New neighbors from 2MASS: Activity and kinematics at the bottom of the main sequence

We have combined 2MASS and POSS II data in a search for nearby ultracool (l ater than M6.5) dwarfs with K-s < 12. Spectroscopic follow-up observations identify 53 M7-M9.5 dwarfs and seven L dwarfs. The observed space density i s 0.0045 +/- 0.0008 M8-M9.5 dwarfs per cubic parsec, without accounting for biases, consistent with a mass function that is smooth across the stellar/ substellar limit. We show the observed frequency of Ha emission peaks at si milar to 100% for M7-M9.5 dwarfs and then decreases for cooler dwarfs. In a bsolute terms, however, as measured by the ratio of Ha to bolometric lumino sity, none of the ultracool M dwarfs can be considered very active compared to earlier RI dwarfs, and we show that the decrease that begins at spectra l type M6 continues to the latest L dwarfs. We find that flaring is common among the coolest M dwarfs and estimate the frequency of flares at 7% or hi gher. We show that the kinematics of relatively active (EW > 6 Angstrom) ul tracool M dwarfs are consistent with an ordinary old disk stellar populatio n, while the kinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr old population. The early L dwarfs in the sample have kinematics consistent with old ages, suggesting that the hydrogen-burning limit is nea r spectral types L2-L4. We use the available data on M and L dwarfs to show that chromospheric activity drops with decreasing mass and temperature and that at a given (M8 or later) spectral type, the younger field (brown) dwa rfs are less active than many of the older, more massive field stellar dwar fs. Thus, contrary to the well-known stellar age-activity relationship, low activity in field ultracool dwarfs can be an indication of comparative you th and substellar mass.