Depth of Maximum of Air-Shower Profiles above 10^17.8 eV Measured with the Fluorescence Detector of the Pierre Auger Observatory and Mass-Composition Implications

After seventeen years of operation, the first phase of measurements at the Pierre Auger Observatory finished and the process of upgrading it began. In this work, we present distributions of the depth of air-shower maximum, X_max, using profiles measured with the fluorescence detector of the Pierre Auger Observatory. The analysis is based on the Phase I data collected from 01 December 2004 to 31 December 2021. The X_max measurements take advantage of an improved evaluation of the vertical aerosol optical depth and reconstruction of the shower profiles. We present the energy dependence of the mean and standard deviation of the X_max distributions above 10^17.8 eV. Both X_max moments are corrected for detector effects and interpreted in terms of the mean logarithmic mass and variance of the masses by comparing them to the predictions of post-LHC hadronic interaction models. We corroborate our earlier findings regarding the change of the elongation rate of the mean X_max at 10^18.3 eV with higher significance. We also confirm, with four more years of data compared to the last results presented in 2019, that around the ankle in the cosmic rays spectrum, the proton component gradually disappears and that intermediate mass nuclei dominate the composition at ultra-high energies.