Impact of helium diffusion and helium-flash-induced carbon production on gravity-mode pulsations in subdwarf B stars

TL;DR

This study examines how helium diffusion, helium-flash-induced carbon production, and other model assumptions influence the gravity-mode pulsations in subdwarf B stars, improving the understanding of their internal structures and pulsation properties.

Contribution

It provides a detailed analysis of the effects of helium diffusion, core-envelope transition profiles, and helium flash processes on g-mode pulsations in subdwarf B stars, highlighting their impact on mode trapping and instability domains.

Findings

Helium settling shifts the blue edge of the instability strip to higher temperatures.

G-mode spectra are highly sensitive to the H-profile in the core-envelope transition zone.

He-flash induces complex mode trapping signatures due to chemical transitions.

Abstract

Realistic stellar models are essential to the forward modelling approach in asteroseismology. For practicality however, certain model assumptions are also required. For example, in the case of subdwarf B stars, one usually starts with zero-age horizontal branch structures without following the progenitor evolution. We analyse the effects of common assumptions in subdwarf B models on the g-mode pulsational properties. We investigate if and how the pulsation periods are affected by the H-profile in the core-envelope transition zone. Furthermore, the effects of C-production and convective mixing during the core helium flash are evaluated. Finally, we reanalyse the effects of stellar opacities on the mode excitation in subdwarf B stars. We find that helium settling causes a shift in the theoretical blue edge of the g-mode instability domain to higher effective temperatures. This results in a closer match to the observed instability strip of long-period sdB pulsators, particularly for l<=3 modes. We show further that the g-mode spectrum is extremely sensitive to the H-profile in the core-envelope transition zone. If atomic diffusion is efficient, details of the initial shape of the profile become less important in the course of evolution. Diffusion broadens the chemical gradients, and results in less effective mode trapping and different pulsation periods. Furthermore, we report on the possible consequences of the He-flash for the g-modes. The outer edge of a flash-induced convective region introduces an additional chemical transition in the stellar models, and the corresponding spike in the Brunt-Vaisala frequency produces a complicated mode trapping signature in the period spacings.