[论文解读] The Formation of Protoplanetary Disks through Pre-Main Sequence Bondi-Hoyle Accretion

Paolo Padoan, Liubin Pan+3 more|arXiv (Cornell University)|May 12, 2024

Astro and Planetary Science被引用 5

一句话总结

论文提出，围绕前主序星的原行星盘主要来自环境气体的 Bondi-Hoyle 吸积，推导湍流流动中的角动量标度，并通过数值模拟验证，以解释观测到的盘尺寸和 j-M 关系。

ABSTRACT

Protoplanetary disks are traditionally described as finite mass reservoirs left over by the gravitational collapse of the protostellar core, a view that strongly constrains both disk evolution and planet formation models. We propose a different scenario where protoplanetary disks of pre-main sequence stars are primarily assembled by Bondi-Hoyle accretion from the parent gas cloud. We demonstrate that Bondi-Hoyle accretion can supply not only the mass, but also the angular momentum necessary to explain the observed size of protoplanetary disks. Additionally, we predict how the specific angular momentum of protoplanetary disks scales with stellar mass. Our conclusions are based on a new analytical derivation of the scaling of the angular momentum in turbulent flows, which we confirm with a numerical simulation of supersonic turbulence. A key outcome of our analysis is the recognition that density fluctuations in supersonic turbulence--previously overlooked in studies of cloud and core rotation--lead to a significant increase in angular momentum at disk-forming scales. This revised understanding of disk formation and evolution alleviates several longstanding observational discrepancies and compels substantial revisions to current models of disk and planet formation.

研究动机与目标

• 提出一种情景，即盘的质量和尺寸由来自母星云的质量坠落决定，而不是在原始恒星坍缩过程中的孤立形成。
• 推导在湍流介质中，星捕获的比角动量如何相对于半径标度，并将其与盘的尺寸联系起来。
• 通过解析标度和数值 MHD 湍流模拟，证明 BH 吸积能够解释观测到的原行星盘角动量及其对恒星质量的依赖。
• 评估观测意义，以及对盘与行星形成模型的潜在修订。

提出的方法

• 推导半径为 R 的湍流球内 rms 比角动量 j 的标度，将来自质心位移与局部速度涨落的贡献分离。
• 获得两种标度区间：在净自转主导的情形下 j ~ (beta/6)^{1/2} sigma_v,rel R；在密度涨落主导的情形下 j ~ (1/2) beta v'^2 R^2；模拟给出 beta = 0.61。
• 计算 Bondi-Hoyle 半径 R_BH，并推导特征盘半径 j_BH 和 R_d 的关系：j_BH = 4.3e20 cm^2 s^-1 (sigma_v,rel/2 km s^-1)^{-1} (M_star/ M_sun) 和 R_d = 3.6e2 AU (sigma_v,rel/2 km s^-1)^{-2} (M_star/ M_sun)。
• 使用随时间变化的速度色散来得到通过 M_d = 3.3e-2 M_sun (t/1 Myr)^{-4} (M_star/ M_sun)^2, j_BH = 9.6e20 cm^2 s^-1 (t/1 Myr)^{-1} (M_star/ M_sun), R_d = 1.8e3 AU (t/1 Myr)^{-2} (M_star/ M_sun) 的演化。
• 在高分辨率的 4 pc、3000 Msun 等温 MHD 湍流模拟中使用吸积粒子来表示恒星，并将 j(M_star) 与密度-速度相关性与解析预测进行比较，从而验证标度。

Figure 1: Specific angular momentum versus size for individual MCs [ 43 , 44 ] and cores [ 45 , 46 ] shown as red circles. A least-squares fit for a large compilation of surveys (partially overlapping with the individual clouds shown here) is given by the red solid line [ 44 ] . The black dashed lin

实验结果

研究问题

• RQ1 Bondi-Hoyle 吸积是否提供足够的质量和角动量来解释观测到的原行星盘大小？
• RQ2 在湍流、超声速云中，坠落气体的比角动量如何相对于半径和恒星质量标度？
• RQ3 湍流中的解析角动量标度是否能被数值 MHD 模拟重复并与 PMS 磁盘的观测 j 与 M_star 趋势相一致？
• RQ4 晚期、非对称 BH 吸积对盘的演化和行星形成模型有何影响？
• RQ5 预测的 j_BH 与 R_d 关系是否在不同的恒星形成区和年龄段的观测数据中保持一致？

主要发现

• Bondi-Hoyle 吸积能够同时提供与观测到的盘半径一致的盘质量和角动量。
• 在高度超音速的湍流中，rms 比角动量随半径线性标度：<j^2>^{1/2} ~ (beta/6)^{1/2} sigma_v,rel R，其中 beta = 0.61。
• 特征盘半径标度为 R_d ~ 3.6e2 AU (sigma_v,rel/2 km/s)^{-2} (M_star/M_sun) 且 R_BH/R_d ~ 4.1。
• 在 t = 1 Myr 之后的时间 t，M_d ~ 3.3e-2 M_sun (t/1 Myr)^{-4} (M_star/M_sun)^2，j_BH ~ 9.6e20 cm^2 s^{-1} (t/1 Myr)^{-1} (M_star/M_sun)，R_d ~ 1.8e3 AU (t/1 Myr)^{-2} (M_star/M_sun)。
• 模拟的 j(M_star) 趋势与幂律 j_BH ∝ M_star^{0.86±0.03} 相符，接近解析预测。

Figure 2: Specific gas angular momentum within the BH radius, $j_{\rm BH}$ , versus stellar mass, $M_{\rm star}$ , measured in 7 snapshots of the simulation, for all sink particles identified as accreting PMS stars (blue dots), with least-squares fit shown by the blue solid line. The two dashed line