Keck时间分辨分光光度法对小卫星2020 CD $_3$的表征
Characterization of Temporarily-Captured Minimoon 2020 CD$_3$ by Keck Time-resolved Spectrophotometry
摘要：
我们提出了minimoon 2020 CD $_3$的时间分辨可见分光光度法，这是已知的第二颗被地月系统引力场暂时捕获的小行星。在2020年3月23日UTC离开地月系统时，使用Keck I/LRIS在$B$、$g$、$V$、$R$、$I$和RG850滤光片中在波长434 nm和912 nm之间进行分光光度测定。2020 CD $_3$的分光光度法与V型小行星和一些月球岩石样品的光谱非常相似，在434 nm和761 nm之间的红色斜率约为18 $\%$/100 nm，对应于$g$-$r$ = 0.62 $\pm$ 0.08的颜色，$r$-$i$ = 0.21 $\pm$ 0.06，在约900 nm处的吸收带对应于$i$-$z$ =-0.54 $\pm$ 0.10。结合我们测得的31. 9 $\pm$ 0. 1绝对星等和典型V型小行星的0. 35绝对星等，我们确定2020 CD $_3$的直径约为0. 9 $\pm$ 0. 1米，使其成为第一个迷你卫星和光谱研究的最小行星之一。我们使用我们的时间序列测光检测周期性的光变曲线变化与$<$ 10 $^{-4}$虚警概率对应的光变曲线周期约为573 s和光变曲线振幅约为1 mag，这意味着2020 CD $_3$具有$b/a$轴比约为2.5。此外，我们将2020 CD $_3$的观测弧延长到2020年2月15日UTC至2020年3月23日UTC之间的37天。根据2020 CD $_3$的改进轨道解，我们估计它被捕获的可能持续时间约为2年，并且我们测量了由于辐射压力引起的轨道上的非引力扰动，面积质量比为6.9 $\pm$ 2.4 $\times$ 10 $^{-4}$ m $^2$/kg，意味着密度为2.3 $\pm$ 0.8 g/cm $^3$，与其他米级小行星和月球岩石的密度大致一致。我们在ZTF档案中检索了早在2018年10月的2020 CD $_3$的发现前检测，但无法找到任何阳性检测。
英文摘要：
We present time-resolved visible spectrophotometry of minimoon 2020 CD$_3$, the second asteroid known to become temporarily captured by the Earth-Moon system's gravitational field. The spectrophotometry was taken with Keck I/LRIS between wavelengths 434 nm and 912 nm in $B$, $g$, $V$, $R$, $I$ and RG850 filters as it was leaving the Earth-Moon system on 2020 March 23 UTC. The spectrophotometry of 2020 CD$_3$ most closely resembles the spectra of V-type asteroids and some Lunar rock samples with a reddish slope of ~18$\%$/100 nm between 434 nm and 761 nm corresponding to colors of $g$-$r$ = 0.62$\pm$0.08, $r$-$i$ = 0.21 $\pm$ 0.06 and an absorption band at ~900 nm corresponding to $i$-$z$ = -0.54$\pm$0.10. Combining our measured 31.9$\pm$0.1 absolute magnitude with an albedo of 0.35 typical for V-type asteroids, we determine 2020 CD$_3$'s diameter to be ~0.9$\pm$0.1 m making it the first minimoon and one of the smallest asteroids to be spectrally studied. We use our time-series photometry to detect periodic lightcurve variations with a $<$10$^{-4}$ false alarm probability corresponding to a lightcurve period of ~573 s and a lightcurve amplitude of ~1 mag implying 2020 CD$_3$ possesses a $b/a$ axial ratio of ~2.5. In addition, we extend the observational arc of 2020 CD$_3$ to 37 days between 2020 February 15 UTC and 2020 March 23 UTC. From the improved orbital solution for 2020 CD$_3$, we estimate its likely duration of its capture to be ~2 y, and we measure the non-gravitation perturbation on its orbit due to radiation pressure with an area-to-mass ratio of 6.9$\pm$2.4$\times$10$^{-4}$ m$^2$/kg implying a density of 2.3$\pm$0.8 g/cm$^3$, broadly compatible with the densities of other meter-scale asteroids and Lunar rock. We searched for pre-discovery detections of 2020 CD$_3$ in the ZTF archive as far back as 2018 October, but were unable to locate any positive detections.
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