Understanding Stellar Convection

所有比氢和氦更重的元素都是在恒星中制造的。宇宙中最常见的元素(碳、氧)是在恒星的核心中形成的，而更重的元素是在恒星死亡的最后阶段产生的，这个过程的细节对主序列末尾的恒星核心的大小非常敏感。对流，特别是对流超调，可以改变可用于恒星核心的物质的数量，但对流超调的数量在很大程度上是未知的。我们将使用恒星脉动来限制六颗恒星样本中的对流超调量(目标1)。测量单个恒星的对流超调是困难的，到目前为止，只对少数主序恒星进行了测量。这个由六颗恒星组成的样本将显著增加具有已知对流过冲参数的恒星数量。随着这种测量在更多的恒星中进行，我们将能够更好地表征对流超调如何随着恒星的质量和年龄而变化。我们还将研究能够测量大量恒星对流超调的技术(目标2)。我最近开发了一种技术，原则上可以用来限制一种类型的变星--德尔塔Scuti星--的对流超调。我们将调查当恒星的组成允许变化时，这项技术是否适用，以及这项技术是否可以扩展到其他类别的变星。同样，这将显著增加具有已知对流超调的恒星的数量，使我们能够表征对流超调参数随质量和年龄的行为。最后，我们将研究演化程度更高的恒星中对流的影响。造父变星是天文学中重要的距离指标，预计会有显著的对流包络。虽然大多数造父变星被观察到只有一个脉动周期，但相当多的造父变星是多周期的。多个周期的起源尚不清楚，但有人提出它与恒星包层内对流和脉动之间的相互作用有关。我们将对造父变星进行二维和三维模拟，以研究这种相互作用，并解释多周期造父变星的起源。这将对造父变星距离尺度的精确度产生重要影响，从而对我们理解宇宙的尺度和历史具有重要意义。

All elements heavier than hydrogen and helium are made in stars. The most common elements in the universe (carbon, oxygen) are made in the cores of stars, while heavier elements are made during the final phases of a stars death, and the details of this process are very sensitive to the size of the stellar core at the end of the main sequence. Convection, especially convective overshoot, can change the amount of material available to stellar cores, but the amount of convective overshoot is largely unknown. We will use stellar pulsation to constrain the amount of convective overshoot in a sample of six stars (Objective 1). Measuring the convective overshoot for individual stars is difficult, and has been done for only a handful of main sequence stars so far. This sample of six stars will significantly increase the number of stars with known convective overshoot parameters. As this measurement is made in more stars, we will be better able to characterize how the convective overshoot varies with the mass and age of a star. We will also investigate techniques that will allow measurements of convective overshoot in large numbers of stars (Objective 2). I have recently developed a technique that can in principle be used to constrain the convective overshoot in a sample of one type of variable star, the delta Scuti stars. We will investigate whether this technique holds when the composition of a star is allowed to vary, and also whether this technique can be extended to other classes of variable stars. Again, this will significantly increase the number of stars with known convective overshoots, allowing us to characterize the behaviour of the convective overshoot parameter with mass and age. Finally, we will investigate the effects of convection in more evolved stars. The Cepheid variables are important distance indicators in astronomy, and are expected to have significant convective envelopes. While most Cepheids are observed to have a single pulsation period, a significant number are multi-periodic. The origin of the multiple periods is unknown, but it has been proposed that it is related to the interaction between convection and pulsation in the stellar envelope. We will perform 2 and 3 dimensional simulations of Cepheid variables to investigate this interaction and explain the origin of the multi-periodic Cepheids. This will have important implications for the accuracy of the Cepheid distance scale, and hence for our understanding of the scale and history of the universe.