Linear analyses for the stability of radial and nonradial oscillations of massive stars

TL;DR

This paper uses linear nonadiabatic stability analyses to explore the oscillation modes of massive stars, revealing new insights into their variability and the connection to stellar winds and the Humphreys-Davidson limit.

Contribution

It introduces the importance of low-degree oscillatory convection modes and links radial instabilities to stellar winds and the Humphreys-Davidson limit.

Findings

Low-degree oscillatory convection modes can be observable.

Radial modes are monotonously unstable in very massive stars.

Instability boundary aligns with the Humphreys-Davidson limit.

Abstract

In order to understand the periodic and semi-periodic variations of luminous O- B- A-type stars, linear nonadiabatic stability analyses for radial and nonradial oscillations have been performed for massive evolutionary models ($8M_\odot - 90M_\odot$). In addition to radial and nonradial oscillations excited by the kappa-mechanism and strange-mode instability, we discuss the importance of low-degree oscillatory convection (nonadiabatic g$^-$) modes. Although their kinetic energy is largely confined to the convection zone generated by the Fe opacity peak near $2\times10^5$K, the amplitude can emerge to the photosphere and should be observable in a certain effective temperature range. They have periods longer than those of the radial strange modes so that they seem to be responsible for some of the long-period microvariations of LBVs (S Dor variables) and $\alpha$ Cyg variables. Moreover, monotonously unstable radial modes are found in some models whose initial masses are greater than or equal to $60M_\odot$ with $Z=0.02$. The monotonous instability probably corresponds to the presence of an optically thick wind. The instability boundary roughly coincides with the Humphreys-Davidson limit.