Astrophysics Group, Keele University, Keele ST5 5BG, UK.
Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, Massachusetts 02138, USA.
Nature. 2016 Jul 28;535(7613):526-8. doi: 10.1038/nature18638.
In solar-type stars (with radiative cores and convective envelopes like our Sun), the magnetic field powers star spots, flares and other solar phenomena, as well as chromospheric and coronal emission at ultraviolet to X-ray wavelengths. The dynamo responsible for generating the field depends on the shearing of internal magnetic fields by differential rotation. The shearing has long been thought to take place in a boundary layer known as the tachocline between the radiative core and the convective envelope. Fully convective stars do not have a tachocline and their dynamo mechanism is expected to be very different, although its exact form and physical dependencies are not known. Here we report observations of four fully convective stars whose X-ray emission correlates with their rotation periods in the same way as in solar-type stars. As the X-ray activity-rotation relationship is a well-established proxy for the behaviour of the magnetic dynamo, these results imply that fully convective stars also operate a solar-type dynamo. The lack of a tachocline in fully convective stars therefore suggests that this is not a critical ingredient in the solar dynamo and supports models in which the dynamo originates throughout the convection zone.
在太阳型恒星中（与我们的太阳具有辐射核和对流包层），磁场为恒星黑子、耀斑和其他太阳现象以及色球和日冕在紫外到 X 射线波长的发射提供动力。产生磁场的发电机取决于内部磁场通过差速旋转的剪切。长期以来，人们一直认为剪切发生在辐射核和对流包层之间的称为转捩区的边界层中。完全对流的恒星没有转捩区，预计它们的发电机机制将非常不同，尽管其确切形式和物理依赖性尚不清楚。在这里，我们报告了对四颗完全对流恒星的观测结果，它们的 X 射线发射与它们的自转周期以与太阳型恒星相同的方式相关。由于 X 射线活动-自转关系是磁场发电机行为的一个成熟指标，这些结果表明完全对流的恒星也运行着太阳型发电机。因此，完全对流恒星中没有转捩区表明这不是太阳发电机的关键因素，并支持发电机起源于整个对流区的模型。
