  p-mode oscillations in solar-like stars are excited by the outer convection
zone in these stars and reflected close to the surface. The p-modes are trapped
inside an acoustic cavity, but the modes only stay trapped up to a given
frequency (known as the acoustic cut-off frequency) as modes with larger
frequencies are generally not reflected at the surface. This means that modes
with frequency larger than the acoustic cut-off frequency must be traveling
waves. The high-frequency modes may provide information about the physics in
the outer layers of the stars and the excitation source and are therefore
highly interesting as it is the estimation of these two phenomena that causes
some of the largest uncertainties when calculating stellar oscillations.
High-frequency modes have been detected in the Sun, beta Hydri and in alpha Cen
A & B by smoothing the so-called echelle diagram and the large frequency
separation as a function of frequency have been estimated. The large frequency
separation has been compared with a simple model of the acoustic cavity which
suggests that the reflectivity of the photosphere is larger at high frequency
than predicted by standard models of the solar atmosphere and that the depth of
the excitation source is larger than what has been estimated by other models
and might depend on the order n and degree l of the modes.
