  Despite their large number in the Galaxy, M dwarfs remain elusive objects and
the modeling of their photospheres has long remained a challenge (molecular
opacities, dust cloud formation). 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. We compare two samples
of optical spectra covering the whole M dwarf sequence with the most recent
BT-Settl synthetic spectra and use a \c{hi}2 minimization technique to
determine Teff . The first sample consists of 97 low-resolution spectra
obtained with NTT at La Silla Observatory. The second sample contains 55
mid-resolution spectra obtained at the Siding Spring Observatory (SSO). The
spectral typing is realized by comparison with already classified M dwarfs. 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{\AA}
and AlH and NaH hydrides in the blue part of the spectra are still missing in
the models. The Teff-scale obtained with the higher resolved SSO 2.3 m spectra
is consistent with that obtained with the NTT spectra. We compare our Teff
scale with those of other authors and to published isochrones using the
BT-Settl colors. We also present relations between effective temperature,
spectral type and colors of the M dwarfs.
