Accretion to stars with non-dipole magnetic fields
Accretion to stars with non-dipole magnetic fields
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DOI：
10.1111/j.1365-2966.2006.11192.x
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发表时间：
2006-10
影响因子：
4.8
通讯作者：
M. Long;M. Romanova;R. Lovelace
中科院分区：
文献类型：
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作者：
M. Long;M. Romanova;R. Lovelace
本文首次在全三维磁流体力学模拟中研究了具有非偶极子磁fi场的旋转星的盘吸积现象。这是首次在全三维磁流体力学模拟中fi。我们研究了(1)纯偶极，(2)纯四极，和(3)偶极加四极fi场的情况。四极磁矩D与偶极磁矩µ平行，且都相对于恒星Ω的自旋轴以Θ角倾斜。模拟表明，在每一种情况下，恒星表面漏斗流和相关热点的结构都具有与磁fi配置有关的特殊特征。在纯偶极子的情况下，物质在两个漏斗流中积累，在磁极附近形成两个拱形斑点。在纯四极fi磁场的情况下，大部分物质fl穿过在磁赤道上形成环形热区的四极“带”。在偶极+四极fi的情况下，磁半球北部的磁flUX大于南部，四极带和磁环向南移动。四极矩越强，磁环就越接近磁赤道。在SUffiScient LargeΘ，物质也向南极扩散，在极点附近形成了一个热点。光曲线具有各种不同的特征，这使得人们从光曲线中推导出磁fi场形。在偶极和四极主导磁场的情况下，存在着不同的fic特征：(1)在偶极ffi磁场的情况下，恒星和盘之间的角动量fi更清楚；(2)在偶极fi磁场的情况下，热点更热和更亮，因为与四极fl磁场相比，物质加速的距离更远。关键词：吸积、吸积盘-磁fi场-磁流体星：磁fi场。
ABSTRACT Disc accretion to a rotating star with a non-dipole magnetic field is investigatedfor the first time in full three-dimensional (3D) magnetohydrodynamic (MHD) simula-tions. We investigated the cases of (1) pure dipole, (2) pure quadrupole, and (3) dipoleplus quadrupole fields. The quadrupole magnetic moment D is taken to be parallelto the dipole magnetic moment µ, and both are inclined relative to the spin axis ofthe star Ω at an angle Θ. Simulations have shown that in each case the structure ofthe funnel streams and associated hot spots on the surface of the star have specificfeatures connected with the magnetic field configuration. In the pure dipole case mat-ter accretes in two funnel streams which form two arch-like spots near the magneticpoles. In the case of a pure quadrupole field, most of the matter flows through thequadrupole “belt” forming a ring-shaped hot region on the magnetic equator. In thecase of a dipole plus quadrupole field, magnetic flux in the northern magnetic hemi-sphere is larger than that in the southern, and the quadrupole belt and the ring aredisplaced to the south. The stronger the quadrupole, the closer the ring is to the mag-netic equator. At sufficiently large Θ, matter also flows to the south pole, forming a hotspot near the pole. The light curves have a variety of different features which makesit difficult to derive the magnetic field configuration from the light curves. There arespecific features which are different in cases of dipole and quadrupole dominated mag-netic field: (1) Angular momentum flow between the star and disc is more efficient inthe case of the dipole field; (2) Hot spots are hotter and brighter in case of the dipolefield because the matter accelerates over a longer distance compared with the flow ina quadrupole case.Key words: accretion, accretion discs - magnetic fields - MHD - stars: magneticfields.