An infrared spectral sequence for M dwarfs

Abstract

We present a spectral sequence from 1 to 2.5 µm of M dwarfs, from GL411 (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 and correlate their strengths with our derived temperature scale. For a given luminosity, this method yields temperatures close to those predicted by evolutionary models for low-mass stars, but not always close to those found by previous investigators. We find that GD165B has a temperature of |$1860 \pm 160 \enspace \text K$| and that it is the only star in the sample that might be classified as a brown dwarf, but to decide its true nature a more accurate parallax and a representative model atmosphere will be necessary.