Post on 15-Jan-2016
description
Asteroseismology of solar-type stars
Jørgen Christensen-Dalsgaard
Institut for Fysik og Astronomi, Aarhus Universitet
Dansk AsteroSeismologisk Center
Solarlike pulsators
Mode damping
Damping rate: amplitude / exp( t), with
Perturbation in convective flux and effect of turbulent pressure perturbation pt appear to yield negative , i.e., damped modes
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
Asymptotics of low-degree p modes
Large frequency separation:
Small frequency separations
Echelle diagramPresent Sun
25 years ago:the start of global helioseismology
Now: the start of solar-like asteroseismology
Cen A (Bedding et al. 2004; ApJ 614, 380)
The observational breakthrough
ESO Very Large Telescope
UVES spectrograph
ESO 3.6 m + HARPS+ HARPS clone?
Very stable spectrographs, motivated by search for exo-planets
The slightly out-of-date situation
Bedding & Kjeldsen (2003; Publ. Astron. Soc. Australia 20, 203)
α Centauri A
α Centauri A
Observations with UVES on VLT
(Butler et al, 2004; ApJ 600, L75)
α Centauri A
VLT(UVES) and AAT(UCLES)
optimally combined
Bedding et al. (2004; ApJ 614, 380)
α Centauri B
UVES (VLT) and UCLES (AAT)
Kjeldsen et al. (2005; submitted to ApJ)
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)
Fitting the α Cen systemObservable 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:
α Centauri system
OPAL EOS, OPAL96 opacity, He, Z settling
(Teixeira et al.)
MA: 1.11111 M¯
MB: 0.92828 M¯
X0: 0.71045
Z0: 0.02870
Age: 6.9848 Gyr
A borderline case
Best-fit model
Model with convective core
α Centauri A
Model problems?
α Centauri B
η Bootis
Observed power spectrum
Kjeldsen et al. (1995; AJ 109, 1313)
Evolutionary state
1.66 M¯
1.6 M¯
Pre-Hipparcos
Post-Hipparcos
Christensen-Dalsgaard et al. (1995; ApJ 443, L29)
Mode trapping
N, Sun
S1
N, Boo
S2
Mixed modes
Echelle diagram
Di Mauro et al. (2003; Astron. Astrophys. 404, 341)
◦ : 0∆ : 1
□ : 2
◊ : 3
Semiregular variables
Semiregular variables
MirasSemiregular variables
AAVSO observations. Mattei et al. (1997; ESA SP-402, 269)
Statistics of stochastically excited oscillators
Energy is exponentially distributed.
Hence amplitude distribution is
Christensen-Dalsgaard et al. (2001; ApJ 562, L141)
Large-scale surveys
Kiss & Bedding (2003; MNRAS 343, L79)
OGLE-II observations of LMC
(Colour indicates J - K)
AGB
RGB