Program: GN-2005B-Q-20

Using 2MASS data, we have completed the first systematic, all-sky survey for ultracool dwarfs within 20 parsecs of the Sun. With follow-up observations of 95\% of our candidates, we have identified 100 new nearby late-M and L dwarfs, tripling the local census. In this proposal, we aim to complete observations of the final 40 of the 44 remaining candidates and, at a lower priority, to get optical spectra of nine nearby L dwarfs identified with NIR (6 objects) and low signal-to-noise optical (3 objects) data. Spectral types provide the most reliable distance estimates ($\sigma_d \sim \pm 15$\%) for L dwarfs, but it is crucial that those types are derived from {\sl optical} spectra. Experience has shown that near-infrared classifications often differ by 2 to 3 spectral classes from the optical benchmarks, corresponding to $\Delta M_J = 1$ to 1.5 magnitudes, or $\delta d \sim$ 60--100\%. Systematic errors of this magnitude obviously result in substantial uncertainties in derived space densities. We therefore propose to finalize our luminosity function analysis by obtaining red spectra of 49 candidates with two nights on the Blanco 4m RC Spectrograph (22 targets, $13<J<15$), 24 hours with GMOS on Gemini South (24 targets, $J>15$, $\delta<-3$), and 3 hours with GMOS on Gemini North (3 targets, $J>15$, $\delta>+15$). This sample is likely to include the coolest, lowest-luminosity L dwarfs in the immediate Solar Neighborhood.