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arXiv 1901.02015astro-ph.EPastro-ph.IM
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Transit最小二乘:搜索小行星周期凌日的优化transit检测算法

Transit Least Squares: Optimized transit detection algorithm to search for periodic transits of small planets

Michael Hippke, René Heller

摘要:

提出了一种新的从时间序列测光中探测行星凌日的方法--凌日最小二乘(TLS)算法。TLS搜索类似凌日的特征,同时考虑恒星边缘变暗和行星进出。我们已经优化了TLS的小行星的信号检测效率和计算速度。TLS分析整个未分组的相位折叠光变曲线。我们通过(i.)使用算法,如“合并排序”(用于试验轨道相位)和(ii.)将试验过境时间限制在一个较小的范围内,包括所有已知的行星,并在可用的情况下使用恒星密度先验。一个典型的K2光变曲线,包括80天的观测,在30分钟的节奏,可以搜索与TLS在约10秒的实时在一个标准的笔记本电脑上,作为广泛使用的框最小二乘(BLS)算法一样快。我们使用合成光变曲线进行了类太阳恒星周围地球大小的行星的过境注入检索实验,每30分钟的节奏与110 ppm的白噪声,对应于开普勒观测到的光度安静的KP=12恒星。我们确定了BLS和TLS在这两种情况下达到1%假阳性率的阈值为0.07。TLS的真阳性(或恢复)率约为93%,BLS约为76%,这意味着TLS的检测更可靠。我们还测试TLS与TRAPPIST-1系统的K2光变曲线,并发现六个地球大小的行星使用迭代搜索越来越低的信号检测效率,相位折叠的第七颗行星的过境受到恒星耀斑的影响。TLS在寻找任何类型的凌日行星方面都比BLS更可靠,但它特别适合于从开普勒,TESS和PLATO长时间序列中检测小行星。我们公开了TLS的Python实现。

英文摘要:

We present a new method to detect planetary transits from time-series photometry, the Transit Least Squares (TLS) algorithm. TLS searches for transit-like features while taking the stellar limb darkening and planetary ingress and egress into account. We have optimized TLS for both signal detection efficiency (SDE) of small planets and computational speed. TLS analyses the entire, unbinned phase-folded light curve. We compensate for the higher computational load by (i.) using algorithms like "Mergesort" (for the trial orbital phases) and by (ii.) restricting the trial transit durations to a smaller range that encompasses all known planets, and using stellar density priors where available. A typical K2 light curve, including 80d of observations at a cadence of 30min, can be searched with TLS in ~10s real time on a standard laptop computer, as fast as the widely used Box Least Squares (BLS) algorithm. We perform a transit injection-retrieval experiment of Earth-sized planets around sun-like stars using synthetic light curves with 110ppm white noise per 30min cadence, corresponding to a photometrically quiet KP=12 star observed with Kepler. We determine the SDE thresholds for both BLS and TLS to reach a false positive rate of 1% to be SDE~7 in both cases. The resulting true positive (or recovery) rates are ~93% for TLS and ~76% for BLS, implying more reliable detections with TLS. We also test TLS with the K2 light curve of the TRAPPIST-1 system and find six of seven Earth-sized planets using an iterative search for increasingly lower signal detection efficiency, the phase-folded transit of the seventh planet being affected by a stellar flare. TLS is more reliable than BLS in finding any kind of transiting planet but it is particularly suited for the detection of small planets in long time series from Kepler, TESS, and PLATO. We make our Python implementation of TLS publicly available.

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