| 摘要: | 近地小行星的特徵在於它的繞行太陽公轉的軌道會和地球的公轉軌道會有交錯且接近地球，因此具有撞擊地球的危險性。根據小行星的軌道性質，可以細分成4個子分類:阿提拉Atira (1.017 < 近日距q < 1.3 AU), 阿登Aten (半長軸 a < 1.0 AU, 遠日距 Q > 0.983 AU), 阿波羅 Apollo (半長軸 a > 1.0 AU, 近日距 q < 1.017 AU), 阿莫Amor (0.718 < 遠日距Q < 0.983 AU)。在2020年1月4月時，國立中央大學與國立清華大學參與Zwicky Transient Facility (ZTF)的曙光計畫(Twilight project)裡，觀測到位於金星軌道以內的小行星，2020 AV2，並且成為第一顆被觀測的凡提拉 Vatira (0.307 AU < 遠日距 Q < 0.718 AU)。這項發現使我們好奇這顆凡提拉Vatira的來源和演化，並有研究推測凡提拉 Vatira很有可能是從近地小行星中的阿提拉Atira遷移到金星軌道以內(Greenstreet et al, 2020; De la Fuente Marcos et al, 2020)。
在本研究中我們使用Mercury6數值模擬, 除了考慮現有行星的重力擾動外，也考慮了非重力效應的亞可夫斯基效應作用在小行星上。在長期的軌道演化中，根據小行星的物理和軌道特徵(例如小行星的大小、自轉傾角、熱慣量和半長軸等)，亞可夫斯基效應造成的微小熱推力會影響小行星漸漸地向內或向外遷移。
從研究結果中，我們估計近地小行星短期和長期的半衰期分別為大約270萬年和2400萬年。而現有觀測到阿提拉Atira根據不同的自轉傾角(即0度、90度和180度)，短期的半衰期分別大約為536萬年、824萬年和330萬年；而長期的半衰期為2510萬年、2870萬年和2410萬年。再者，我們發現凡提拉Vatira和阿提拉Atira兩者的軌道演化主要由行星的重力擾動所導致，非重力效應的亞可夫斯基效應造成的改變極其微小。接著從小行星的遷移機率機算中，根據不同的自轉傾角，阿提拉Atira遷移到凡提拉Vatira的機率分別為~3.87±0.263,~4.79±0.365和 ~6.14±0.149 %,。這代表著亞可夫斯基效應在小行星長期遷移演化中仍佔了某部分的影響。最後，我們在統計中考慮了2020AV2的光譜類型(S-型小行星)及絕對星等大小(15.625 < H < 17.175)，並推估S-型的凡提拉Vatira現在大約有0.9 ± 0.8個，而阿提拉Atira大約有7.8 ± 4.47個，這個數量級和我們現今在JPL資料庫找到的數量很接近。;Asteroids having perihelion distance q < 1.3 AU and aphelion distance Q > 0.983 AU are classified as near-Earth objects (NEOs). And they are divided into different groups: Atira-class (1.017 < q < 1.3 AU), Aten-class (a < 1.0 AU, Q > 0.983 AU), Apollo-class (a > 1.0 AU, q < 1.017 AU), and Amor-class (0.718 < Q < 0.983 AU). There are 23 known Atiras with perihelion distance 1.017 < q < 1.3 AU. The first Vatira (its orbits totally inside Venus′ orbit), 2020 AV2, was discovered by the Twilight project of the Zwicky Transient Facility (ZTF) on January 4, 2020. A couple of orbital studies of the short-term orbital evolution of 2020 AV2 were performed soon after its discovery and indicated that 2020 AV2 was an Atira-class asteroid before entering the orbital region of the Vatira-class asteroid (de la Fuente Marcos et al, 2020; Greenstreet, 2020).
In this study, we performed three numerical simulations by using the Mercury6 N-body code with the hybrid symplectic integrator. We considered not only planetary gravitational perturbation but also the non-gravitational Yarkovsky effect. In addition, the tiny thermal force acting on asteroids characteristically by the Yarkovsky effect would cause gradual drift inward/outward in long-term evolution depending on the physical and orbital characteristics (e.g. an asteroid′s size, obliquity, thermal inertia, semi-major axis).
Our calculation shows that the NEOs have generally two dynamical populations, one short-lived and the other long-lived. The dynamical short-term half-lifetime is ~ 2.71 Myr, while the long-term half-lifetime is ~ 23.84 Myr. As for the Atria-class asteroids, the short-term half-lifetime for different values of the Yarkovsky force (i.e. obliquity of 0, 90, and 180 deg.) are ~ 5.36, 8.24, and 3.30 Myr, respectively, and the long-term part ~ 25.1, 28.7, and 24.1 Myr, respectively. The dynamical evolution of Atira-class and Vatira-class asteroids under the Yarkovsky force are similar because orbit evolution are dominated by planetary gravitational perturbation instead of non-gravitational thermal force. From the calculation of the transfer probabilities of Atira-class asteroids to Vatira-class asteroids are ~ 3.87±0.263, ~ 4.79±0.365, and ~ 6.14±0.149%, respectively. It suggests that the radiation force plays some role in the long-term evolution of this asteroid population. Finally, our statistical study implicates that there should be 7.8 ± 4.47 Atira-class asteroids and 0.9 ± 0.8 Vatira-asteroids of the S-type taxonomy and in the absolute magnitude range of 15.625 < H < 17.175. The values are close to the known number of the Vatira-class and Atira-class asteroids from the JPL database. |
