ANALYSIS OF KECK HIGH-RESOLUTION SPECTRA OF VB-10

We use a preliminary version of our 'NextGen' grid of cool star model atmospheres to compute synthetic line profiles which fit high-resoluti on Keck spectra of the cool M dwarf VB 10 satisfactorily well. We show that the parameters derived from the Keck data are consistent with th e parameters derived from lower resolution spectra with larger wavelen gth coverage. We discuss the treatment of van der Waals broadening in cool stellar atmospheres that are dominated by molecules (mostly H-2) The line profiles are dominated by van der Waals pressure broadening a nd are a sensitive indicator for the gravity and metallicity. Therefor e the high-resolution Keck spectra are useful for determining the para meters of M dwarfs. There is some ambiguity between the metallicity an d gravity. For VB 10, we find from the high-resolution spectra that 5. 0 < log g < 5.5 and 0 < [M/H] < +0.5 for an adopted fixed effective te mperature of 2700 K, which is consistent with recent interior calculat ions.