Asteroseismology of α Cen A

Asteroseismology, which consists in measuring properties of p-mode oscillations, provides a unique tool for studying stellar interiors . Recently p modes have been detected on the bright solar-like star α Cen A thanks to high- precision radial- velocity measurements. However a better characterization of p modes is clearly needed to constrain theoretical models. We observed α Cen A during five nights using the high precision spectrograph HARPS in order to improve our knowledge of the seismic properties of this star and to investigate its mode lifetimes and rotational splitting. We obtained high- precision radial- velocity sequences and computed the acoustic spectrum of α Cen A . We estimate the scatter in the frequencies for each angular degree ( ℓ) and analyse it to constrain the rotational splitting and mode lifetimes. We identify 34 p modes with angular degree ℓ = 0 - 3 in the frequency range 1.8 - 2 .9 mHz and amplitude range 13 - 4 8 cms, in agreement with the previous seismic studies. We find a clear enhancement of the frequency scatter with the angular degree ( ℓ) , which indicates rotational splitting and explains the low value of mode lifetimes previously suggested by Kjeldsen et al. (2005). Combining all of the existing seismic measurements, we determine a mode lifetime of days and a rotational rate. We also derive new values for the small separations that take into account the effect of rotational splitting. Our seismic study of α Cen A leads to a list of now well- identified p-mode frequencies and shows the importance of taking into account the rotation in order to properly characterize the p modes even for quite short campaigns