X-shooter spectroscopy of young stellar objects IV. Accretion in low-mass stars and substellar objects in Lupus

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 similar to 0.03 to similar to 1.2 M-circle dot, but mostly with 0.1 M-circle dot < M-star < 0.5 M-circle dot. 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 ((M) over dot(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 Ca II, observed simultaneously in the range from similar to 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 Pa beta and Br gamma 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 (M) over dot(acc) yield significantly different results: H alpha line profile modelling may underestimate (M) over dot(acc) by 0.6 to 0.8 dex with respect to (M) over dot(acc) derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships between (M) over dot(acc) and other YSOs properties reported in the literature. We derived (M) over dot(acc) in the range 2 x 10(-12)-4 x 10(-8) M-circle dot yr(-1) and conclude that (M) over dot(acc) proportional to M-star(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 (M) over dot(acc), confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.