Institute for Advanced Study Informal Astrophysics Seminar

A stellar mode achieves approximate energy equipartition with the kinetic energy of convective eddies whose correlation times are comparable to its period. The energies of the most visible modes are similar to those of individual granules, or equivalently to the stellar ux that passes through a granule during its lifetime. Interactions that excite and damp an acoustic mode take place above the mode's acoustic cavity; they merely tickle the mode's evanescent tail. As a consequence, the mode's linewidth is much smaller than its frequency and the modal peak rises above the level of the convective noise in velocity and intensity power spectra. Scaling observational properties of stochastically excited modes from helio- seismology to asteroseismology implicity assumes that the maximum convective Mach number, M, is the same in all stars with convective envelopes. For some properties, such as the frequency of maximum visibility, this works pretty well; M is expected to be of order a few tenths and only enters to the rst power. Other observational quantities depend more sensitively on M. Peaks of low degree modes rise by of order M????4 above the convective noise in velocity and intensity power spectra formed from observations of an unresolved star. The ratio of linewidth to mode frequency depends even more sensitively on M. Scaling of velocity amplitudes is more direct than those of intensity since the latter are sensitive to variations in the opacity. The -mechanism operating just below the photosphere mutes ux variations associated with excited modes thereby decreasing signal to noise in intensity power spectra relative to those in velocity.