The depth of maximum shower development and its fluctuations: cosmic ray mass composition at E_0 >= 10^17 eV

We present new data on Cherenkov light observations obtained during the period 1994-2009, after a modernization of the Yakutsk EAS array. A complex analysis of $\text{xmax}$ and its fluctuations $\sigma (\text{xmax})$ was performed over a wide energy range. With the new data, according to QGSJet II model, an estimation was made of the cosmic ray mass composition for $\E \sim 10^{17} - 3 \times 10^{19}$ eV. The result points towards a mixed composition with a large portion of heavy nuclei at $\text{E}\sim {10}^{17}$ eV and the dominance of light nuclei at $\text{E}\sim {10}^{19}$ eV. The analysis of $\sigma (\text{xmax})$ energy dependence for the same energies qualitatively confirms this result. The shape of the $\text{xmax}$ distribution at fixed energy ${10}^{18}$ eV is analysed to make more precise conclusions on cosmic ray mass composition.