New constraints on the formation and settling of dust in the atmospheres of young M and L dwarfs .

We obtained medium-resolution ISAAC near-infrared spectra of seven young M9.5-L3 dwarfs classified at optical wavelengths. We compared our targets spectra to those of mature and young BDs, and to young late-type companions to nearby stars with known ages, to identify and study gravity-sensitive features. We computed spectral indices weakly sensitive to the surface gravity to derive near-infrared spectral types. Finally, we found the best fit between each spectrum and synthetic spectra from the BT-Settl 2010 and 2013 atmospheric models. Using the best fit, we derived the atmospheric parameters of the objects and {identified} which spectral characteristics the models do not reproduce. BT-Settl models fit the spectra and the 1-5 mu m spectral energy distribution of the L0-L3 dwarfs for temperatures between 1600-2000 K. But the models fail to reproduce the shape of the H band and the near-infrared slope of some of our targets. This fact, and the best-fit solutions found with super-solar metallicity, are indicative of a lack of dust, in particular at high altitude, in the cloud models.