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-resolution Keck spectra of the cool M dwarf VB 10 satisfactorily well. We show that the parameters derived from the Keck data are consistent with the parameters derived from lower resolution spectra with larger wavelength coverage. We discuss the treatment of van der Waals broadening in cool stellar atmospheres that are dominated by molecules (mostly H2). The line profiles are dominated by van der Waals pressure broadening and are a sensitive indicator for the gravity and metallicity. Therefore the high-resolution Keck spectra are useful for determining the parameters of M dwarfs. There is some ambiguity between the metallicity and 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 temperature of 2700 K, which is consistent with recent interior calculations.