Effective temperatures of late L dwarfs and the onset of methane signatures

We present a spectral analysis of a sample of late L dwarfs. We use our latest model atmospheres and synthetic spectra and optical and K-band spectra to determine effective temperatures. We derive effective temperatures of 1400-1700 K for L8-L6 dwarfs. The analysis demonstrates that our recent models that rain out the formed dust completely are applicable to optical spectra of late L dwarfs and that more consistent models are needed for intermediate L dwarfs and for infrared spectra. We compare the results for the effective temperatures with the temperatures of the onset of methane formation. Our models predict methane absorption at 3.3 mum to occur at temperatures about 400 K higher than methane absorption at 2.2 mum. This is consistent with our data and previous observations, which show methane absorption at 3.3 mum, but not at 2.2 mum, in late L dwarfs.