X-shooter spectroscopy of young stellar objects

IV. Accretion in low-mass stars and substellar objects in Lupus ^⋆,⋆⋆

¹ INAF – Osservatorio Astronomico di Capodimonte, via Moiariello 16, 80131 Napoli, Italy
e-mail: alcala@oacn.inaf.it
² INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
³ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
⁴ INAF – Osservatorio Astrofisico di Catania, via S. Sofia 78, 95123 Catania, Italy
⁵ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
⁶ INAF – Osservatorio Astronomico di Roma, via Frascati 33, 00040 Monteporzio Catone, Italy
⁷ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching, Germany
⁸ Department of Planetary Science, Lunar and Planetary Lab. University of Arizona, 1629, E. University Blvd, 85719, Tucson, AZ, USA
⁹ DIAS/Dublin Institute for Advanced Studies, Burlington Road, 15 Dublin 4, Ireland
¹⁰ ASI-Science Data Centre, via Galileo Galilei, 00044 Frascati, Italy
¹¹ Excellence Cluster Universe, Boltzmannstr. 2, 85748 Garching, Germany

Received: 10 July 2013
Accepted: 3 October 2013

Abstract

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 ~1.2 M_⊙, but mostly with 0.1 M_⊙<M_⋆< 0.5 M_⊙. 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 (Ṁ_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 ~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 Ṁ_acc yield significantly different results: Hα line profile modelling may underestimate Ṁ_acc by 0.6 to 0.8 dex with respect to Ṁ_acc derived from continuum-excess measures. These differences may explain the probably spurious bi-modal relationships between Ṁ_acc and other YSOs properties reported in the literature. We derived Ṁ_acc in the range 2 × 10^-12–4 × 10^-8 M_⊙ yr^-1 and conclude that Ṁ_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 Ṁ_acc, confirming previous suggestions that the geometry of the accretion flow controls the rate at which the disc material accretes onto the central star.