EXCITATION OF PRESSURE MODES IN COMMON ENVELOPES

We study the excitation of oscillatory modes by a low-mass star orbiting inside a common envelope with a more massive star. We treat adiabatic high spherical harmonic degree (l >> 1) modes propagating outward in the envelope. The dominant oscillatory modes are those for which the spherical harmonic order is large m approximately l, and thus the amplitudes are large close to the equatorial plane and small closer to the poles. A secondary of mass approximately 1% of the primary mass excites modes with relative surface amplitudes of a few x 10%. Even a Jupiter-like brown dwarf when it is very deep in the envelope of an asymptotic giant branch star can cause perturbations of relative surface amplitudes of approximately 10% near the equatorial plane. If mass loss is influenced by oscillation then this mechanism can lead to higher mass loss in the equatorial plane. In case of an asymptotic giant branch primary this might lead to the formation of an elliptical planetary nebulae. Oscillation excitation by a low-mass secondary might also be relevant to the formation of the ring around SN 1987A.