Two Dimensional Stellar Evolution

*ConextStars在很大程度上是球形物体。出于这个原因，恒星的理论和计算模型通常被假定为：它们是球体。这样的模型工作得非常好，从事恒星演化领域的人，在一个世纪左右的时间里，已经非常成功地利用了它们。例如，我们现在了解了恒星如何将氢燃烧成氦，使它们在数十亿年的时间里发光。然而，在过去的十年里，越来越清楚的是，许多最令人兴奋的恒星并不是我们习惯的简单的球形天体。旋转是关键。自转的恒星围绕赤道膨胀，对其内部结构产生重要影响。一些被称为BE的恒星自转速度如此之快，以至于它们几乎要飞离了。这些恒星更像是蛋形的，而不是球形的！现有的这些旋转恒星的模型对它们的内部结构做了一些假设，以构建扭曲的球体模型，这些模型是基于带有小的非球面扰动的球体模型。例如，这些假设假设恒星在内部像近乎球形的壳体一样旋转。将其与太阳的内部结构进行比较。乍一看，它是球形的，但我们从对太阳黑子的观察中知道，它在赤道每24天自转一次。这是缓慢的自转，因为太阳表面的变形非常小。尽管如此，内部旋转并不在壳体上--在表面可以看到纬向的差异旋转，日震观测证实了这种差异旋转延伸到对流区。在恒星内部，它的自转似乎更稳定。许多恒星的自转速度比太阳快得多，并且表现出严重得多的物理症状。例如，自转导致从恒星中心到其表面的物质混合，导致化学污染，这是我们的望远镜可以观察到的。如果像太阳一样，自转速度随深度变化，我们预计发电机将在恒星中产生磁场。在双星系统中，物质可以从一颗恒星移动到另一颗恒星。这可以使恒星达到最大自转、碎裂、自转速度，并引起显著的化学混合和磁场形成。磁场反过来传输角动量。*目的、目标和应用这些过程可以用简单的球体来模拟。恒星的多维模型更合适，这也是这个项目的最终目标。虽然三维恒星演化目前还不可能，因为计算机不够快或足够强大，但二维恒星演化是在我们掌握之中的。在两个维度中，旋转和磁场的基本过程可以被适当地建模，而不需要目前恒星演化模型中采用的许多通常不合理的假设。从一维到二维是一个巨大的飞跃。它需要仔细制作。第一个可实现的目标是通过编写一种新的代码来自洽地求解二维恒星的结构，从而对二维恒星演化进行建模。这将包括恒星自转、发电机产生的磁场、恒星核心的核燃烧、整个恒星的混合以及基本的双星相互作用。这些模型将涵盖从氢气燃烧(如太阳)到红巨星和超巨星(已知的最大最亮的恒星)再到恒星死亡的演化阶段。在这个项目中构建的模型将向合作者提供。这包括英国的BRIDGCE项目，该项目研究恒星自转对星系化学演化的影响。在国际上，VLT-火焰合作需要恒星演化模型来解释和理解他们观察到的快速旋转的恒星。这个项目的结果将是这项任务的理想选择，也是与其他调查(如NASA的开普勒卫星)进行比较的理想选择。

* ContextStars are, for the most part, spherical objects. For this reason, theoretical and computational models of stars are usually assumed to be just that: they are spheres. Such models work amazingly well and the those working in the field of stellar evolution have, for a century or so, exploited them very successfully. We now understand how stars burn hydrogen to helium to make them shine for billions of years, for example.However, in the last decade, it has become increasingly clear that many of the most exciting stars are not the simple, spherical objects we are used to. Rotation is key. Stars that rotate expand around their equator with important consequences for their internal structure. Some stars, known as the Be stars, rotate is so fast that they are nearly flying apart. These stars are more egg-shaped than spherical!Existing models of these rotating stars make some assumptions about their internal structure to construct distorted sphere models which are based on spherical models with small, non-spherical perturbations. These assume, for example, that stars rotate as nearly-spherical shells internally. Compare this to the internal structure of our Sun. At first glance it is spherical but we know from observing sunspots that it rotates once every 24 days at the equator. This is slow rotation because the distortion at the solar surface is very small. Even so the internal rotation is not on shells - latitudinal differential rotation is visible at the surface and helioseismolgy confirms that this differential rotation extends through the convective zone. In the interior it appears to rotate more solidly.Many stars rotate much faster than the Sun and show far more severe physical symptoms. For example, rotation causes mixing of material from the centre of the star to its surface, leading to chemical pollution which can be observed with our telescopes. If, as in the Sun, the rotation rate changes with depth, we expect a dynamo to generate magnetic fields in the star. In binary-star systems material can move from one star to the other. This can spin up a star to its maximum, break up, rotation rate and induce significant chemical mixing and magnetic field formation. In turn magnetic fields transport angular momentum.* Aims, objectives and applicationsThese processes can be modelled with simple spheres. A multi-dimensional model of a star is much more apt and that is the ultimate aim of this project. While three-dimensional stellar evolution is not currently possible because computers are simply not fast or powerful enough, two-dimensional stellar evolution is within our grasp. In two dimensions the fundamental processes of rotation and magnetic fields can be modelled properly without the many, often poorly justified, assumptions currently employed in stellar evolution models. The leap from one to two dimensions is a big. It needs to be made carefully. The first achievable goal is to model two-dimensional stellar evolution by writing a new code to self-consistently solve for the structure of stars in two dimensions. This will include, as standard features, stellar rotation, magnetic field generation by dynamos, nuclear burning in the core of the star, mixing throughout the star and basic binary star interactions. The models will cover evolutionary phases from hydrogen burning, like our Sun, through red giants and supergiants, the biggest and brightest stars known, to stellar death.The models constructed in this project will be made available to collaborators. This include the BRIDGCE project in the UK, to study the impact of rotation in stars on the chemical evolution of galaxies. Internationally the VLT-FLAMES collaboration needs stellar evolutionary models to interpret and understand the rapidly rotating stars they observe. The results of this project will be ideal for this task and for comparison with other surveys such as that made by NASA's Kepler satellite.