Testing disk evolution through mass accretion rates and its variability: The case of XX ChA

Young stars accrete mass from the protoplanetary disk that surrounds them. Studying the accretion process is key to understand protoplanetary disk evolution and therefore planet formation. In particular, the spread in mass accretion rate is a way to test and constrain disk evolution models. Previous works found that the typical extent of accretion variability on the observed spread is limited (≲ 0.5 dex). Here, I will present results obtained for the T Tauri star XX Cha. X-shooter spectra were obtained in 2010 and 2021, allowing us to derive stellar and accretion properties by fitting the UV excess, redding and stellar photosphere. We observed an accretion variability of almost 1.5 dex between the two epochs. This extreme variability of XX Cha is unlikely to fully explain the observed accretion rate spread. Other processes, affecting disk evolution, must therefore be explored as the cause. A HST-STIS observation confirms a short variability timescale for XX Cha. This is but one example of how combining X-Shooter and other instruments contributes to our understanding of the accretion process in Classical T Tauri stars. Relating accretion rates to characteristics obtained with e.g. ALMA provides valuable constraints on disk evolution. I am hence working to improve the determination and accuracy of mass accretion rates from X- shooter spectra, which I will also present in this talk