Abstract
We use a seismic diagnostic, based on the derivative of the phase shift of the reflected by the surface acoustic waves, to probe the outer layers of the star HD 49933. This diagnostic is particularly sensitive to partial ionization processes occurring above the base of the convective zone. The regions of partial ionization of light elements, hydrogen and helium, have well known seismological signatures. In this work we detect a different seismic signature in the acoustic frequencies, that we showed to correspond to the location where the partial ionization of heavy elements occurs. The location of the corresponding acoustic glitch lies between the region of the second ionization of helium and the base of the convective zone, approximately 5% below the surface of the stars.
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