Mass-Spectral Class Relationship for M Dwarfs

We derive mass-spectral class relationships for M dwarfs in the mass range 0.075 ≤ m/M☉ ≤ 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. We show 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.

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