Can Giant Planets Form by Direct Gravitational Instability?
Can Giant Planets Form by Direct Gravitational Instability?
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DOI：
10.1086/428899
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发表时间：
2004-06
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影响因子：
--
通讯作者：
R. Rafikov
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文献类型：
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作者：
R. Rafikov
引力不稳定性被认为是在原行星盘中产生巨行星的一种可能机制。在这里，我们批判性地修正了它的可行性，指出要直接形成行星，原行星盘的引力不稳定是不够的。它们还必须能够有效地冷却（在与当地盘轨道周期相当的时间尺度上），以允许通过碎裂形成结合的团块。动力学和热约束的结合对能够分裂成自引力物体的盘的性质提出了非常严格的下限：为了使引力不稳定性在辐射转移冷却的盘中形成10 Au的巨行星，气体温度必须超过103 K，最小盘质量为0.7 M，光度为40 L。虽然这些要求在盘的较远部分被放宽，但由于不稳定性而形成的束缚物体的质量太大，即使在100 Au（~ 10 MJ）也无法解释已知的太阳系外巨行星的特征。这样的原行星盘（以及在其中形成的行星）具有非常不寻常的观测特性，这严重限制了通过直接引力不稳定性形成巨行星的可能性。
Gravitational instability has been invoked as a possible mechanism of the giant planet production in protoplanetary disks. Here we critically revise its viability by noting that to form planets directly, it is not enough for protoplanetary disks to be gravitationally unstable. They must also be able to cool efficiently (on a timescale comparable to the local disk orbital period) to allow the formation of the bound clumps by fragmentation. A combination of the dynamical and thermal constraints puts very stringent lower limits on the properties of disks capable of fragmenting into the self-gravitating objects: for the gravitational instability to form giant planets at 10 AU in the disk cooled by the radiation transfer, the gas temperature must exceed 103 K with a minimum disk mass of 0.7 M☉ and a luminosity of 40 L☉. Although these requirements are relaxed in the more distant parts of the disk, masses of the bound objects formed as a result of instability are too large even at 100 AU (~10MJ) to explain the characteristics of known extrasolar giant planets. Such protoplanetary disks (and planets formed in them) have very unusual observational properties, and this severely constrains the possibility of giant planet formation by direct gravitational instability.