellar structure

• normal mode frequencies parallel structural properties

• simple example: radial fundamental is one mode, 1st overtone (a node within) is another mode

towards the wave equation I • continuity equation:

• equation of motion (HSE when =0):

• perturb r, P, and ρ:

• and assume δx << x so we can linearize

@t2= �GMr

r2� 4⇡r2 @P

@r= 4⇡r2⇢

x(t, Mr

�x(t, Mr

• replace x with x+δx in the two equations, subtract off the equilibrium equations, and keep only 1st-order terms to find:

• linearized continuity equation

• linearized equation of motion

�⇢

= �3�r

� ro

@(�r/ro

d2�r/ro

dt2= �

✓4�r

� Po

@(�P/Po

towards the wave equation II

• assume adiabatic relationship between P and ρ

• combine continuity and equation of motion:

• assume exponential (complex) time dependence

towards the wave equation III

�r(t, ro

=�r(r

ei�t = ⌘(ro

)ei�t

�1 =

✓@ lnP

@ ln ⇢

= �1�⇢

1⇢r4

✓�1Pr4 @⌘

◆+ ⌘

@r[(3�1 � 4) P ]

• substitute to yield the Linear Adiabatic Wave Equation (LAWE):

• This is a wave equation: L(η) = σ2η in the displacement η.

• the eigenvalue σ2 corresponds to the oscillation frequency

towards the wave equation IV

L(⌘) = � 1⇢r4

✓�1Pr4 @⌘

◆� ⌘

@r[(3�1 � 4) P ]

�= �2⌘

the LAWE: a simple case• assume Γ1 and η both constant throughout

the star (homologous motion)

• LAWE becomes

• now, assume a constant density, and use HSE to replace the pressure derivative to find

• look familiar?!

�⌘1⇢r

(3�1 � 4)@P

@r= �2⌘

⇧ =2⇡

G⇢̄ (�1 � 43 )

the LAWE: standing wave solutions

• boundary conditions:

• center: zero displacement (η = 0)

• surface: perfect wave reflection [d(δP/P)/dr = 0]

• asymptotic analysis:

• clever change of variables renders LAWE as:

• recognizing the sound speed when we see it:

d2w(r)dr2

�2⇢

�1P� �(r)

�w(r) = 0

d2w(r)dr2

� �(r)�

w(r) = 0

the LAWE: asymptotic solution

• represent the eigenfunction as: where kr is the (local) radial wavenumber and varies slowly with radius so, locally:

• for a standing wave, we need an integral number of half-wavelengths between inner and outer reflection points:

d2w(r)dr2

� �(r)�

w(r) = 0

w(r) / eikrr

c2s(r)

� �(r)

akr dr = (n + 1)⇡

the LAWE: asymptotic solution

• if then

• i.e. high-frequency (high overtone, n) radial modes are equally spaced in frequency, with

σo2 ≈ G<ρ>

akr dr = (n + 1)⇡

� �

� = (n + 1)⇡

= (n + 1) �o

c2s(r)

� �(r)where

Nonradial oscillations• preserve angular derivatives in LAWE

• similar operator structure for radial part (as before), now along with angular part

where the quantity A is:

A < 0 when radiative A > 0 when convective

(the ‘Schwarzschild A’)

A =d ln ⇢

dr� 1

�⇢[r�rad]

d2�rdt2

= �r✓

⇢+ 0

◆+ A

⇢r · �r

Decompose into Spherical Harmonics

• position perturbation decomposition

• produces (after some work):

where the operator L2 (the Legendrian) is:

• which has eigenstates Ylm such that:

�r = �r er + r�✓ e✓ + r sin ✓�� e�

r · �r =1r2

@(r2�r)@r

� 1�2

✓P 0

⇢+ 0

L2 = � 1sin ✓

✓sin ✓

◆� 1

sin2 ✓

L2Y ml (✓, �) = l(l + 1)Y m

l (✓, �)

l=1, m=0 l=1, m=1

l=3, m=0 l=3, m=1 l=3, m=3

Spherical Harmonics.... courtesy asteroseismology.org (Travis Metcafe)

• now we have

• expanding into components:

using Spherical Harmonics�24

r · �r =1r2

@(r2�r)@r

� l(l + 1)�2r2

✓P 0

⇢+ 0

r2 d⌘t

�r⌘r +

(�Ag)

g� 1

�r⌘t

r2 d⌘r

� 2�

r⌘r + r2 l(l + 1)r2

1� �2r2

1l(l + 1)

�r⌘t

frequency-2

another frequency2

• so:

• where we’ve defined 2 “structural” frequencies

• the acoustic (Lamb) frequency Sl :

• the Brunt-Väisälä (buoyancy) frequency N:

the two characteristic frequencies�25

r2 d⌘r

gl(l + 1)

� 2r

�⌘r + l(l + 1)

1� �2

�r⌘t

r2 d⌘t

1� N2

�r⌘r +

g� 1

�r⌘t

l(l + 1)r2

N2 = �Ag = �g

d ln ⇢

dr� 1

Propagation diagram, ZAMS solar model�26

Sl2, l=1

CZbase

NRP dispersion relation• identify the horizontal wave number(s)

• allows the wave equation(s) to reduce to a local dispersion relation, as with the radial case, to provide relationship between kr and σ:

l(l + 1)r2

1�2c2

(�2 �N2)(�2 � S2l )

asymptotic analysis

• kr2 > 0 (kr real) when

• σ2 > N2, Sl2 - or - σ2 < N2, Sl2

• kr real means oscillatory eigenfunctions

• kr2 < 0 (kr imaginary) when

• Sl2 > σ2 > N2 or Sl2 < σ2 < N2

• kr real means evanescent (exponentially decreasing or increasing) eigenfunctions

1�2c2

(�2 �N2)(�2 � S2l )

Propagation diagram, ZAMS solar model�29

Sl2, l=1

CZbase

l=1 n=1l=1 n=2l=1 n=3l=1 n=4

page �30

Sl2, l=1

CZbase

l=1 n=1l=1 n=2l=1 n=3l=1 n=4

Propagation diagram, ZAMS solar model

evanescentzonesin gray

page �31

Sl2, l=1

CZbase

l=1 n=1l=1 n=2l=1 n=3l=1 n=4

“low” σoscillatory

zonesin green

“high” σoscillatory

zonesin pink

the NRP LAWE: asymptotic solutions

• again, integrate dispersion relation over propagation regions:

• two classes of solutions:

• σ2 > N2, Sl2:

• “p-modes”; pressure as the restoring force

• σ2 < N2, Sl2:

• “g-modes”: buoyancy as the restoring force

1�2c2

(�2 �N2)(�2 � S2l )

akr dr = (n + 1)⇡ where

= (n + l/2)�o

; �o

= n⇧

opl(l + 1)

; ⇧o

= 2⇡2

page �33

Sl2, l=1

CZbase

l=1 n=1l=1 n=2l=1 n=3l=1 n=4

“low” σoscillatory

zonesin green

“high” σoscillatory

zonesin pink

l=1 n=1l=1 n=2l=1 n=3l=1 n=4

Pulsation PeriodsPeriod of ‘radial fundamental’ ~ tff

g-modes p-modes

Periods Π > tff Π < tffrestoring force buoyancy pressure

asymptotic behavior

Π ∝ Πo x n σ ∝ σo x n

examples white dwarfs Cepheids,the Sun

p-modes: ~ equally spaced in frequency�35

n n+1 n+2 n+3 n+4 n+5 n+6n-1n-2

......frequency

= (n + l/2)�o

; �o

l l l l l l l l l

n-2l+1

n-1l+1

n+1l+1

n+2l+1

n+3l+1

n+4l+1

n+5l+1

• if modes of different l present, observed spacing ~ σo / 2

an example: solar-like oscillations in RV of α Cen A (Bouchy et al. 2002)

frequency spacing = 0.1055 mHz

p. �37

How to observe all these low amplitude modes?

have your friends buy a $600,000,000 photometer!

16 Cyg A and B (Metcalfe et al. 2012)�38

asteroseismic radius determination (i.e. Chaplin et al. 2011)

• νmax scales with acoustic cutoff frequency ~ gTe-1/2

• Δν measures mean density:�

��

� �R

��3

��max

�max,�

��

� �R

��2 �Te�

Te�,�

��0.5

Kepler 93b (Ballard et al. 2014)

�41 p. �42

Kepler 93b (Ballard et al. 2014)

g-modes: ~ equally spaced in period�43

n n+1 n+2 n+3 n+4 n+5 n+6n-1n-2

......Period

n n+1 n+2 n+3 n+4 n+5 n+6n-1n-2

.........

...Period

= n⇧

opl(l + 1)

; ⇧o

= 2⇡2

• spacing depends on l

PG 1159-035: a g-mode pulsator

P P-390

424 34 ?

451 61 3x20.3

495 105 5x21.0

516 126 6x21.0

539 149 7x21.3

645 255 12x21.3

832 442 21x21.0

an example: hot white dwarf PG 1159-035 (Corsico et al. [WET] 2008)

in white dwarfs: Πodepends on total stellar mass

PG 1159 as an example(Winget et al. 1991)

Rotational splitting of nonradial oscillations (uniform, slow rotation)

equal frequency spacing: triplets (l=1), quintuplets (l=2) etc.

Pulsating stars in the HR diagram

from J. Christensen-Dalsgaard

“Solar-like” oscillations• globally stable (damped) but constantly excited

• damping time τ generally ~ days

• continuously re-excited by turbulence

• frequencies locked to normal modes of star

• excited mode periods ~ minutes

• broad mode selection, low amplitude

• (integrated) velocity amplitude < meters / second

• photometric amplitude ~ parts per million

Consequences of stochasitic excitation

• lots of modes present . . . but

• coherence time of (only) days lowers peak amplitudes• reduces detectability

• phase instability broadens FT peaks• Lorentzian envelope

• reduces frequency accuracy

• confuses mode identification and rotational splitting effects

from J. C.-D.

Stochastically excited modeheat-engine mode

1/(observing time)

1/lifetime

Observational Differences

1980 1990 2000 2010 2020

1.00T = 5d

1980 1990 2000 2010 2020

T = 10d

1980 1990 2000 2010 2020

T = 30d

coherent pulsators: frequency precision ~ 1/T

1980 1990 2000 2010 2020

1.00T = 5d

1980 1990 2000 2010 2020

T = 10d

1980 1990 2000 2010 2020

T = 30d

1.00T = 5d T = 10d T = 30d

stochastic pulsators - freq. precision poorer than 1/T Asymptotics of low-degree p modes

Large frequency separation:

Frequency separations:

Small frequency separations Asteroseismic HR diagram

from J. Christensen-Dalsgaard

A Search for Habitable Planets

Borucki et al. 2009 -

Kepler’s Optical Phase Curve of the Exoplanet HAT-P-7b

HAT-P7 asteroseismology

1.40 Mo

1.45 Mo

Asteroseismic HR diagram

from J. Christensen-DalsgaardSunα Cen A

HAT-P-7

HAT-P7 asteroseismology

spectroscopyPal et al. 2008

M=1.40±0.02

t = 1.6±0.4 Gyr

r = 1.94±0.05 Rsun

Xc=0.19

quick seismic fitISUEVO

Solar-like pulsators: KIC 6603624 Chaplin et al. 2010

Solar-like - FTs

Sunα Cen A

HAT-P-7

Asteroseismic HR diagram

KIC 11026764 is post-MS!

p.oscillations beyond the MS w/ Kepler(Chaplin & Miglio 2013)

�67 p.Solar-like oscillations w/ Kepler(Chaplin & Miglio 2013)

MS (16 Cyg A)

Subgiant

Base of RGB

500 1000 2000 3000 4000 uHz

p.Solar-like oscillations w/ Kepler(Chaplin & Miglio 2013)

MS (16 Cyg A)

Subgiant

Base of RGB

500 1000 2000 3000 4000 uHz

p. �70

First-ascent RGB

Red Clump

20 40 60 80 100 uHz

oscillations beyond the MS w/ Kepler(Chaplin & Miglio 2013)

scaling relations MS to RGB�71

R/Rsun log g Δν [uHz] νmax

MS 1 4.44 135 3300

RGB base 5 3.04 12.1 140

RGB top 20 1.84 1.5 9 < 1/d1/wk

p.internal rotation of simple RGB models:conservation of local angular momentum

1.9 Rsun 3.0 Rsun 3.8 Rsun 4.2 Rsun

12 Rsun

p. �73KIC7341231:at the base of the RGB

(Deheuvels et al. 2012)

R ~ 2.6 Rsun

at the base of the RGB(Deheuvels et al. 2012)

l = 1 modes (core)l = 2 modes (envelope)

splitting -> global averaged rotation rate�75

l=1, p-mode

splitting -> global averaged rotation rate�76

l=1, core(mixed) mode

p. �77

solid bodyrotation model two-zone

rotation model(5 x faster core than

envelope)

effective

p. �78

p. �79

25 hM=1.24 Mo

R=2.92 Ro

Deheuvels et al. (2014): Kepler RG differential rotation - inversion

p. �80

• reveals initial spin-up of contracting RG core

Deheuvels et al. (2014): Kepler RG differential rotation sample

10 day

100 day

Deheuvels et al. (2014), Mosser et al (2012):Kepler RG differential rotation

• reveals initial spin-up of contracting RG core

• … and subsequent spin-down via coupling with envelope?

spin-up

spin-down (coupling)?

Chaplin et al., Science, 7 April 2011

• Δν measures mean density:

• νmax scales with acoustic cutoff frequency ~ gTe-1/2

• then one can determine M and R:

• and, with Teff from multicolor photometry, one gets L

��

� �R

��3

��max

�max,�

��

� �R

��2 �Te�

Te�,�

��0.5

R��

��max

�max,�

� ��

��

��2 �Te�

Te�,�

�0.5

M��

��max

�max,�

�3 ��

��

��4 �Te�

Te�,�

�1.5

dynamical Asteroseismology - Iowa State Universitycourse.physastro.iastate.edu/astro580/... ·...

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