Study of sdO models. Pulsation Analysis

TL;DR

This study investigates pulsation modes in sdO star models, discovering the first models capable of driving high-radial order g-modes through various opacity mechanisms, enhancing understanding of stellar pulsations.

Contribution

It introduces the first sdO models that can drive high-radial order g-modes, analyzing multiple driving mechanisms including iron-peak opacity and ionization zones.

Findings

Discovered sdO models capable of driving high-radial order g-modes.

Identified kappa-mechanism due to iron-peak elements as a driver.

Found multiple regions contributing to mode excitation.

Abstract

We have explored the possibility of driving pulsation modes in models of sdO stars in which the effects of element diffusion, gravitational settling and radiative levitation have been neglected so that the distribution of iron-peak elements remains uniform throughout the evolution. The stability of these models was determined using a non-adiabatic oscillations code. We analysed 27 sdO models from 16 different evolutionary sequences and discovered the first ever sdO models capable of driving high-radial order g-modes. In one model, the driving is by a classical kappa-mechanism due to the opacity bump from iron-peak elements at temperature ~200,000 K. In a second model, the driving result from the combined action of kappa-mechanisms operating in three distinct regions of the star: (i) a carbon-oxygen partial ionization zone at temperature ~2 10^6 K, (ii) a deeper region at temperature ~2 10^7 K, which we attribute to ionization of argon, and (iii) at the transition from radiative to conductive opacity in the core of the star.