Meeting the cool neighbors. IX. The luminosity function of M7-L8 ultracool dwarfs in the field

We present a 20 pc, volume-limited sample of M7-L8 dwarfs created through spectroscopic follow-up of sources selected from the Two Micron All Sky Survey Second Incremental Release Point Source Catalog. In this paper we present optical spectroscopy of 198 candidate nearby ultracool dwarfs, including 12 late-M and L dwarfs likely to be within 20 pc of the Sun and 94 more distant late-type dwarfs. We have also identified five ultracool dwarfs with spectral signatures of low gravity. Combining these data with previous results, we define a sample of 99 ultracool dwarfs in 91 systems within 20 pc. These are used to estimate the J- and K-band luminosity functions for dwarfs with optical spectral types between M7 and L8 (10.5 < M-J < 15, 9.5 < M-KS < 13). We find a space density of 4.9 x 10(-3) pc(-3) for late-M dwarfs (M7-M9.5) and a lower limit of 3.8 x 10(-3) pc(-3) for L dwarfs.