The optical+infrared L dwarf spectral sequence of young planetary-mass
  objects in the Upper Scorpius association
  We present the results of photometric and spectroscopic follow-ups of the
lowest mass member candidates in the nearest OB association, Upper Scorpius
(5-10 Myr; 145+/-17 pc), with the Gran Telescopio de Canarias (GTC) and
European Southern Observatory (ESO) Very Large Telescope (VLT). We confirm the
membership of the large majority (>80%) of the candidates selected originally
photometrically and astrometrically based on their spectroscopic features, weak
equivalent widths of gravity-sensitive doublets, and radial velocities.
Confirmed members follow a sequence over a wide magnitude range (J=17.0-19.3
mag) in several colour-magnitude diagrams with optical, near-, and mid-infrared
photometry, and have near-infrared spectral types in the L1-L7 interval with
likely masses below 15 Jupiter masses. We find that optical spectral types tend
to be earlier than near-infrared spectral types by a few subclasses for
spectral types later than M9. We investigate the behaviour of spectral indices
defined in the literature as a function of spectral type and gravity by
comparison with values reported in the literature for young and old dwarfs. We
also derive effective temperatures in the 1900-1600K from fits of synthetic
model-atmosphere spectra to the observed photometry but we caution the
procedure carries large uncertainties. We determine bolometric corrections for
young L dwarfs with ages of ~5-10 Myr (Upper Sco association) and find them
similar in the J-band but larger by 0.1-0.4 mag in the K-band with respect to
field L dwarfs. Finally, we discovered two faint young L dwarfs,
VISTAJ1607-2146 (L4.5) and VISTAJ1611-2215 (L5) that have H$\alpha$ emission
and possible flux excesses at 4.5 microns, pointing towards the presence of
accretion from a disk onto the central objects of mass below ~15 Jupiter masses
at the age of 5-10 Myr.
