X-Shooter survey of disc accretion in Upper Scorpius

Context. The evolution of protoplanetary discs is intertwined with the process of planet formation, growth and migration. Studies of nearby star-forming regions of different ages and properties provide the information needed to understand the processes governing their evolution. Aims. This paper presents the results of a spectroscopic study of the stellar and accretion properties of a large sample of 127 stars with protoplanetary discs in the Upper Scorpius region, a relatively old (5-10 Myr), nearby (similar to 145 pc) star-forming region, with disc dust masses inferred from ALMA continuum measurements. Methods. We derived the accretion luminosity from the excess UV continuum emission with respect to the photospheric and chromospheric emission self-consistently with the stellar spectral types, extinction, and luminosity, using the FitteR for Accretion ProPErties of T Tauri stars (FRAPPE) code. We applied a new method to evaluate upper limits on the accretion luminosity. In similar to 50% of cases, we could only evaluate upper limits on the accretion luminosity, either because the signal-to-noise ratio of the data was insufficient or because the measured value of the accretion luminosity was below the statistical estimate of the emission due to chromospheric activity. Results. The mass accretion rate shows a weak correlation with stellar mass, while we find no correlation with disc properties such as dust mass or gaseous disc radius. The dispersion is larger than that found in younger star-forming regions such as Lupus and Chamaeleon I, and suggests fading of the correlations with age. We find no evidence that the observed dispersion can be explained by membership in Upper Scorpius sub-groups, or by the properties of known binary systems or transition discs. Conclusions. The lack of correlation and the large dispersion of accretion rates challenge the current expectations of evolutionary models. The observed properties point to a decoupling of the inner and outer discs by the age of Upper Scorpius and a fading of the relations observed in younger star-forming regions, which calls for further development of current theoretical frameworks to be explained.