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
(EWLi = 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 Ṁacc [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 Ṁout [M⊙/yr]
<-10.4. The mass outflow and inflow rates can be combined to yield
Ṁout/Ṁacc ~ 0.3, a value that is in
agreement with jet-launching models.
Based on observations collected at the Very Large Telescope of the
European Southern Observatory under program 086.C-0173(A).
Original language | English |
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Article number | A106 |
Number of pages | 12 |
Journal | Astronomy & Astrophysics |
Volume | 551 |
DOIs | |
Publication status | Published - Mar 2013 |
Keywords
- brown dwarfs
- stars: pre-main sequence
- accretion
- accretion disks
- stars: individual: FU Tau A