cTTS V2129 Oph的冠状结构
Coronal structure of the cTTS V2129 Oph
摘要：
金牛座T星与其盘之间的磁耦合性质不仅决定了质量吸积过程，而且可能决定了中心恒星的自旋演化。我们已经采取了最近发表的表面磁图的一个中度吸积金牛座T星（V2129 Oph），并用它来推断其大尺度场的几何形状。我们确定的结构的开放（风轴承）场线，封闭（X射线亮）场线和那些潜在的吸积场线，通过赤道平面内的开普勒共同旋转半径。我们考虑了一系列的模型，其中恒星磁场是开放的向外压力的热日冕气体在一定范围内的半径。随着这个半径的增加，吸积沿着更简单的场结构发生，并在恒星表面的更少的地点发生影响。这是一致的，在钙II IRT和氦线，这表明吸积在可见的半球被限制在一个单一的高纬度点观察到的变化。通过确定吸积流的密度和速度，我们发现，为了使大部分的总质量吸积率影响到一个单一的高纬度地区，我们需要磁盘材料吸积约7R*，接近开普勒的共同旋转半径在6.8R*。我们还计算了日冕密度和X射线辐射量。我们发现，无论是幅度和旋转调制的排放措施增加源表面的增加。对于V2129 Oph的场结构，它主要是八极的，发射形成一个明亮的高纬度环，当恒星旋转时总是可见的。由于吸积漏斗的密度不足以引起日冕X射线光子的显著散射，它们只能提供最多约10%的低旋转调制。
英文摘要：
The nature of the magnetic coupling between T Tauri stars and their disks determines not only the mass accretion process but possibly the spin evolution of the central star. We have taken a recently-published surface magnetogram of one moderately-accreting T Tauri star (V2129 Oph) and used it to extrapolate the geometry of its large-scale field. We determine the structure of the open (wind-bearing) field lines, the closed (X-ray bright) field lines and those potentially accreting field lines that pass through the equatorial plane inside the Keplerian co-rotation radius. We consider a series of models in which the stellar magnetic field is opened up by the outward pressure of the hot coronal gas at a range of radii. As this radius is increased, accretion takes place along simpler field structures and impacts on fewer sites at the stellar surface. This is consistent with the observed variation in the Ca II IRT and HeI lines which suggests that accretion in the visible hemisphere is confined to a single high-latitude spot. By determining the density and velocity of the accretion flows, we find that in order to have most of the total mass accretion rate impacting on a single high-latitude region we need disk material to accrete from approximately 7R*, close to the Keplerian co-rotation radius at 6.8R*. We also calculate the coronal density and X-ray emission measure. We find that both the magnitude and rotational modulation of the emission measure increase as the source surface is increased. For the field structure of V2129 Oph which is dominantly octupolar, the emission forms a bright, high-latitude ring that is always in view as the star rotates. Since the accretion funnels are not dense enough to cause significant scattering of coronal X-ray photons, they provide only a low rotational modulation of around 10% at most.
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