Fe-bump instability: the excitation of pulsations in subdwarf B and other low-mass stars

Abstract

We consider the excitation of radial and non-radial oscillations in low-mass B stars by the iron-bump opacity mechanism. The results are significant for the interpretation of pulsations in subdwarf B stars, helium-rich subdwarfs and extreme helium stars, including the EC14026 and PG1716 variables. We demonstrate that, for radial oscillations, the driving mechanism becomes effective by increasing the contrast between the iron-bump opacity and the opacity from other sources. The location of the iron-bump instability boundary depends on the mean molecular weight in the envelope and also on the radial order of the oscillation. A bluer instability boundary is provided by increasing the iron abundance alone, explaining the observed EC14026 variables, and by higher radial order oscillations. We show that the coolest EC14026 variables may vary in the fundamental radial mode, but the hottest variables must be of higher radial order. In considering non-radial oscillations, we demonstrate that g-modes of high radial order and low spherical degree (l<4) may be excited in some blue horizontal branch stars with near-normal composition (Z=0.02). Additional iron enhancement extends the g-mode instability zone to higher effective temperatures and also creates a p-mode instability zone. With sufficient iron, the p-mode and g-mode instability zones overlap, allowing a small region where the EC14026 and PG1716-type variability can be excited simultaneously. However its location is roughly 5000 K too low compared with the observed boundary between EC14026 and PG1716 variables.