A Theoretical Study of the Outer Layers of Eight Kepler F-stars: The Relevance of Ionization Processes

We have analyzed the theoretical model envelopes of eight Kepler F-stars by computing the phase shift of the acoustic waves, $\alpha \left(\omega \right)$, and its related function, $\beta \left(\omega \right)$. The latter is shown to be a powerful probe of the external stellar layers since it is particularly sensitive to the partial ionization zones located in these upper layers. We found that these theoretical envelopes can be organized into two groups, each of which is characterized by a distinct $\beta \left(\omega \right)$ shape that we show to reflect the differences related to the magnitudes of ionization processes. Since $\beta \left(\omega \right)$ can also be determined from the experimental frequencies, we compared our theoretical results with the observable $\beta \left(\omega \right)$. Using the function $\beta \left(\omega \right)$, and with the purpose of quantifying the magnitude of the ionization processes occurring in the outer layers of these stars, we define two indexes, $\Delta {\beta }_1$ and $\Delta {\beta }_2$. These indexes allow us to connect the microphysics of the interior of the star with macroscopic observable characteristics. Motivated by the distinct magnetic activity behaviors of F-stars, we studied the relation between the star's rotation period and these indexes. We found a trend, in the form of a power-law dependence, that favors the idea that ionization is acting as an underlying mechanism, which is crucial for understanding the relation between rotation and magnetism and even observational features such as the Kraft break.