Pulsations in B-type supergiants with masses $\boldsymbol {M<20}\,\mathbf {M}_{\odot }$ before and after core helium ignition

Abstract

Evolutionary tracks and pulsational analysis of models with masses of 13-18 M ⊙ are presented. We address two important questions. The first one deals with one of the most unresolved problems in astrophysics, i.e., the existence of a blue loop after core helium ignition; the so called "to loop or not to loop" problem. We show that inward overshooting from the outer convective zone in the red giant phase is prerequisite for the development of the blue loop. Our second question concerns pulsational instability of models in the core helium burning phase. We present for the first time that models on the blue loop can have unstable modes driven by the κ mechanism operating in the Z−bump. Contrary to post-main sequence models in the shell hydrogen burning phases, pulsational instability of the blue loop models depends mainly on effective temperature and metallicity is of secondary importance. Finally, we try to interpret the oscillation spectrum of the blue supergiant HD 163899, the only member of the SPBsg class, and to get some clue on the evolutionary status of the star.

Determining the evolutionary stage of HD163899 on the basis of its oscillation spectrum

“…Saio et al (2006) claimed that some pulsational modes can be partially reflected at the intermediate convective zone (ICZ) and hence they do not penetrate the damping region and can be driven in the outer layers. This explanation has been also confirmed by Ostrowski & Daszyńska-Daszkiewicz (2015).…”

“…It was not the case for previously studied, less massive models (e.g. Ostrowski & Daszyńska-Daszkiewicz 2015). Such similarity makes it impossible to discriminate between evolutionary stages.…”

“…The very rough estimates of T eff and log L/L ⊙ gave the mass in the range [13,20] M ⊙ and the three possibilities for the evolutionary stage: the main sequence, the shell hydrogen burning phase and the core helium burning phase. The most supported hypothesis was that the star is a blue supergiant in the phase of shell hydrogen burning during the first crossing towards red giant branch and the core helium burning phase on the blue loop was rejected by Ostrowski & Daszyńska-Daszkiewicz 2015. In order to advance the studies of HD 163899, the proper determinations of its basic physical properties were needed.…”

Wider pulsation instability regions forβCephei and SPB stars calculated using new Los Alamos opacities

“…The post-main sequence g-type instability regions are connected with so-called slowly pulsating B-type supergiants (SPBsg). For more detailed discussions on this type of oscillation, see Saio et al (2006), Godart et al (2009), Daszyńska-Daszkiewicz et al (2013a, Ostrowski & Daszyńska-Daszkiewicz (2015).…”

Triennial Report (2012-2015): The Legacy Issue

“…Saio et al (2006) reported the first case of a pulsating B-type supergiant, which can be explained by state-of-the-art models (Saio et al 2013, Ostrowski & Daszyńska-Daszkiewicz 2015. For main-sequence pulsators, large numbers of oscillation modes were detected from space (e.g.…”