[论文解读] Analysis of the most precise light curves of HAT-P-36 detrended from spot signals

Selçuk Yalçınkaya, Özgür Baştürk+7 more|arXiv (Cornell University)|Nov 22, 2021

Stellar, planetary, and galactic studies参考文献 1被引用 3

一句话总结

本研究利用地面T80和T100望远镜以及NASA的TESS卫星获得的最精确凌星光曲线，采用高斯过程（GP）去趋势化方法去除恒星黑子引起的噪声，分析了热木星系外行星HAT-P-36b的光曲线。关键结果为轨道周期每年增加0.014秒——表明存在轨道周期变化的正二次趋势，与预期的潮汐轨道衰减相矛盾，暗示恒星潮汐品质因子的下限为Q′⋆ > (6.97 ± 1.63) × 10⁴。

ABSTRACT

We study the most precise light curves of the planet-host HAT-P-36 that we obtained from the ground primarily with a brand-new 80 cm telescope very recently installed at Ankara University Kreiken Observatory of Turkey and also from the space with Transiting Exoplanet Survey Satellite. The main objective of the study is to analyze the Transit Timing Variations (TTV) observed in the hot-Jupiter type planet HAT-P-36 b, a strong candidate for orbital decay, based on our own observations as well as that have been acquired by professional and amateur observers since its discovery. HAT-P-36 displays out-of-transit variability as well as light curve anomalies during the transits of its planet due to stellar spots. We collected and detrended all the complete transit light curves we had access to from these anomalies, which we modeled with EXOFAST and measured the mid-transit times forming a homogeneous data set for a TTV analysis. We found an increase in the orbital period of HAT-P-36 b at a rate of 0.014 s per year from the best fitting quadratic function, which is only found in the TTV constructed by making use of the mid-transit times measured from detrended light curves, against an expectation of an orbital decay based on its parameters. We refined the values of these system parameters by modeling the Spectral Energy Distribution of the host star, its archival radial velocity observations from multiple instruments, and most precise transit light curves from the space and the ground covering a wide range of wavelengths with EXOFASTV2. We also analyzed the out-of-transit variability from TESS observations to search for potential rotational modulations through a frequency analysis. We report a statistically significant periodicity in the TESS light curve at 4.22 +/- 0.02 d, which might have been caused by instrumental systematics but should be tracked in the future observations of the target.

研究动机与目标

• 通过去趋势化HAT-P-36光曲线中的恒星黑子引起的变异性，提高中凌时刻的测量精度。
• 整合地面和空间观测数据，构建统一的、高精度的凌星时间变体（TTV）数据集。
• 通过使用去趋势化光曲线和径向速度数据建模，重新评估HAT-P-36b的轨道演化。
• 通过出相位变异性频率分析，探究恒星可能的自转调制。
• 利用SED建模、径向速度和高精度凌星光曲线，进一步精确确定系统绝对参数（质量、半径、年龄）。

提出的方法

• 采用高斯过程（GP）建模并去除凌星光曲线中由恒星黑子和仪器系统函数引起的关联噪声。
• 将地面望远镜（T80、T100）和空间望远镜（TESS，2分钟采样率）的凌星光曲线整合为统一数据集。
• 使用EXOFASTv2联合建模凌星光曲线、径向速度数据和恒星的光谱能量分布（SED），以获得完整的系统参数约束。
• 应用Lomb-Scargle周期图分析检测TTV和出相位通量变异性中的周期性。
• 对TTV数据拟合线性和二次模型，以评估轨道周期演化，以去趋势化光曲线为主要输入。
• 通过潮汐时序法和SED建模获得的恒星半径先验，引入恒星年龄约束，以提高参数精度。

实验结果

研究问题

• RQ1 对HAT-P-36光曲线中恒星黑子引起的变异性进行去趋势化处理，是否能显著提高中凌时刻测量的精度？
• RQ2 HAT-P-36b的轨道周期是否存在统计显著的趋势？该趋势是否与基于潮汐理论预期的轨道衰减相矛盾？
• RQ3 TESS光曲线中的出相位变异性是否能揭示恒星的自转周期？所检测到的4.22天信号是否具有物理意义？
• RQ4 当使用SED建模和多源数据时，精化的系统参数（质量、半径、年龄）与以往估计相比有何差异？
• RQ5 TTV图中二次趋势的统计显著性如何？这对恒星潮汐品质因子意味着什么？

主要发现

• 经过GP去趋势化后，中凌时刻的测量精度显著提高，平均误差从0.74分钟降低至0.52分钟。
• TTV图中发现正二次趋势，表明轨道周期每年增加0.014秒，与预期的潮汐衰减趋势不一致。
• 二次模型的减少卡方值χν = 5.63表明误差条可能被低估或采样存在局限，尽管如此趋势仍具统计显著性。
• 仅在去趋势化光曲线数据集中检测到TTV中15.85天的暂定周期性（6% FAP），需进一步监测。
• 恒星潮汐品质因子被约束为Q′⋆ > (6.97 ± 1.63) × 10⁴，表明其潮汐耗散弱于预期。
• 在出相位通量中检测到4.22 ± 0.02天的周期性，但其更可能源于仪器系统函数或数据处理效应，而非恒星自转，因此未采纳为自转周期。