• atomic data;
  • molecular data;
  • stars: late-type


We analyse medium-resolution spectra (R∼ 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 μm for stars with spectral classes from G8V to M9.5V were modelled. We find that the Na i (2.2062- and 2.2090-μm) and 12CO 2–0 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 (∼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.