Mass–Spectral Class Relationship for M Dwarfs

Abstract

We derive mass–spectral class relationships for M dwarfs in the mass range 0.075  m/MJ  0.6, which corresponds to a spectral range of M0–M10. The stellar models are based on the most recent nongray atmosphere models presently available. The results are in excellent agreement with the observed relationship derived by Kircpatrick&McCarthy for dwarfs of spectral type earlier than M7.Weshow that the spectral subclass is definitely not a linear function of mass and increases abruptly as the substellar mass limit is approached. This behavior reflects the observed trends in color, magnitude, and luminosity as a function of the mass and stems from the very physical properties of very low mass stars. Objects older than 1 Gyr with a spectral type earlier than M10 (for solar metallicity) are predicted to be main-sequence very low mass stars. For objects with such a spectral class to be bona fide brown dwarfs, they must be younger than 1 Gyr.