Observations of M dwarfs beyond 2.2 m

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Abstract
This paper presents the first systematic spectroscopic observations of M dwarfs beyond 2.2 μm. The coolest dwarfs show strong water absorption in the 3 μm window, and beyond 4 μm , the energy distributions of all the stars fall slightly less steeply than the Rayleigh–Jeans tail of a blackbody. Spectra between 1 and 4 μm are essential in deriving accurate luminosities of M dwarfs, and possibly in deriving accurate effective temperatures too. New values reported here are not in general well explained by theoretical models of hydrogen burning stars. This is especially true for those cooler than 3000 K: in the HR diagram they lie closer to brown dwarfs, in contrast to recent results based only on photometry. This result revives the old issues of the nature of the coolest stars and of the accuracy of the theoretical models.

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