Peanuts in Space: solving the thermodynamical conundrum in contact binary stars

接触双星系统中的两颗恒星轨道非常接近，以至于它们共享一个共同的包络，导致它们独特的“花生”形状。接触双星是普遍存在的，由于其0.2-1.2天的轨道周期和大的亮度变化，很容易观察到，而且由于其观察到的物理性质而具有吸引力。最令人困惑的是，尽管它们的质量和半径不同，但两颗恒星的温度基本相同，这表明它们处于热平衡状态，需要在两颗恒星的共同外壳之间有大量的质量和能量流动。然而，目前的接触二元模型不考虑质量/能量混合。主要研究者（PI）团队将通过升级其建模软件PHOEBE来纠正这种情况，以通过引入混合参数来解释质量/能量转移。他们将分析数千个接触式二元光变曲线，并探索混合参数的相关性，以推断驱动这种质量和能量转移的物理过程，从而更好地理解这些系统。该奖项每年将支持一名博士后和三名本科生研究人员，以及支持定期通过IAU外联办公室在线分发的普通读者水平的文章。该团队将探索PHOEBE的三个简单的单参数模型扩展：径向混合，横向混合和磁活动。在每一种情况下，都假定混合只影响较冷的星星，而且混合的量可以用幂律很好地近似。在每种情况下，模型将由单一混合参数“p”参数化，作为包层中质量和能量流动的流体动力学基础的代理。他们将评估“p”对光曲线的影响，由于“p”至少在某种程度上与其余的主要参数退化，因此将考虑其他参数以生成与数据的良好拟合并解释混合。通过对公共档案中的数千个接触二元光变曲线进行同样的分析，他们将能够研究“p”与热接触和几何接触程度之间的相关性，以及每个单参数模型的参数“p”的分布。这些相关性将为团队提供基础物理学的信息，而“p”将为他们提供如何正确制定更高级混合模型的基本指导。该计划已经在PHOEBE中实施，用于计算恒星通量，使接触双星大气成为PHOEBE现有能力的自然延伸。该奖项反映了NSF的法定使命，并通过使用基金会的知识价值和更广泛的影响审查标准进行评估，被认为值得支持。

The two stars of a contact binary system orbit so close to each other that they share a common envelope, resulting in their distinctive "peanut" shape. Contact binaries are ubiquitous, easily observable thanks to their 0.2-1.2-day orbital periods and large brightness variations, and moreover appealing due to the physics governing their observed properties. Most perplexing is the fact that despite their generally different masses and radii, the two stars possess essentially identical temperatures, indicating that they are in thermal equilibrium and requiring a significant flow of mass and energy between the two stars' common envelope. However, current contact binary models do not account for mass/energy mixing. The principal investigator's (PI) team will correct this situation by upgrading their modeling software PHOEBE to account for mass/energy transfer by the introduction of a mixing parameter. They will analyze thousands of contact binary light curves and explore mixing parameter correlations to infer the physical processes driving this mass and energy transfer, leading to a better understanding of these systems. This award will support a postdoc and three undergraduate researchers each year, as well as supporting regular general audience level articles to be distributed online through the IAU Outreach office.The team will explore three simple 1-parameter model extensions to PHOEBE: radial mixing, lateral mixing, and magnetic activity. In each case mixing will be assumed to affect only the cooler star, and that the amount of mixing is approximated well enough by a power law. In each case the model will be parametrized by the single mixing parameter "p", serving as a proxy to the hydrodynamical underpinnings of mass and energy flows in the envelope. They will evaluate the effect of “p” on the light curve, and since “p” is at least somewhat degenerate with the remaining principal parameters, other parameters will be considered to generate a good fit to the data and account for mixing. Carrying out this same analysis on thousands of contact binary light curves available in public archives, they will be able to study the correlations between “p” and the degree of thermal and geometrical contact, and the distributions of parameter “p” for each of the single-parameter models. Those correlations are what will inform the team of the underlying physics, and “p” will provide them with an essential guidance on how to properly formulate a more advanced mixing model. This scheme is already implemented in PHOEBE for computing stellar fluxes, making contact binary atmospheres a natural extension to PHOEBE’s existing capabilities.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.