The very low mass multiple system LHS 1070. A testbed for model atmospheres for the lower end of the main sequence

Context. LHS 1070 is a nearby multiple system of low mass stars. It is an important source of information for probing the low mass end of the main sequence, down to the hydrogen-burning limit. The primary of the system is a mid-M dwarf and two components are late-M to early L dwarfs, at the star-brown dwarf transition. Hence LHS 1070 is a valuable object to understand the onset of dust formation in cool stellar atmospheres. Aims. This work aims at determining the fundamental stellar parameters of LHS 1070 and to test recent model atmospheres: BT-Dusty, BT-Settl, DRIFT, and MARCS models. Methods. Unlike in previous studies, we have performed a chi(2)-minimization comparing well calibrated optical and infrared (IR) spectra with recent cool star synthetic spectra leading to the determination of the physical stellar parameters T-eff, radius, and log g for each of the three components of LHS 1070. Results. With exception of the MARCS models which do not include dust formation, the models are able to reproduce the observations and describe the main features of the visible to IR spectra. This is consistent with the fact that dust formation prevails in the B and C component atmospheres. The parameters obtained with the DRIFT models confirm the values determined in earlier studies. But important differences between models are observed, where the MARCS model is too bright in the H and K bands, and the BT-Settl and BT-Dusty models systematically yield up to 100 K higher T-eff in the case of the B and C components. This confirms a trend for models without, or with less efficient cloud formation, to predict higher T-eff than models richer in dust (DRIFT). Even models including cloud physics however still produce slightly too bright J band flux, showing as too blue J - K colors. The onset of dust formation remains therefore a particularly challenging regime to understand.