Differential rotation of giants and fast-rotating dwarf stars

在最近的太阳发电机模型中，微分旋转和经向流动都起着重要的作用。因此，了解这些现象对于恒星活动理论至关重要。因此，提出了一种新的并行计算机MHD代码，对巨星和快速旋转的MS星的微分旋转进行了数值研究。基于一个成功的太阳旋转模型（见封面图），我们开发了一个代码，可以预测恒星对流区的旋转和经向流动模式。到目前为止，我们的计算仅限于缓慢和中等旋转的太阳型恒星。这些恒星的模型总是产生太阳型的自转规律，赤道的自转速度比两极快，赤道-两极的自转速率Q差几乎不依赖于自转周期。最近对年轻的太阳型恒星的卫星观测支持了这一理论，这些恒星的自转周期约为10天。然而，对HD 31993的多普勒成像发现，这颗赤道减速的k2巨星具有反太阳旋转规律。角动量守恒定律预测了在非常快的子午流情况下的这种模式，这确实是观察到的。因此，我们建议计算HD 31993和其他巨星的自转。另一类具有强烈经向循环的恒星是非常年轻的快速旋转的MS星。由于对旋转周期短至≈1d的f型恒星的观测也越来越多，我们计划将该模型也应用于这些具有相当薄对流区的天体。这种全新的结果只能通过满足分辨率和数值稳定性的最高约束的代码推导出来。

In recent models of the solar dynamo both differential rotation and meridional flow play important parts. Understanding these phenomena is therefore essential for the theory of stellar activity. A numerical study of the differential rotation of giants and rapidly rotating MS stars by means of a new MHD code for parallel computers is thus proposed.Based on a successful model of the solar rotation (see the coverplot) we have developed a code that predicts the rotation and meridional flow pattern of stellar convection zones. So far our computations have been restricted to solar-type stars of slow and intermediate rotation. Models for these stars always produce solar-type rotation laws with the equator rotating faster than the poles where the equator-pole difference of the rotation rate Q hardly depends on the rotation period. The theory for such stars is supported by recent satellite-based observations of young solar-type stars with about 10 days rotation period.Doppler imaging of HD 31993 finds, however, an anti-solar rotation law for this K2-giant with decelerated equator. The angular momentum conservation law predicts such a pattern in case of very fast meridional flow which is indeed observed. We therefore propose to compute the rotation of HD 31993 and other giant stars. Another class of stars with intense meridional circulation are very young fast-rotating MS stars.As there is also an increasing amount of observations of F-type stars with rotation periods as short as ≈ 1d we plan to apply the model also to these objects with their rather thin convection zones. Such completely new results can only be derived with a code which fulfills highest constraints for resolution and numerical stability.