The effective temperature scale of M dwarfs

Context. Despite their large number in the Galaxy, M dwarfs remain elusive objects and the modeling of their photosphere has long remained a challenge (molecular opacities, dust cloud formation). Aims. Our objectives are to validate the BT-Settl model atmospheres, update the M dwarf T-eff-spectral type relation, and find the atmospheric parameters of the stars in our sample. Methods. We compare two samples of optical spectra covering the whole M dwarf sequence with the most recent BT-Settl synthetic spectra and use a chi(2) minimization technique to determine T-eff. The first sample consists of 97 low-resolution spectra obtained with New Technology Telescope (NTT) at La Silla Observatory. The second sample contains 55 medium-resolution spectra obtained at the Siding Spring Observatory (SSO). The spectral typing is realized by comparison with already classified M dwarfs. Results. We show that the BT-Settl synthetic spectra reproduce the slope of the spectral energy distribution and most of its features. Only the CaOH band at 5570 angstrom and AlH and NaH hydrides in the blue part of the spectra are still missing in the models. The T-eff scale obtained with the higher resolved SSO 2.3 m spectra is consistent with that obtained with the NTT spectra. We compare our T-eff scale with those of other authors and with published isochrones using the BT-Settl colors. We also present relations between effective temperature, spectral type, and colors of the M dwarfs.