AN INFRARED SPECTRAL SEQUENCE FOR M-DWARFS

We present a spectral sequence from 1 to 2.5 mum of M dwarfs, from GL4 11 (M2V) to the best brown dwarf candidate GD165B which is classified as cooler than M9V. This sequence shows the progressive importance of water absorption in the atmospheres of M dwarfs. We take the strength of the water absorption bands as the basis of a new method to derive a temperature from which we calculate effective temperatures and radii. We also identify many of the stronger atomic and molecular features a nd correlate their strengths with our derived temperature scale. For a given luminosity, this method yields temperatures close to those pred icted by evolutionary models for low-mass stars, but not always close to those found by previous investigators. We find that GD165B has a te mperature of 1860 +/- 160 K and that it is the only star in the sample that might be classified as a brown dwarf, but to decide its true nat ure a more accurate parallax and a representative model atmosphere wil l be necessary.