Overstability of acoustic modes in late-type stars and its observational implication

The stability against linear nonadiabatic nonradial oscillations is investigated for acoustic modes in the envelopes of stars in various regions of the H-R diagram. The method of computation developed by ANDO and OSAKI (1975) is applied. The results are presented in a diagnostic (l, sigma)-diagram, where l and sigma are the spherical harmonic number and the angular frequency of oscillations, respectively. Some of acoustic modes are found to be overstable in stars that lie to the right of the Cepheid instability strip in the H-R diagram. The driving of oscillations is mainly due to the kappa-mechanism of the hydrogen ionization zone in stars with relatively low effective temperatures and that of the second helium ionization zone in stars with relatively high effective temperatures. The maximum growth rate eta = -sigma sub(I)/sigma sub(R) in stars with relatively low effective temperatures is found to increase with decreasing surface gravity g and with increasing effective temperature T sub(e); eta proportional to gsup(-0.5)T sub(e)sup(6). It is suggested that these overstable acoustic modes may be responsible for the formation of the chromosphere and corona and for the WILSON and BAPPU (1957) effect in late-type stars. (auth.)