SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS
SURPRISINGLY WEAK MAGNETISM ON YOUNG ACCRETING BROWN DWARFS
复制标题
年轻的吸积褐矮星的磁性非常弱
DOI：
--
复制
发表时间：
2009
期刊：
影响因子：
--
通讯作者：
U. Christensen
中科院分区：
文献类型：
--
作者：
A. Reiners;G. Basri;U. Christensen
我们利用FeH分子吸收谱线的高分辨率光谱测量了四颗吸积年轻褐矮星和一颗非吸积年轻极低质量（VLM）恒星的表面磁通量。据推测，其中一颗褐矮星J1207334−393254的磁场为1-2 kG，因为它与高质量的金牛座T恒星相似，表现为吸积和喷流的存在。我们没有在任何年轻的褐矮星中找到明确的证据，但在年轻的VLM恒星上发现了一个2kg的场。2MASS J1207334−393254磁通量的3σ上限刚好达到1 kG。根据观测到的旋转，我们估计了年轻褐矮星吸积所需的磁场，并发现只有几百高斯的磁场就足以使磁层吸积。这个预测值小于我们观察到的上限。我们的结论是，年轻的棕矮星的磁场比大约一个太阳质量的年轻恒星的磁场低5倍或更多，而在光谱类型与我们年轻的棕矮星相似的更老的恒星中。有趣的是，在最初的几百万年里，磁场随着质量的下降而缩小，这与磁层吸积的需要是一致的，然而在相同的有效温度范围内，没有观察到这种缩小。这种尺度与自转的趋势相反，与太阳质量的天体相比，非常年轻的吸积棕矮星的自转周期更短（所有情况下的罗斯比数都很低）。我们推测，在年轻的天体中，磁层吸积和磁场强度之间可能存在更深层次的内在联系，或者褐矮星中的磁场产生效率可能低于恒星。这两种现象目前都没有一个简单的物理解释。
We have measured the surface magnetic flux on four accreting young brown dwarfs and one nonaccreting young very low mass (VLM) star utilizing high-resolution spectra of absorption lines of the FeH molecule. A magnetic field of 1–2 kG had been proposed for one of the brown dwarfs, Two Micron All Sky Survey (2MASS) J1207334−393254, because of its similarities to higher mass T Tauri stars as manifested in accretion and the presence of a jet. We do not find clear evidence for a kilogauss field in any of our young brown dwarfs but do find a 2 kG field on the young VLM star. Our 3σ upper limit for the magnetic flux in 2MASS J1207334−393254 just reaches 1 kG. We estimate the magnetic field required for accretion in young brown dwarfs given the observed rotations, and find that fields of only a few hundred gauss are sufficient for magnetospheric accretion. This predicted value is less than our observed upper limit. We conclude that magnetic fields in young brown dwarfs are a factor of 5 or more lower than in young stars of about one solar mass, and in older stars with spectral types similar to our young brown dwarfs. It is interesting that, during the first few million years, the fields scale down with mass in line with what is needed for magnetospheric accretion, yet no such scaling is observed at later ages within the same effective temperature range. This scaling is opposite to the trend in rotation, with shorter rotation periods for very young accreting brown dwarfs compared with accreting solar-mass objects (and very low Rossby numbers in all cases). We speculate that in young objects a deeper intrinsic connection may exist between magnetospheric accretion and magnetic field strength, or that magnetic field generation in brown dwarfs may be less efficient than in stars. Neither of these currently has an easy physical explanation.