根据光度凌日时间变化确定“外卫星”的大小、质量和密度
Determination of the size, mass, and density of "exomoons" from photometric transit timing variations
摘要：
对即将到来的太空任务进行精确的光度测量，可以确定系外行星卫星的大小，质量和密度。在这里，我们提出了这样一个分析使用测光凌日定时变化（$TTV_p$）。我们研究了凌日行星及其卫星的光变曲线效应。我们定义了相当于一个固定的感光中心的凌日的光度中心时间。这一点围绕质心运动，并导致测光凌日时间的变化。$TTV_p$的精确值取决于卫星和行星的密度、质量和大小的比值。由于这些参数中的两个是独立的，因此对密度比的可靠估计导致了对系外卫星的大小和质量的估计。在$TTV_p$的上限已知的情况下，参数的上限估计是可能的。在密度比不能可靠地估计的情况下，我们提出了一个近似与假设等密度。提出的感光中心$TTV_p$分析预测的卫星的大小比质量。我们模拟了地月系统在太阳前的凌日。月球的大小和质量估计为0.020地球质量和0.274地球质量，如果假设密度相等。该结果与实际值相差2倍。如果我们考虑真实密度比（约0.6），则结果为0.010地球质量和0.253地球大小，这与真实值在20%以内一致。
英文摘要：
Precise photometric measurements of the upcoming space missions allow the size, mass, and density of satellites of exoplanets to be determined. Here we present such an analysis using the photometric transit timing variation ($TTV_p$). We examined the light curve effects of both the transiting planet and its satellite. We define the photometric central time of the transit that is equivalent to the transit of a fixed photocenter. This point orbits the barycenter, and leads to the photometric transit timing variations. The exact value of $TTV_p$ depends on the ratio of the density, the mass, and the size of the satellite and the planet. Since two of those parameters are independent, a reliable estimation of the density ratio leads to an estimation of the size and the mass of the exomoon. Upper estimations of the parameters are possible in the case when an upper limit of $TTV_p$ is known. In case the density ratio cannot be estimated reliably, we propose an approximation with assuming equal densities. The presented photocenter $TTV_p$ analysis predicts the size of the satellite better than the mass. We simulated transits of the Earth-Moon system in front of the Sun. The estimated size and mass of the Moon are 0.020 Earth-mass and 0.274 Earth-size if equal densities are assumed. This result is comparable to the real values within a factor of 2. If we include the real density ratio (about 0.6), the results are 0.010 Earth-Mass and 0.253 Earth-size, which agree with the real values within 20%.
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