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J/AJ/133/2258 Activity and kinematics of ultracool dwarfs (Schmidt+, 2007)
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Beginning of ReadMe :J/AJ/133/2258 Activity and kinematics of ultracool dwarfs (Schmidt+, 2007)
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Activity and kinematics of ultracool dwarfs, including an amazing
flare observation.
Schmidt S.J., Cruz K.L., Bongiorno B.J., Liebert J., Reid I.N.
<Astron. J., 133, 2258-2273 (2007)>
=2007AJ....133.2258S
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ADC_Keywords: Stars, dwarfs ; Stars, late-type ; Equivalent widths
Keywords: Galaxy: stellar content - solar neighborhood - stars: activity -
stars: flare - stars: late-type - stars: low-mass, brown dwarfs
Abstract:
We present the activity and kinematics of a representative
volume-limited (20pc) sample of 152 late-M and L dwarfs (M7-L8)
photometrically selected from the Two Micron All Sky Survey (2MASS).
Using new proper-motion measurements and spectrophotometric distance
estimates, we calculate tangential velocities. The sample has a mean
tangential velocity of <V_tan_>=31.5km/s, a velocity dispersion of
sigma_tan_=20.7km/s, and a maximum tangential velocity of
V_tan_=138.8km/s. These kinematic results are in excellent agreement
with previous studies of ultracool dwarfs in the local solar
neighborhood. Halpha emission, an indicator of chromospheric
activity, was detected in 63 of 81 late-M dwarfs and in 16 of 69
L dwarfs examined. We find a lack of correlation between activity
strength, measured by log(FHalpha/Fbol), and V_tan_, although
velocity distributions suggest that the active dwarfs in our sample
are slightly younger than the inactive dwarfs. Consistent with
previous studies of activity in ultracool dwarfs, we find that the
fraction of H-emitting objects per spectral type peaks at spectral
type M7 and declines through mid-L dwarfs. Activity strength is
similarly correlated with spectral type for types later than M7.
Eleven dwarfs out of 150 show evidence of variability, ranging from
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,
variable activity on the L1 dwarf 2MASS J10224821+5825453 and an
amazing flare event on the M7 dwarf 2MASS J1028404-143843 are
discussed.