Pulsation of High Luminosity Helium Stars

A study has been made of the radial pulsation of high luminosity helium stars in the region of the HR diagram occupied by the R Coronae Borealis and other hydrogen-deficient carbon stars. As well as instability of the fundamental mode when log |Tₑff<3· 88|⁠, overtone instability is found in the region |3· 93\,Tₑff<4· 12|⁠, and possibly when log |Tₑff>4·2| if the luminosity to mass ratio is high enough. Non-adiabatic effects are very large and there is no one-to-one correspondence between linear adiabatic and nonadiabatic modes. This can cause the periods of two overtones in the same star to be identical at certain effective temperatures. Non-linear calculations show the fundamental pulsation to be very violent but the full amplitude overtone pulsation is well behaved.