New constrains on the formation and settling of dust in the atmosphere of young M and L dwarfs

At young ages, low surface gravity affects the atmospheric properties of ultracool dwarfs. The impact on medium-resolution near-infrared (NIR) spectra has only been slightly investigated at the M-L transition sofar. We obtain 6 near-infrared, medium resolution (R~1500), spectra of young M9.5-L3 dwarfs. We use them to study age-sensitive features. We build the first age-sequence of spectra of optically classified M9.5 dwarfs. We also use the spectra and complementary photometry to test the 2010 and 2012 releases of the BT-SETTL models (Allard et al. 2012). We find that the models can reproduce the spectral energy distributions (SED) of young L2-L3 dwarfs. But we find/confirm that 1/ these models give a spread of Teff at the M-L transition 2/ that they do not fit the SEDs and the individual J, H, and K band spectra simultaneously. This inconsistency likely arises from a deficit of dust at high altitude in the cloud models (in particular in the BT-Settl 2012). A modification of the nucleation rate might solve the issue.