The photosphere

4.8 Summary

Atmospheric parameters of M and L dwarfs may be found by matching synthetic spectra from model atmosphere calculations to observed data. High resolution spectral data allow quite accurate determinations, but even photometry (particularly in the infrared) can be used with some success. The models are now good enough that fine analysis of very high-resolution spectra allows metallicity, gravity and temperature to be determined with good precision. Lacking this data, gravity may be found by indirect methods using luminosity and temperature information to infer mass and radius. Metallicity may be determined by comparing the molecular band-strengths of hydrides (CaH) to double metal species (TiO). Thus, the temperature, gravity and metallicity of a low-mass star are now accessible using optical and infrared photometry and spectroscopy together with the current generation of atmospheric models. Improvements in the next few years will come with the acquisition of higher-resolution optical and infrared spectra, and from continuing work on the molecular and dust opacity data and the treatment of the opacities in the models.