The Evolution of Gravitationally Unstable Protoplanetary Disks: Fragmentation and Possible Giant Planet Formation
The Evolution of Gravitationally Unstable Protoplanetary Disks: Fragmentation and Possible Giant Planet Formation
复制标题
引力不稳定的原行星盘的演化：碎片和可能的巨行星形成
DOI：
--
复制
发表时间：
2003
期刊：
影响因子：
--
通讯作者：
J. Stadel
中科院分区：
文献类型：
--
作者：
L. Mayer;T. Quinn;J. Wadsley;J. Stadel
我们进行了大量的高分辨率、三维、平滑粒子流体动力学模拟，描述了引力不稳定的气态原行星盘的演化。我们考虑了广泛的初始磁盘参数。 20 天文单位的盘质量范围为 0.075 到 0.125 M☉，大致与金牛座 T 星周围的盘推断的质量分布的高端一致。最低外部温度范围为 30 至 100 K，正如对早期原太阳星云的研究所预期的那样，以及原行星盘光谱建模所表明的那样。中心恒星的质量也各不相同，尽管通常假设它等于太阳的质量。总体而言，初始磁盘的最小 Q 参数在 0.8 到 2 之间，大多数模型的 Q 值约为 1.4。盘的演化假设是局部等温状态方程或具有变化 γ 的绝热状态方程。当使用绝热状态方程时，包括（人工）粘度和冲击引起的加热。当局部等温计算中由于重力不稳定而出现高于特定密度阈值的凝结时，状态方程将切换为绝热方程以考虑光学深度的增加。我们表明，当盘的最小 Q 参数小于 1.4 时，强烈的尾随螺旋不稳定性（通常是三臂或四臂模式）会形成并增长，直到大约 5 个平均盘轨道时间后沿臂发生碎片。由此产生的团块迅速收缩，其密度比初始圆盘密度高出几个数量级，并且其中最致密的团块即使在绝热条件下也能存活。这些团块对于潮汐破坏很稳定，并且会快速合并，在大约 103 年之后，在相当偏心的轨道上留下两到三个原行星（平均偏心率约为 0.2）。碎片并不强烈依赖于磁盘是从稍微不稳定的状态开始还是逐渐达到不稳定状态；我们表明，如果允许圆盘从非常轻、非常稳定的状态经过数十个轨道时间增加质量，它仍然会在与标准圆盘模型大致相同的质量和温度下破裂。我们表明，不稳定的第一阶段，直到出现过密度，可以用最大不稳定托姆雷波长和局部牛仔裤长度来理解。需要高质量和力分辨率来正确解析这两个尺度并适当地遵循破碎过程。不同的盘质量和温度会影响这种物理尺度，从而影响所形成的原行星的典型质量。比土星小的物体或比木星大几倍的物体都可以通过碎片产生。它们的最终质量将取决于随后与其他团块的相互作用和合并以及圆盘材料的吸积。吸积率取决于盘的热力学，并且在绝热条件下可以忽略不计。大约 103 年后，质量范围从略低于 1MJup 到超过 7MJup，与检测到的太阳系外行星的质量非常一致。
We carry out a large set of very high resolution, three-dimensional, smoothed particle hydrodynamics simulations describing the evolution of gravitationally unstable gaseous protoplanetary disks. We consider a broad range of initial disk parameters. Disk masses out to 20 AU range from 0.075 to 0.125 M☉, roughly consistent with the high end of the mass distribution inferred for disks around T Tauri stars. Minimum outer temperatures range from 30 to 100 K, as expected from studies of the early protosolar nebula and suggested by the modeling of the spectra of protoplanetary disks. The mass of the central star is also varied, although it is usually assumed to be equal to that of the Sun. Overall, the initial disks span minimum Q-parameters between 0.8 and 2, with most models having Q ~ 1.4. The disks are evolved assuming either a locally isothermal equation of state or an adiabatic equation of state with varying γ. Heating by (artificial) viscosity and shocks is included when the adiabatic equation of state is used. When condensations above a specific density threshold appear as a result of gravitational instability in a locally isothermal calculation, the equation of state is switched to adiabatic to account for the increased optical depth. We show that when a disk has a minimum Q-parameter less than 1.4, strong trailing spiral instabilities, typically three- or four-armed modes, form and grow until fragmentation occurs along the arms after about 5 mean disk orbital times. The resulting clumps contract quickly to densities several orders of magnitude higher than the initial disk density, and the densest of them survive even under adiabatic conditions. These clumps are stable to tidal disruption and merge quickly, leaving two to three protoplanets on fairly eccentric orbits (the mean eccentricity being around 0.2) after ~103 yr. Fragmentation is not strongly dependent on whether the disk starts from a marginally unstable state or gradually achieves it; we show that if the disk is allowed to grow in mass from a very light, very stable state over tens of orbital times, it still fragments at roughly the same mass and temperature as in the standard disk models. We show that the first stages of the instability, until the appearance of the overdensities, can be understood in terms of the maximum unstable Toomre wavelength and the local Jeans length. A high mass and force resolution are needed to correctly resolve both scales and follow the fragmentation process appropriately. Varying disk mass and temperature affects such physical scales and hence the typical masses of the protoplanets that form. Objects smaller than Saturn or a couple of times bigger than Jupiter can both be produced by fragmentation. Their final masses will then depend on the subsequent interactions and mergers with other clumps and on the accretion of disk material. The accretion rate depends on the disk thermodynamics and is negligible with adiabatic conditions. After ~103 yr the masses range from just below 1MJup to more than 7MJup, well in agreement with those of detected extrasolar planets.