Asteroseismology of solartype stars

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Asteroseismology of solar-type stars

• Jørgen Christensen-Dalsgaard
• Institut for Fysik og Astronomi, Aarhus
  Universitet
• Dansk AsteroSeismologisk Center

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Solarlike pulsators
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Mode damping
Damping rate amplitude / exp(h t), with
Perturbation in convective flux and effect of
turbulent pressure perturbation d pt appear to
yield negative h, i.e., damped modes
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Characteristics of solar-like oscillations

• Intrinsically damped by the effects of
  convection
• Stochastically excited by the effects of
  convection
• Typically very small amplitudes (20 cm/sec or 5
  ppm for main-sequence stars)

Found in stars that are not very solar-like
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Asymptotics of low-degree p modes
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Small frequency separations
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Asteroseismic HR diagram
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Echelle diagram
Present Sun
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25 years agothe start of global helioseismology
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Now the start of solar-like asteroseismology
a Cen A (Bedding et al. 2004 ApJ 614, 380)
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The observational breakthrough
Very stable spectrographs, motivated by search
for exo-planets
ESO Very Large Telescope
UVES spectrograph
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The slightly out-of-date situation
Bedding Kjeldsen (2003 Publ. Astron. Soc.
Australia 20, 203)
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a Centauri A
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a Centauri A
Observations with UVES on VLT (Butler et al,
2004 ApJ 600, L75)
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a Centauri A
VLT(UVES) and AAT(UCLES) optimally
combined Bedding et al. (2004 ApJ 614, 380)
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a Centauri B
UVES (VLT) and UCLES (AAT) Kjeldsen et al. (2005
submitted to ApJ)
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Classical observables
(a) Pourbaix et al. (2002 Astron. Astrophys.
386, 280) (b) Pijpers (2003 Astron. Astrophys.
400, 241) (c) Kervella et al. (2003 Astron.
Astrophys. 404 1087)
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Fitting the a Cen system
Observable quantities for the system
Model parameters
Fit using Marquardt method, with centred
differences, using an 8-processor Linux cluster,
implemented by T. C. Teixeira
Choice of oscillation variables, from Bedding et
al. fits to Butler et al. observations
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a Centauri system
MA 1.11111 M MB 0.92828 M X0 0.71045 Z0
0.02870 Age 6.9848 Gyr
OPAL EOS, OPAL96 opacity, He, Z
settling (Teixeira et al.)
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A borderline case
Best-fit model
Model with convective core
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a Centauri A
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a Centauri B
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? Bootis
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Observed power spectrum
Kjeldsen et al. (1995 AJ 109, 1313)
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Evolutionary state
Christensen-Dalsgaard et al. (1995 ApJ 443, L29)
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Mode trapping
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Mixed modes
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Echelle diagram
? 0 ? 1 ? 2 ? 3
Di Mauro et al. (2003 Astron. Astrophys. 404,
341)
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Semiregular variables
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Semiregular variables
AAVSO observations. Mattei et al. (1997 ESA
SP-402, 269)
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Statistics of stochastically excited oscillators
Energy is exponentially distributed. Hence
amplitude distribution is
Christensen-Dalsgaard et al. (2001 ApJ 562, L141)
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Large-scale surveys
OGLE-II observations of LMC
Kiss Bedding (2003 MNRAS 343, L79)
(Colour indicates J - K)