Analytic Theory of the YORP Effect for Near-Spherical Objects
Analytic Theory of the YORP Effect for Near-Spherical Objects
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近球形物体YORP效应的解析理论
DOI：
10.1086/521651
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发表时间：
2007
期刊：
影响因子：
--
通讯作者：
D. Vokrouhlický
中科院分区：
文献类型：
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作者：
D. Nesvorný;D. Vokrouhlický
当行星际空间中的小物体的表面被太阳光加热并以热波长重新辐射吸收的能量时，就会产生YORP效应。被吸收、反射和发射的光子在小天体上产生微小的扭矩，可以在行星的时间尺度上改变它的自转速率和自转速度。以前的YORP效应理论依赖于辐射力矩的数值或非数值评估。在这里，我们开发了一种替代方法，并计算YORP扭矩分析。我们的理论仅限于近球形物体。虽然不适合精确确定的扭矩细长和/或高度不规则的对象，分析理论有助于解释几个一般性质的YORP扭矩，确定在以前的数值工作。例如，我们证明了影响自旋速率s的YORP扭矩的分量可以在双折射率值为55°（和125°）时消失。正如Vokrouhlickerg及其同事所讨论的那样，s的这种性质对于建立所谓的Slivan态很重要，Slivan态是太阳系小行星等小天体自旋矢量的演化终态。我们表明，s（平均自旋和轨道周期）是一个二阶量的小参数，描述了偏离理想的球的形状。我们将s显式地计算为cos φ的多项式。这些表达式表明，YORP扭矩来自于物体形状在地形经度和纬度上的耦合变形。此外，通过引入一个小的相位滞后来模拟光子的吸收和再发射之间的延迟，我们证明了s对表面热导率的精确值不敏感。这里描述的这些和其他分析结果提供了一个基线，用于理解比这里考虑的更复杂的表面形状和性质的物体上的YORP效应。我们讨论了分析理论在近球形小行星上的应用，如1998 KY 26和更细长和/或不规则的物体，如（1862）Apollo和（25143）Itokawa。
The YORP effect is produced when the surface of a small object in interplanetary space is heated by sunlight and reradiates the absorbed energy in thermal wavelengths. The absorbed, reflected, and emitted photons produce tiny torques on the small body that can change its spin rate and obliquity over planetary timescales. Previous theories of the YORP effect relied on numerical or seminumerical evaluation of the radiation torques. Here we develop an alternative approach and calculate the YORP torques analytically. Our theory is limited to near-spherical objects. While unsuitable for a precise determination of torques on elongated and/or highly irregular objects, the analytic theory helps to explain several general properties of the YORP torques that were identified in previous numerical works. For example, we demonstrate that the component of the YORP torque that affects the spin rate, s, can vanish for obliquity values ϵ ≈ 55° (and ϵ ≈ 125°). As discussed by Vokrouhlický and coworkers, this property of s is important for establishing the so-called Slivan states, which arise as evolutionary end states of spin vectors of small solar system bodies such as asteroids. We show that s (averaged over spin and orbit periods) is a second-order quantity in the small parameter that describes the deviation of the shape from an ideal sphere. We calculate s explicitly as polynomials of cos ϵ. These expressions show that the YORP torque arises from coupled deformations of the body's shape in topographic longitude and latitude. Moreover, by introducing a small phase lag to mimic the delay between the absorption and reemission of photons we demonstrate that s is insensitive to the exact value of the surface thermal conductivity. These and other analytic results described here provide a baseline for understanding the YORP effect on bodies with more complicated surface shapes and properties other than the ones considered here. We discuss applications of the analytic theory on near-spherical asteroids like 1998 KY26 and on more elongated and/or irregular objects like (1862) Apollo and (25143) Itokawa.