Fundamental parameters of M dwarfs

We introduce a new technique which exploits the flux ratio in different bands as a proxy of both effective temperature and metallicity. Our temperature scale for late-type dwarfs extends well below 3000K (almost to the brown dwarf limit) and is supported by interferometric angular diameter measurements above 3000K.

Our metallicities are in excellent agreement (usually within 0.2dex) with recent determinations via independent techniques. A subsample of cool M dwarfs with metallicity estimates based on hotter Hipparcos common proper motion companions indicates our metallicities are also reliable below 3000K, a temperature range unexplored until now. The high quality of our data allows us to identify a striking feature in the bolometric luminosity versus temperature plane, around the transition from K to M dwarfs.