Trapped and excited

The trapped w modes of stars with a first order phase transition (a density discontinuity) are computed and the excitation of some of the modes of these stars by a perturbing shell is investigated. Attention is restricted to odd parity (''axial'') w modes. With R the radius of the star, M its mass, R{sub i} the radius of the inner core and M{sub I} the mass of such core, it is shown that stars with R/M{ge}5 can have several trapped w modes, as long as R{sub i}/M{sub i}{lt}2.6. Excitation of the least damped w mode is confirmed for a few models. All of these stars can only exist, however, for values of the ratio between the densities of the two phases, greater than {approximately}46. We also show that stars with a phase transition and a given value of R/M can have far more trapped modes than a homogeneous single density star with the same value of R/M, provided both R/M and R{sub i}/M{sub i} are smaller than 3. If the phase transition is very fast, most of the stars with trapped modes are unstable to radial oscillations. We compute the time of instability, and find it to be comparable to the damping of the w mode excited in most cases where w-mode excitation is likely. If on the other hand the phase transition is slow, all the stars are stable to radial oscillations.