北落师门、HR 8799和HL金牛座是否通过引力不稳定性形成行星？对所需磁盘质量设置限制
Did Fomalhaut, HR 8799, and HL Tauri Form Planets via the Gravitational Instability? Placing Limits on the Required Disk Masses
摘要：
由引力不稳定性引起的盘破裂被认为是形成巨行星的有效机制。我们使用行星北落师门b，三行星系统HR 8799，以及与HL Tau相关的潜在原行星来测试这种机制的可行性。我们选择上述系统是因为它们拥有质量和轨道特征受碎裂机制青睐的行星。我们并不主张这些行星一定是由于碎裂而形成的，相反：如果行星可以从盘碎裂中形成，那么这些系统与我们应该期望看到的是一致的。我们使用这些最近发现的行星的轨道特征，以及一种新的技术，以更准确地确定磁盘冷却时间，将下限和上限的磁盘表面密度-从而质量-所需的磁盘碎片形成这些对象。我们的冷却时间比Rafikov（2005）的冷却时间短一个数量级，这使得磁盘碎片对这些物体更可行。我们发现，北落师门b，原行星轨道HL Tau和HR 8799的最外层行星，在行星轨道半径内部所需的质量约为0.1 Msun。HR 8799的两颗内行星很可能不是在原位置由盘破裂形成的。
英文摘要：
Disk fragmentation resulting from the gravitational instability has been proposed as an efficient mechanism for forming giant planets. We use the planet Fomalhaut b, the triple-planetary system HR 8799, and the potential protoplanet associated with HL Tau to test the viability of this mechanism. We choose the above systems since they harbor planets with masses and orbital characteristics favored by the fragmentation mechanism. We do not claim that these planets must have formed as the result of fragmentation, rather the reverse: if planets can form from disk fragmentation, then these systems are consistent with what we should expect to see. We use the orbital characteristics of these recently discovered planets, along with a new technique to more accurately determine the disk cooling times, to place both lower and upper limits on the disk surface density--and thus mass--required to form these objects by disk fragmentation. Our cooling times are over an order of magnitude shorter than those of Rafikov (2005),which makes disk fragmentation more feasible for these objects. We find that the required mass interior to the planet's orbital radius is ~0.1 Msun for Fomalhaut b, the protoplanet orbiting HL Tau, and the outermost planet of HR 8799. The two inner planets of HR 8799 probably could not have formed in situ by disk fragmentation.
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