Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation
Activity and Kinematics of Ultracool Dwarfs, Including an Amazing Flare Observation
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DOI：
10.1086/512158
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发表时间：
2007-01
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通讯作者：
Sarah J. Schmidt;K. Cruz;Bethany J. Bongiorno;J. Liebert;I. N. Reid
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作者：
Sarah J. Schmidt;K. Cruz;Bethany J. Bongiorno;J. Liebert;I. N. Reid
我们展示了从2微米全天空巡天（2MASS）中通过光度法选取的152颗晚M型和L型矮星（M7 - L8）的一个具有代表性的体积有限（20秒差距）样本的活动情况和运动学特征。利用新的自行测量结果和分光光度距离估算值，我们计算了切向速度。该样本的平均切向速度为⟨Vtan⟩ = 31.5千米/秒，速度弥散为σtan = 20.7千米/秒，最大切向速度为Vtan = 138.8千米/秒。这些运动学结果与之前对本地太阳邻域中超冷矮星的研究非常吻合。在检测的81颗晚M型矮星中的63颗以及69颗L型矮星中的16颗中检测到了作为色球活动指标的Hα发射。我们发现，以log(FHα/Fbol)衡量的活动强度与Vtan之间缺乏相关性，尽管速度分布表明我们样本中的活跃矮星比不活跃矮星略年轻。与之前对超冷矮星活动的研究一致，我们发现每个光谱型中发射Hα的天体所占比例在光谱型M7达到峰值，并在中L型矮星中下降。对于M7之后的光谱型，活动强度与光谱型也有类似的相关性。150颗矮星中有11颗显示出变异性的证据，从微小的波动到大的耀斑事件都有。我们估计晚M型矮星的耀斑周期约为5%，L型矮星约为2%。文中还讨论了对L1型矮星2MASS J10224821 + 5825453的强烈可变活动的观测以及M7型矮星2MASS J1028404 - 143843上令人惊叹的耀斑事件。
We present the activity and kinematics of a representative volume-limited (20 pc) 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 ⟨Vtan⟩ = 31.5 km s-1, a velocity dispersion of σtan = 20.7 km s-1, and a maximum tangential velocity of Vtan = 138.8 km s-1. These kinematic results are in excellent agreement with previous studies of ultracool dwarfs in the local solar neighborhood. Hα 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(FHα/Fbol), and Vtan, 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 Hα-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.