Calibrating models of ultralow-mass stars

Evolutionary and atmospheric models have become available for young ultralow-mass objects. These models are being used to determine fundamental parameters from observational properties. TiO bands are used to determine effective temperatures in ultralow-mass objects, and together with Na- and K-lines to derive gravities at the substellar boundary. Unfortunately, model calibrations in (young) ultralow-mass objects are rare. As a first step towards a calibration of synthetic spectral features, I show molecular bands of TiO, which is a main opacity source in late M-dwarfs. The TiO $\epsilon$-band at 8450\AA is systematically too weak. This implies that temperatures determined from that band are underestimated, and I discuss implications for determining fundamental parameters from high resolution spectra.