PERIODIC RADIO AND Hα EMISSION FROM THE L DWARF BINARY 2MASSW J0746425+200032: EXPLORING THE MAGNETIC FIELD TOPOLOGY AND RADIUS OF AN L DWARF
PERIODIC RADIO AND Hα EMISSION FROM THE L DWARF BINARY 2MASSW J0746425+200032: EXPLORING THE MAGNETIC FIELD TOPOLOGY AND RADIUS OF AN L DWARF
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L 矮星双星 2MASSW J0746425+200032 的周期性无线电和 Hα 发射：探索 L 矮星的磁场拓扑和半径
DOI：
10.1088/0004-637x/695/1/310
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发表时间：
2008
期刊：
影响因子：
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通讯作者：
Thomas A. Fleming
中科院分区：
文献类型：
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作者：
E. Berger;R. Rutledge;N. Phan;G. Basri;M. Giampapa;J. Gizis;James Liebert;Eric Martin;Eric Martin;Thomas A. Fleming
我们对L矮星双星J0746425+200032进行了8.5小时的同步射电、X射线、紫外和光学观测。我们在4.86 GHz处探测到了以短周期脉冲为主的强射电发射，P=124.32±0.11min。脉冲分布和到达时间的稳定性表明，它们是由于1.7kgB≈磁场的旋转调制所致。还可以检测到静止的不变分量，这很可能是由于均匀大尺度场的发射。Hα辐射表现出相同的周期，但与射电脉冲不同的是，它是正弦变化的，并且恰好偏移了1/4个相位。这种正弦变化需要大尺度场结构的色球发射，射电脉冲可能从磁极发出。虽然这两条光曲线都可以用旋转失调磁场来解释，但1/4的相位滞后排除了对称偶极拓扑，因为它会导致1/2(极向场)或零(环向场)的相位滞后。因此，我们得出结论：(1)场是由四极构型主导的，这可以自然地解释1/4位相滞后；(2)Hα和/或射电发射区与场不是平凡地对齐的。不管场拓扑如何，我们使用测量的周期以及已知的自转速度(v Sin I≈27 KM S−1)和双星轨道倾角(I≈142°)来推导出主星的半径为0.078±0.010 R☉。这是首次测量一颗L矮星的半径，质量为0.085±0.010 M☉时，它将质量半径关系限制在0.1M☉以下。我们发现，即使对于几个Gyr年龄，半径也比理论模型预期的小30%左右。这种差异的根源要么是主序列底部的模型崩溃，要么是旋转轴和轨道轴之间严重的不对准。
We present an 8.5 hr simultaneous radio, X-ray, UV, and optical observation of the L dwarf binary 2MASSW J0746425+200032. We detect strong radio emission, dominated by short-duration periodic pulses at 4.86 GHz with P = 124.32 ± 0.11 min. The stability of the pulse profiles and arrival times demonstrates that they are due to the rotational modulation of a B ≈ 1.7 kG magnetic field. A quiescent nonvariable component is also detected, likely due to emission from a uniform large-scale field. The Hα emission exhibits identical periodicity, but unlike the radio pulses it varies sinusoidally and is offset by exactly 1/4 of a phase. The sinusoidal variations require chromospheric emission from a large-scale field structure, with the radio pulses likely emanating from the magnetic poles. While both light curves can be explained by a rotating misaligned magnetic field, the 1/4 phase lag rules out a symmetric dipole topology since it would result in a phase lag of 1/2 (poloidal field) or zero (toroidal field). We therefore conclude that either (1) the field is dominated by a quadrupole configuration, which can naturally explain the 1/4 phase lag; or (2) the Hα and/or radio emission regions are not trivially aligned with the field. Regardless of the field topology, we use the measured period along with the known rotation velocity (v sin i ≈ 27 km s−1), and the binary orbital inclination (i ≈ 142°), to derive a radius for the primary star of 0.078 ± 0.010 R☉. This is the first measurement of the radius of an L dwarf, and along with a mass of 0.085 ± 0.010 M☉ it provides a constraint on the mass–radius relation below 0.1 M☉. We find that the radius is about 30% smaller than expected from theoretical models, even for an age of a few Gyr. The origin of this discrepancy is either a breakdown of the models at the bottom of the main sequence, or a significant misalignment between the rotational and orbital axes.