Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation

We present the activity and kinematics of a representative volume-limited (20pc) sample of 152 late-M and L dwarfs (M7--L8) photometrically selected fromthe Two Micron All-Sky Survey (2MASS). Using new proper motion measurements andspectrophotometric distance estimates, we calculate tangential velocities. Thesample has a mean tangential velocity of = 31.5 km/s, a velocitydispersion of sigma_tan = 20.7 km/s, and a maximum tangential velocity of V_tan= 138.8 km/s. These kinematic results are in excellent agreement with previousstudies of ultracool dwarfs in the local solar neighborhood. H_alpha emission,an indicator of chromospheric activity, was detected in 63 of 81 late-M dwarfsand 16 of 69 L dwarfs examined. We find a lack of correlation between activitystrength, measured by log(F_H_alpha/F_bol), and V_tan, though velocitydistributions suggest that the active dwarfs in our sample are slightly youngerthan the inactive dwarfs. Consistent with previous studies of activity inultracool dwarfs, we find that the fraction of H$\alpha$ emitting objects perspectral type peaks at spectral type M7 and declines through mid-L dwarfs.Activity strength is similarly correlated with spectral type for spectral typeslater than M7. Eleven dwarfs out of 150 show evidence of variability, rangingfrom small fluctuations to large flare events. We estimate a flare cycle of ~5%for late-M dwarfs and ~2% for L dwarfs. Observations of strong, variableactivity on the L1 dwarf 2MASS J10224821+5825453 and an amazing flare event onthe the M7 dwarf 2MASS J1028404$-$143843 are discussed.Comment: 30 pages, 12 figures, accepted to A