标题： 一种关于边缘不对称的解析表征——掩星时间退化 显示英文标题

标题： An Analytic Characterization of the Limb Asymmetry -- Transit Time Degeneracy

摘要： 大气并非空间均匀的。 这对于具有较大昼夜温差的热潮汐锁定系外行星尤其如此，这可能导致晨昏线之间的显著差异——称为边缘不对称性。 目前詹姆斯·韦布空间望远镜（JWST）的凌星观测精度足以在某些情况下区分这些晨昏线对整体透射光谱的单独贡献。 然而，凌星光曲线中边缘不对称性的特征与行星合时的不确定性高度退化。 这引发了如何精确测量凌星时间以实现对边缘不对称性的准确研究的问题，特别是对于JWST而言。 尽管这种退化已经在文献中讨论过，但尚未提出一般的描述。 在本工作中，我们展示了这种退化如何源于当行星盘具有不对称边缘尺寸时，凌星接触时间的明显变化。 我们推导了一个一般公式，将边缘不对称性的大小与它会导致 apparent 合时变化的程度联系起来，这种变化可达几十秒。 将我们的公式与模拟观测进行比较，我们发现数值拟合技术会增加额外的偏差，通常小于一秒，这是由于掩食几何引起的。 我们还推导了这种额外数值偏差的解析公式。 这些公式可用于规划新的观测或解释文献中的测量结果，并我们展示了常见研究的系外行星的例子。 显示英文摘要

摘要： Atmospheres are not spatially homogeneous. This is particularly true for hot, tidally locked exoplanets with large day-to-night temperature variations, which can yield significant differences between the morning and evening terminators -- known as limb asymmetry. Current transit observations with the James Webb Space Telescope (JWST) are precise enough to disentangle the separate contributions of these morning and evening limbs to the overall transmission spectrum in certain circumstances. However, the signature of limb asymmetry in a transit light curve is highly degenerate with uncertainty in the planet's time of conjunction. This raises the question of how precisely transit times must be measured to enable accurate studies of limb asymmetry, in particular with JWST. Although this degeneracy has been discussed in the literature, a general description of it has not been presented. In this work, we show how this degeneracy results from apparent changes in the transit contact times when the planetary disk has asymmetric limb sizes. We derive a general formula relating the magnitude of limb asymmetry to the amount by which it would cause the apparent time of conjunction to vary, which can reach tens of seconds. Comparing our formula to simulated observations, we find that numerical fitting techniques add additional bias to the measured time, of generally less than a second, resulting from the occultation geometry. We also derive an analytical formula for this extra numerical bias. These formulae can be applied to planning new observations or interpreting literature measurements, and we show examples for commonly studied exoplanets.