2014A&A...561A...2A

We present VLT/X-shooter observations of a sample of 36 accreting low-mass stellar and substellar objects (YSOs) in the Lupus star-forming region, spanning a range in mass from ∼0.03 to ∼1.2M_☉, but mostly with 0.1M_☉ < M_* < 0.5M_☉. Our aim is twofold: firstly, to analyse the relationship between excess-continuum and line emission accretion diagnostics, and, secondly, to investigate the accretion properties in terms of the physical properties of the central object. The accretion luminosity (L_acc), and in turn the accretion rate ({dot}(M)_acc), was derived by modelling the excess emission from the UV to the near-infrared as the continuum emission of a slab of hydrogen. We computed the flux and luminosity (L_line) of many emission lines of H, He, and CaII, observed simultaneously in the range from ∼330 nm to 2500 nm. The luminosity of all the lines is well correlated with L_acc. We provide empirical relationships between L_acc and the luminosity of 39 emission lines, which have a lower dispersion than relationships previously reported in the literature. Our measurements extend the Paβ and Brγ relationships to L_acc values about two orders of magnitude lower than those reported in previous studies. We confirm that different methodologies of measuring L_acc and {dot}(M)_acc yield significantly different results: Hα line profile modelling may underestimate {dot}(M)_acc by 0.6 to 0.8 dex with respect to {dot}(M)_acc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships between {dot}(M)_acc and other YSOs properties reported in the literature. We derived {dot}(M)_acc in the range 2x10^–12-4x10^–8M_☉/yr and conclude that {dot}(M)_acc ∝ M_*^1.8(±0.2), with a dispersion lower by a factor of about 2 than in previous studies. A number of properties indicate that the physical conditions of the accreting gas are similar over more than 5 orders of magnitude in {dot}(M)_acc, confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.