Metallicity of M dwarfs IV. A high-precision [Fe/H] and Teff technique from high-resolution optical spectra for M dwarfs

Aims. In this work we develop a technique to obtain high precision determinations of both metallicity and effective temperature of M dwarfs in the optical. Methods. A new method is presented that makes use of the information of 4104 lines in the 530-690 nm spectral region. It consists in the measurement of pseudo equivalent widths and their correlation with established scales of [Fe/H] and T-eff. Results. Our technique achieves a rms of 0,08 +/- 0.01 for [Fe/H], 91 +/- 13 K for T-ett, and is valid in the (-0.85,0.26 dex), (2800, 4100 K), and (M0.0, M5.0) intervals for [Fe/H], T-etf and spectral type respectively. We also calculated the RMSEV which estimates uncertainties of the order of 0.12 dex for the metallicity and of 293 K for the effective temperature. The technique has an activity limit and should only be used for stars with log L-H alpha/L-bol < -4.0.