Two Dimensional Stellar Evolution

* ContextStars在很大程度上是球形物体。由于这个原因，恒星的理论和计算模型通常被假设为：它们是球体。这样的模型运行得非常好，一个世纪左右以来，研究恒星演化的科学家们已经非常成功地利用了这些模型。例如，我们现在知道恒星如何将氢燃烧成氦，从而使它们发光数十亿年。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所习惯的简单的球形物体。旋转是关键。恒星在其赤道附近旋转，对其内部结构产生重要影响。一些恒星，被称为Be星，旋转得如此之快，以至于它们几乎要分开了。这些恒星与其说是球形，不如说是蛋形！这些旋转恒星的现有模型对其内部结构进行了一些假设，从而构建了畸变球模型，这些模型是基于具有小的非球摄动的球形模型。例如，这些理论假设恒星在内部以接近球形的壳体形式旋转。将其与太阳的内部结构进行比较。乍一看它是球形的，但我们通过观察太阳黑子知道它在赤道每24天旋转一次。这是缓慢的自转，因为太阳表面的畸变非常小。即使如此，内部旋转也不是在壳层上——在表面可以看到纬度上的差异旋转，而且日震学证实这种差异旋转延伸穿过对流区。在内部，它似乎旋转得更牢固。许多恒星的旋转速度比太阳快得多，并且表现出更严重的身体症状。例如，旋转导致恒星中心的物质混合到表面，导致化学污染，这可以用望远镜观察到。如果像太阳一样，旋转速率随深度变化，我们预计发电机会在恒星中产生磁场。在双星系统中，物质可以从一颗恒星移动到另一颗恒星。这可以使恒星旋转到最大，分裂，旋转速度，并诱导显著的化学混合和磁场形成。磁场反过来传递角动量。*目的、目标和应用这些过程可以用简单的球体来建模。一个明星的多维模型是更合适的，这是这个项目的最终目的。由于计算机不够快或不够强大，三维恒星演化目前还不可能实现，但二维恒星演化是在我们掌握之中的。在二维空间中，旋转和磁场的基本过程可以适当地建模，而不需要目前在恒星演化模型中使用的许多（通常是缺乏证据的）假设。从一维到二维的飞跃是巨大的。这需要小心谨慎。第一个可实现的目标是通过编写新的代码来自洽地求解二维恒星的结构，从而模拟二维恒星的演化。这将包括，作为标准特征，恒星旋转，发电机产生磁场，恒星核心的核燃烧，整个恒星的混合和基本的双星相互作用。这些模型将涵盖从氢燃烧（如我们的太阳）到红巨星和超巨星（已知最大、最亮的恒星）到恒星死亡的进化阶段。在这个项目中构建的模型将提供给合作者。其中包括英国的bridge项目，该项目旨在研究恒星自转对星系化学演化的影响。在国际上，vlt - flame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。这个项目的结果对于这项任务来说是理想的，并且可以与其他调查进行比较，比如美国宇航局的开普勒卫星。

* 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.