X-shooter spectroscopy of FU Tauri A^⋆

¹ INAF – Osservatorio Astronomico di Palermo, Piazza del Parlamento 1, 90134 Palermo, Italy
e-mail: stelzer@astropa.inaf.it
² INAF – Osservatorio Astronomico di Capodimonte, Via Moiariello 16, 80131 Napoli, Italy
³ School of Cosmic Physics, Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, Dublin 2, Ireland
⁴ INAF – Osservatorio Astrofisico di Arcetri, Large E. Fermi 5, 50125 Firenze, Italy

Received: 15 November 2012
Accepted: 19 December 2012

Abstract

We have analyzed a broad-band optical and near-infrared spectrum of FU Tau A, which is a presumed young brown dwarf in the Taurus star forming region that has intrigued both theorists and observers by its overluminosity in the Hertzsprung-Russell (HR) diagram with respect to standard pre-main sequence evolutionary models. FU Tau A is brighter than any other Taurus member of the same or similar spectral type, and various phenomena (accretion, activity, binarity) have been put forth as possible explanations. The new data, obtained with the X-shooter spectrograph at the Very Large Telescope, include an unprecedented wealth of information on stellar parameters and simultaneously observed accretion and outflow indicators for FU Tau A. We present the first measurements of gravity (log g = 3.5 ± 0.5), radial velocity (RV = 22.5 ± 2.9 km s^-1), rotational velocity (vsini = 20 ± 5 km s^-1), and lithium equivalent width (EW_Li = 430 ± 20 mÅ) for FU Tau A. From the rotational velocity and the published period we infer a disk inclination of i ≳ 50°. The lithium content is much lower than theoretically expected for such a young very low-mass object, adding another puzzling feature to this object’s properties. We determine the mass accretion rate of FU Tau A from comparison of the luminosities of 20 emission lines to empirical calibrations from the literature and find a mean of log Ṁ_acc  [M_⊙/yr]  = −9.9 with standard deviation σ = 0.2. The accretion rate determined independently from modeling of the excess emission in the Balmer and Paschen continua is consistent with this value. The corresponding accretion luminosity is too low to make a significant contribution to the bolometric luminosity. Strong magnetic activity affecting the stellar parameters or binarity of FU Tau A, both combined with extreme youth, may be responsible for its position in the HR diagram. The existence of an outflow in FU Tau A is demonstrated through the first detection of forbidden emission lines from which we obtain an estimate for the mass loss rate, log Ṁ_out  [M_⊙/yr]  < −10.4. The mass outflow and inflow rates can be combined to yield Ṁ_out/Ṁ_acc ~ 0.3, a value that is in agreement with jet-launching models.