THE SPECTROSCOPIC PROPERTIES OF EXTREME M DWARFS IN THE 2 MICRON REGION

We compare the spectroscopic properties of the late M dwarfs VB 8, VB 10, G1 569B, and LHS 2924 in the 1.5-2.3 mum wavelength interval. Conspicuous star-to-star differences are seen in the strengths of many features, such as the Na I 4530 cm-1 blend and the absorption bands of CO, in marked contrast to the much more subtle differences that are seen shortward of 1 mum. The strength of the first-overtone CO bands is correlated with spectral type derived shortward of 1 mum and I-K color, while this is not the case for H2O absorption, which is strongest in VB 10 and G1 569B, and then weakens in LHS 2924. The spectra of the coolest stars, Gl 569B and LHS 2924, are very different from those of VB 8 and VB 10 in that a number of strong unidentified absorption features are present; we speculate that these features may be the signatures of as yet unidentified polyatomic molecules. Despite the uncertainties in the origin of these features, they nevertheless constitute a significant source of opacity, which should be considered in future attempts to model the atmospheres of very cool low-mass objects.