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(later than M6.5) dwarfs with K_s<12. Spectroscopic follow-up observationsidentify 53 M7 to M9.5 dwarfs and seven L dwarfs. The observed space density is0.0045 +- 0.0008 M8-M9.5 dwarfs per cubic parsec, without accounting forbiases, consistent with a mass function that is smooth across thestellar/substellar limit. We show the observed frequency of H alpha emissionpeaks at ~100% for M7 dwarfs and then decreases for cooler dwarfs. In absoluteterms, however, as measured by the ratio of H alpha to bolometric luminosity,none of the ultracool M dwarfs can be considered very active compared toearlier M dwarfs, and we show that the decrease that begins at spectral type M6continues to the latest L dwarfs. We find that flaring is common among thecoolest M dwarfs and estimate the frequency of flares at 7% or higher. We showthat the kinematics of relatively active (EW_{H alpha}>6 Angstroms) ultracool Mdwarfs are consistent with an ordinary old disk stellar population, while thekinematics of inactive ultracool M dwarfs are more typical of a 0.5 Gyr oldpopulation. The early L dwarfs in the sample have kinematics consistent withold ages, suggesting that the hydrogen burning limit is near spectral typesL2-L4. We use the available data on M and L dwarfs to show that chromosphericactivity drops with decreasing mass and temperature, and that at a given (M8 orlater) spectral type, the younger field (brown) dwarfs are less active thanmany of the older, more massive field stellar dwarfs. Thus, contrary to thewell-known stellar age-activity relationship, low activity in field ultracooldwarfs can be an indication of comparative youth and substellar mass.Comment: 39 pages including tables and figures, to appear in August 2000 A.