From cool to hot F-stars: the influence of two ionization regions in the acoustic oscillations

TL;DR

This study investigates how two distinct ionization regions in F-type stars influence their acoustic oscillation spectra, revealing that the deeper heavy element ionization region becomes increasingly significant in hotter stars, aiding stellar outer layer analysis.

Contribution

It identifies and characterizes two key ionization regions affecting F-star oscillations, highlighting the increasing impact of heavy element ionization in hotter stars, which was previously less understood.

Findings

Two ionization regions significantly influence F-star oscillations.

The impact of heavy element ionization increases with stellar temperature.

Deeper ionization regions become comparable to helium ionization in hotter stars.

Abstract

The high-precision data available from the Kepler satellite allows us to venture in the study of the complex outer convective envelopes of solar-type stars. We use a seismic diagnostic, specialized for investigating the outer layers of solar-type stars, to infer the impact of the ionization processes on the oscillation spectrum, for a sample of Kepler stars. These stars, of spectral type F, cover all of the observational seismic domain of the acoustic oscillation spectrum in solar-type stars. They also cover the range between a cool F-dwarf (\sim 6000 K) and a hotter F-star (\sim 6400 K). Our study reveals the existence of two relevant ionization regions. One of these regions, which is located closer to the surface of the star, is commonly associated with the second ionization of helium, although other chemical species also contribute to ionization. The second region, located deeper in the envelope, is linked with the ionization of heavy elements. Specifically, in this study, we analyze the elements carbon, nitrogen, oxygen, neon, and iron. Both regions can be related to the K electronic shell. We show that, while for cooler stars like the Sun, the influence of this second region on the oscillation frequencies is small; in hotter stars, its influence becomes comparable to the influence of the region of the second ionization of helium. This can guide us in the study of the outer layers of F-stars, specifically with the understanding of phenomena related to rotation and magnetic activity in these stars.