吸积热点密度结构的测量
Measuring the Density Structure of an Accretion Hot Spot
摘要：
磁层吸积模型预测，来自原行星盘的物质通过漏斗流吸积到恒星上，漏斗流沿着恒星场线和恒星表面的冲击，留下一个具有密度梯度的热点。以前的工作已经推断出不同的密度在热点，但一直不敏感的径向密度分布。人们试图用X射线观测来测量这一点，但X射线发射只能追踪到热点的一小部分，还有日冕发射。在这里，我们报告的吸积星GM Aur的周期性紫外光和光学光变曲线，显示其峰值之间的时间差约为1天。当紫外线和光发射源随着恒星旋转而进入和离开视野时，周期性就会出现。时间滞后表明在恒星表面的紫外线和光学亮度的空间分布的差异。在磁层吸积模型的框架内，这表明在恒星表面的热点径向密度梯度，因为热点的不同密度的部分预计会发出不同波长的辐射。这些结果是磁层吸积模型预测的热点密度梯度的第一次观测确认，并展示了从关注可以观察到大部分吸积能量的波长中获得的见解。
英文摘要：
Magnetospheric accretion models predict that matter from protoplanetary disks accretes onto the star via funnel flows which follow the stellar field lines and shock on the stellar surface leaving a hot spot with a density gradient. Previous work has inferred different densities in the hot spot, but has not been sensitive to the radial density distribution. Attempts have been made to measure this with X-ray observations, but X-ray emission only traces a fraction of the hot spot and also coronal emission. Here we report periodic ultraviolet and optical light curves of the accreting star GM Aur that display a time lag of about 1 day between their peaks. The periodicity arises as the source of the ultraviolet and optical emission moves into and out of view as it rotates along with the star. The time-lag indicates a difference in the spatial distribution of ultraviolet and optical brightness over the stellar surface. Within the framework of the magnetospheric accretion model, this indicates a radial density gradient in a hot spot on the stellar surface since different density parts of the hot spot are expected to emit radiation at different wavelengths. These results are the first observational confirmation of the magnetospheric accretion model's prediction of a density gradient in the hot spot and demonstrate the insights gained from focusing on the wavelengths where the bulk of the accretion energy can be observed.
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