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

Abstract

Non-adiabatic linear pulsation models have been calculated for low-mass stars with effective temperatures between 16 000 and 35 000 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 of 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∼0.4 M⊙) but is suppressed either by a reduction in metallicity Z or by a selective enhancement of the carbon abundance. An unexpected result 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 region 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 the prototype V652 Her.