THE ACTIVE CENTAURS
THE ACTIVE CENTAURS
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DOI：
10.1088/0004-6256/137/5/4296
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发表时间：
2006-09
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影响因子：
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通讯作者：
D. Jewitt
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作者：
D. Jewitt
半人马座是最近从柯伊伯带逃离的星体，它们要么注定要在太阳系炽热的内部区域被炽热的遗忘，要么被巨大行星的引力散射抛到星际介质中。动态演化的半人马座彗星，当被木星捕获并足够靠近太阳使近表面的水冰升华时，通常被标记为“短周期”彗星（特别是木星家族）。值得注意的是，一些半人马座行星即使在远离木星轨道的地方也表现出类似彗星的活动，这表明质量损失是由水冰升华以外的过程引起的。我们观察了23个半人马的样本，发现其中9个是活跃的，其质量损失率从几公斤到几吨不等。作为一个整体，我们发现我们样本中“活跃的半人马座”的近日点小于不活跃的半人马座（中位数5.9 AU对8.7 AU），小于所有已知半人马座的近日点距离中位数（12.4 AU）。这表明它们的活动是由热驱动的。我们考虑了来自活跃的半人马的质量损失的几种可能性。大多数都太冷，无法在观测到的水平上通过结晶水冰的升华产生活动。固体一氧化碳和二氧化碳则有相反的问题：它们极易挥发，会在比观测到的更远的距离上驱动半人马座的活动。我们认为半人马座的活动可能是由无定形冰转化为晶体形式，同时释放出包括一氧化碳在内的被困气体引起的。通过施加结晶时间短于轨道周期时应发生结晶的条件，我们发现了与活动半人马座近日点分布的定性匹配，并得出结论，该数据与半人马座含有无定形冰的假设一致。
The Centaurs are recent escapees from the Kuiper Belt that are destined either to meet fiery oblivion in the hot inner regions of the solar system or to be ejected to the interstellar medium by gravitational scattering from the giant planets. Dynamically evolved Centaurs, when captured by Jupiter and close enough to the Sun for near-surface water ice to sublimate, are conventionally labeled as “short-period” (specifically, Jupiter-family) comets. Remarkably, some Centaurs show comet-like activity even when far beyond the orbit of Jupiter, suggesting mass loss driven by a process other than the sublimation of water ice. We observed a sample of 23 Centaurs and found nine to be active, with mass-loss rates measured from several kg s−1 to several tonnes s−1. Considered as a group, we find that the “active Centaurs” in our sample have perihelia smaller than the inactive Centaurs (median 5.9 AU versus 8.7 AU), and smaller than the median perihelion distance computed for all known Centaurs (12.4 AU). This suggests that their activity is thermally driven. We consider several possibilities for the origin of the mass loss from the active Centaurs. Most are too cold for activity at the observed levels to originate via the sublimation of crystalline water ice. Solid carbon monoxide and carbon dioxide have the opposite problem: they are so volatile that they should drive activity in Centaurs at much larger distances than observed. We consider the possibility that activity in the Centaurs is triggered by the conversion of amorphous ice into the crystalline form accompanied by the release of trapped gases, including carbon monoxide. By imposing the condition that crystallization should occur when the crystallization time is shorter than the orbital period we find a qualitative match to the perihelion distribution of the active Centaurs and conclude that the data are consistent with the hypothesis that the Centaurs contain amorphous ice.