of the effects of surface gravity on the K band spectra of very young brown dwarfs.

Our aim was to use medium resolution spectroscopy to investigate the sensitivity of the NaI doublet at 2.21$\mu$m to surface gravity, and hence age, while using the CaI line at 2.26$\mu$m and the CO absorption bands at 2.29$\mu$m and 2.32$\mu$m to correct for any metallicity variations or veiling by hot dust. It was also intended to use these spectra to provide more precise spectral types than had been possible previously, and also to confirm that the IMF extends below 10M$_{\rm{Jup}}$. An unexpected by-product of the investigation was the discovery that the slope of the K band also has strong sensitivity to gravity, which appears to provide a useful age indicator. While the effect of surface gravity on the K band spectra of pre-main sequence brown dwarfs has previously been noted (@luhman04a; A07), it has not been studied in detail.

Structure of this Paper
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Section 3 of this paper describes how our sample of brown dwarf calibrators and PMOs were chosen.

Section 4 describes how these objects were observed, and how their spectra were extracted.

Results are reported in Section 5. Section 5.1 contains 