Values of v sin i-s for late-type stars from spectral synthesis in the K-band region.

We analyse medium-resolution spectra (R similar to 18 000) of 19 late type dwarfs in order to determine v sin i values using synthetic rather than observational template spectra. For this purpose, observational data around 2.2 mu m for stars with spectral classes from G8V to M9.5V were modelled. We find that the Na i (2.2062- and 2.2090-mu m) and 12CO 20 band features are modelled well enough to use for v sin i determination without the need for suitable observational template spectra. Within the limit of the resolution of our spectra, we use synthetic spectra templates to derive v sin i values consistent with those derived in the optical regime using observed templates. We quantify the errors in our v sin i determination due to incorrect choice of model parameters Teff, log g, vmicro, [Fe/H] or full width at half-maximum and show that they are typically less than 10 per cent. We note that the spectral resolution of our data (similar to 16 km s-1) limited this study to relatively fast rotators and that a resolution of 60 000 will be required to access most late-type dwarfs.