  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.
