Inside a quaking star: asteroseismic analysis of alpha Centauri

Jørgen Christensen-Dalsgaard

Institut for Fysik og Astronomi, Aarhus Universitet

Teresa Teixeira, Timothy R. Bedding, Hans Kjeldsen ,

Frank P. Pijpers

Back to the future:the solar case

Grec et al., Nature 288, 541; 1980

α Centauri A

Observations with UVES on VLT

(Butler et al, 2004; ApJ 600, L75)

Asymptotics of p modes

Frequency separations:

Near-surface frequency effectsStellar structure and oscillation modelling deal inadequately with

• Treatment of convection in modelling (thermal structure, turbulent pressure)

• Mode damping excitation

• Dynamical effects of convection on oscillations

• Atmospheric structure

These effects are concentrated very near the surface

Near-surface effects (II)General frequency differences between star and model can be represented as

Note:

Near-surface effects are contained in H2

Solar data,

l = 0 - 100

Solar data,

l = 0 - 100

Near-surface effects (III)

From solar observations over a range of degrees, H2(¯)() is known.

Can that be used for other stars, somehow?

Use as ansatz

On physical grounds we expect

α Centauri A

α Centauri A

VLT(UVES) and AAT(UCLES)

optimally combined

Bedding et al., ApJ, submitted

Fitting observed frequencies Cen A

α Centauri B

UVES (VLT) and UCLES (AAT)

Kjeldsen et al. (in preparation)

Classical variables

(a) Pourbaix et al. (2002)

(b) Pijpers (2003)

(c) Kervella et al. (2003)

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 fits to observations and model frequencies:

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

α Centauri system

: A: B

α Centauri A

Observations: use Bedding et al. fits

Models: surf = 0.75

α Centauri B

Observations: use Bedding et al. fits

Models: surf = 0.75

α Centauri A

Observations: use Bedding et al. fits

Models: surf = 0.75

α Centauri B

Observations: use Bedding et al. fits

Models: surf = 0.75

What is next?

Better data

Better models

Better analysis techniques

More stars

The future

Rømer: a Danish micro-satellite project

?

A swarm of Rømers