4.6 Asteroseismology

Magnetic activity influences the internal stellar structure. In the case of the Sun, this is observed as shifts of eigenfrequencies of global modes of solar oscillations (Woodard and Noyes, 1985 ). Mode frequencies increase with magnetic activity: for the 11- $yr$ cycle, low-degree mode frequencies are shifted by the order of $10-4$ . The study of high-degree modes has revealed that frequency shifts are caused by structural perturbations near the solar surface which are localised in latitude (Libbrecht and Woodard, 1990 ). Thus, measurements of frequency shifts on solar-type stars is a potential tool for studying starspot latitudes. Lanza and Rodonò (2002 ) studied the possibility for asteroseismic tests of the internal structure of magnetically active close binaries. Gizon (2002 ) investigated prospects for detecting stellar activity through asteroseismology. He concluded that observations of solar-like oscillations contain measurable information about the latitudinal distribution of stellar activity under the following conditions: The stellar rotation is high enough to resolve frequencies of different azimuthal numbers; stellar activity is strong enough to produce significant spotted area, and the inclination angle of the stellar rotation axis is larger than $o 40$ . Still, observations should be long and continuous, which can be provided by the future space missions COROT (

http://iaa13u.iaa.csic.es/~corot/mainE.html) and Kepler (

http://www.kepler.arc.nasa.gov/).

Stellar differential rotation for solar-type stars can be measured through asteroseismology as well (Gizon and Solanki, 2004 ). Rotationally split frequencies of global oscillation provide information about rotation at different latitudes depending on the azimuthal order $m$ of the mode of pulsation. Since the $m = ±1$ and $m = ± 2$ components of quadrupole oscillations can be observed simultaneously in asteroseismology, rotational frequency splittings can be inverted to provide an estimate of the difference in stellar angular velocity between the equator and $45o$ latitude. The precision of the method depends on the value of the mean rotation and on the inclination angle between the rotation axis and the line of sight.