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 effects Stellar 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 H 2 Solar data, l = Solar data, l =

Near-surface effects (III) From solar observations over a range of degrees, H 2 (¯) ( ) is known. Can that be used for other stars, somehow? Use as ansatz On physical grounds we expect

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 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 fits to observations and model frequencies:

α Centauri system OPAL EOS, OPAL96 opacity, He, Z settling (Teixeira et al.) M A : M ¯ M B : M ¯ X 0 : Z 0 : Age: 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