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arXiv 1302.1253astro-ph.EP
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外行星系统的组成和起源:来自Roche临界密度的见解

Compositions and origins of outer planet systems: Insights from the Roche critical density

Matthew S. Tiscareno, Matthew M. Hedman, Joseph A. Burns, Julie Castillo-Rogez

摘要:

我们考虑洛希临界密度(rho_Roche),即轨道物体在距其行星给定距离处能够通过自引力保持自身的最小密度。它与我们更熟悉的“洛希极限”直接相关,洛希极限是指一个给定密度的无强度轨道物体被潮汐拉开时与行星的距离。一个实质性的环的存在要求瞬态团块具有小于rho_Roche的内部密度。相反,在存在大量吸积物质的情况下,密度大于rho_Roche的轨道物体将增长。比较土星和天王星系统从破裂主导(环)迅速过渡到吸积主导(卫星)的rho_Roche值,我们推断组成天王星环的材料可能比组成土星环的材料更多岩石,以及更少的多孔性。从木星和海王星最内侧的环卫星的高rho_Roche值,我们推断这些卫星可能由比预期更致密的物质组成,或者更有可能是它们是在远离行星的地方形成的闯入者,并且已经向内迁移,现在被内部物质强度保持在一起。最后,由八颗紧密排列的天王星卫星组成的“波西亚群”的整体表面密度与土星的A环相似。因此,它可以被看作是一个吸积主导的环系统,除了其物质的特征密度大于当地的rho_Roche之外,它与标准环系统具有相似的特征。

英文摘要:

We consider the Roche critical density (rho_Roche), the minimum density of an orbiting object that, at a given distance from its planet, is able to hold itself together by self-gravity. It is directly related to the more familiar "Roche limit," the distance from a planet at which a strengthless orbiting object of given density is pulled apart by tides. The presence of a substantial ring requires that transient clumps have an internal density less than rho_Roche. Conversely, in the presence of abundant material for accretion, an orbiting object with density greater than rho_Roche will grow. Comparing the rho_Roche values at which the Saturn and Uranus systems transition rapidly from disruption-dominated (rings) to accretion-dominated (moons), we infer that the material composing Uranus' rings is likely more rocky, as well as less porous, than that composing Saturn's rings. From the high values of rho_Roche at the innermost ring-moons of Jupiter and Neptune, we infer that those moons may be composed of denser material than expected, or more likely that they are interlopers that formed farther from their planets and have since migrated inward, now being held together by internal material strength. Finally, the "Portia group" of eight closely-packed Uranian moons has an overall surface density similar to that of Saturn's A ring. Thus, it can be seen as an accretion-dominated ring system, of similar character to the standard ring systems except that its material has a characteristic density greater than the local rho_Roche.

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