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

Abstract

We present the activity and kinematics of a representative volume-limited (20 pc) sample of 152 late-M and L dwarfs (M7YL8) 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 hV tan i ¼ 31:5 km s À1 , a velocity dispersion of tan ¼ 20:7 km s À1 , and a maximum tangential velocity of V tan ¼ 138:8 km s À1 . These kinematic results are in excellent agreement with previous studies of ultracool dwarfs in the local solar neighborhood. H 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 (F H /F bol ), 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 J1028404À143843 are discussed.

Spectroscopic characterization of 78 DENIS ultracool dwarf candidates in the solar neighborhood and the Upper Scorpii OB association

“…In a sample of 152 late-M and L dwarfs, Schmidt et al (2007) found a slightly lower frequency of Hα emitters among the M8 dwarfs and confirmed the decline of Hα emission in L dwarfs. Our data indicates a frequency of Hα emitters that is consistent with that of Schmidt et al (2007) in the range M8-M9, but is significantly lower for the L dwarfs. We do not attach high significance to our results because it is possible that our lower fraction of Hα emitters in the L dwarfs is caused by the low spectral resolution and modest signal-tonoise ratio of our spectra.…”

“…We did not detect any obvious flares in our observations. Schmidt et al (2007) estimated a flare cycle of 5% for late-M dwarfs and of 2% for L dwarfs. Those numbers may need to be revised slightly downwards.…”

Ultra-cool dwarfs: new discoveries, proper motions, and improved spectral typing from SDSS and 2MASS photometric colors

“…Figure 3 shows 8 spectra of previously found M, L and T dwarfs as a comparison. SDSS J1428+5923 was recently confirmed and spectral-typed using its infrared spectrum as an L5 dwarf using the Two Micron Proper Motion (2MUPM) survey, under the name 2MASS J14283132+5923354 (Schmidt et al 2007). SDSS J0249−0034, SDSS J1048+0111, SDSS J1653+6231 and SDSS J1331−0116 were classified by Hawley et al (2002).…”

X-ray emission from the M9 dwarf 1RXS J115928.5-524717

“…Other indicators of magnetic activity are most prominently their often strong and variable Hα emission (Gizis et al 2000;Mohanty & Basri 2003;Schmidt et al 2007), suggesting the ubiquity of chromospheres and radio gyrosynchrotron emission, which requires the presence of fast electrons and magnetic fields (Berger 2002). Recently, strong magnetic fields with B f -values of up to a few kG have been detected on several ultracool dwarfs, utilising the profiles of their FeH lines (Reiners & Basri 2007).…”