The metallicity of CM Draconis

We compare observations of the eclipsing binary system CM Draconis (hereafter CM Dra) with synthetic spectra computed using the stellar atmosphere code phoenix. High resolution infrared spectroscopic observations of six 0.05 μm wide regions between 1.51 and 2.45μm, combined with previous work, particularly CM Dra's accurately known surface gravity, enable us to estimate its metallicity using detailed spectral synthesis. We find significant discrepancies between the observed and synthetic spectra throughout most of the region emphasizing the need for higher quality atomic data in the infrared. Nevertheless, the CO bands beyond 2.3μm seem to be well modelled and metal sensitive, and thus high resolution spectra should be a most powerful diagnostic tool for spectroscopic analyses for M dwarfs and brown dwarfs. The CO bands indicate a metallicity of around −1dex for CM Dra. This result is supported by observations of two M dwarfs of similar spectral type, GJ 699 (Barnard's star) and GJ 725B. This result supports inferences from previous infrared work, although it does not agree with standard evolutionary models or optical analyses, which both suggest an abundance for CM Dra close to that of the Sun.