Radial and nonradial oscillations of massive supergiants

TL;DR

This paper investigates the stability and oscillation modes of massive supergiants, revealing various instabilities, potential wind indicators, and observable convection modes linked to opacity peaks, with implications for stellar variability.

Contribution

It introduces the existence of low-degree convection modes associated with Fe-opacity peaks that can be observed in supergiants, expanding understanding of stellar oscillations.

Findings

Strange-mode and kappa-mechanism driven oscillations occur in supergiants.

Monotonously unstable modes suggest possible optically thick winds.

Low-degree convection modes may be observable in supergiants.

Abstract

Stability of radial and nonradial oscillations of massive supergiants is discussed. The kappa-mechanism and strange-mode instability exciteoscillations having various periods in wide ranges of the upper part of the HR diagram. In addition, in very luminous ($\log L/L_\odot \gtrsim 5.9$) models, monotonously unstable modes exist, which probably indicates the occurrence of optically thick winds. The instability boundary is not far from the Humphreys-Davidson limit. Furthermore, it is found that there exist low-degree($\ell = 1, 2$) oscillatory convection modes associated with the Fe-opacity peak convection zone, and they can emerge to the stellar surface so that they are very likely observable in a considerable range in the HR diagram. The convection modes have periods similar to g-modes, and their growth-times are comparable to the periods. Theoretical predictions are compared with some of the supergiant variables.