Non-adiabatic linear pulsation models for low-mass helium stars

Non-adiabatic linear pulsation models have been calculated for low-mass sta rs with effective temperatures between 16000 and 35000 K, and with surface gravities in the range 3 < log g < 5. The radial pulsation models assume a homogeneous stellar envelope which is deficient in hydrogen and display the well-known Z-bump instability to radial pulsations. The aim of this paper has been to explore the behaviour of the Z-bump instability as a function o f mass and composition around a reference model with M = 0.5 M., X = 0.00, Z = 0.02. It is shown that the Z-bump instability persists to low masses (M similar to 0.4 M.) but is suppressed either by a reduction in metallicity Z or by a selective enhancement of the carbon abundance. An unexpected resu lt is the discovery that Z-bump instability persists at hydrogen abundances X > 0.3, although the position of the red edge is sensitive to X. We have found that non-radial pulsations are also excited in the same instability r egion as radial pulsations. The implications of these results for individual low-mass helium stars are discussed. It is concluded that Z-bump driven pulsations (radial and/or non -radial) may be excited in some helium-rich subdwarf B stars, representing a possible major extension to the class of variable stars represented by th e prototype V652 Her.