Pulsational instability in B-type supergiant stars

TL;DR

This study analyzes pulsational instabilities in B-type supergiant stars, revealing that both p- and g-modes can be excited without the need for an intermediate convective zone, challenging previous assumptions.

Contribution

It demonstrates that g-modes are excited by the κ-mechanism without requiring an intermediate convective zone, revising earlier models of stellar pulsation in supergiants.

Findings

Both p- and g-modes are excited by the κ-mechanism.

G-modes can be reflected at a minimum of the Brunt-Väisälä frequency.

No regular oscillation patterns are expected in spectra, but mode identification is possible.

Abstract

We present results of the instability analysis of the post-main sequence massive star models against radial and nonradial pulsations. We confirm that both p- and g-modes can be excited by the $\kappa$-mechanism acting in the metal opacity bump. However, as opposed to the previous claims, we find that an intermediate convective zone (ICZ) related to the hydrogen burning shell is not necessary for excitation of g-modes. These modes can be reflected at a minimum of the Brunt-V\"ais\"al\"a frequency, located at the top of the chemical composition gradient region surrounding the radiative helium core. This minimum is associated with the change of actual temperature gradient from the adiabatic value in the semiconvective zone to the radiative value above it. Thus, the existence of pulsations at this evolutionary stage does not prove the existence of the convective zone but only some reflective layer. Finally, we show that no regular patterns can be expected in oscillation spectra of blue supergiant pulsators but there is a prospect for identification of the mode degree, $\ell$, from multicolour photometry.