SEISMOLOGICAL EFFECTS OF CONVECTIVE-CORE OVERSHOOTING IN STARS OF INTERMEDIATE-MASS

We study the effects of convective-core overshooting on acoustic and gravity modes of low degree l for stars of intermediate mass. Using the CESAM code (Morel 1993) we have computed sequences of stars of 1.7 and 2 M. along the main sequence; both ''classical'' models, without overshooting, and models including convective overshooting from the core over 0.1 and 0.2 pressure scale-height are considered. We compare the structure and the oscillation properties ofmodels of the same effective temperature. At given effective temperature, overshooting increases the stellar radius, and the main effect on oscillations is therefore to decrease the frequencies. This could give rise to a problem of the identification of the radial order of modes. The effects of overshooting on the deep stellar structure can be examined through different frequency combinations. In particular, the ratio (v(n,0) + v(n-1,0) - 2 v(n-1,1))/(v(n,0) - v(n-1,2)) (where v(n,l) is the cyclic frequency of a mode of radial order n and degree l) is very sensitive to the structure of the stellar: core throughout the whole main sequence, and could therefore be a good indicator of the stellar age and of the extent of overshooting. The estimation of stellar mass and age from seismological observations is modified by convective core penetration, which must thus be taken into account for asteroseismological calibration. We furthermore study the evolution of p and g modes of low radial order along the main sequence, and exhibit the exchange of physical nature between pairs of modes whose frequencies approach very closely, related to the so-called avoided crossing. Convective core penetration makes avoided crossing occur at lower effective temperature, substantially changing the distribution of the kinetic-energy density of mixed modes throughout the stellar interior at a given effective temperature. Independently of overshooting, these modes have kinetic energy both in the central and external parts, and their observation would provide a powerful tool for probing the deep interior of stars and for investigating the importance of convective-core overshooting.