Growth and Evolution of Planets in protoplanetary Disks

到目前为止，已经发现了2600多个系外行星系统，其中包含3500多颗行星。偏心观测表明，最丰富的行星是轨道周期短于100天的超级地球（质量为地球1-20倍的行星），其次是距离母恒星1-3个天文单位（AU）的巨行星。后者的数量至少是热木星（距离恒星约0.1天文单位）数量的十倍。巨行星的质量分布峰值约为1-3个木星质量；质量比这更大的行星是存在的，但非常罕见。从理论的角度来看，这些观察结果很难理解。行星向恒星的迁移可以很容易地解释近距离超级地球的存在，但要理解为什么只有少数巨行星的轨道在半长轴上小于1天文单位是一个问题。此外，气体吸积到行星核心的速度应该非常快。因此，是什么阻止了超级地球成为巨行星，是什么限制了巨行星的增长，只有几个木星的质量，这一点还不清楚。这一建议是基于这样一种观点，即理解太阳系外行星的质量和轨道分布的困难是由于对原行星盘结构的错误假设。传统的粘性圆盘的观点认为，粘性是由磁振子旋转不稳定性驱动的强湍流产生的，这一观点受到了现代磁流体动力学模拟的挑战。圆盘的粘性可能比以前认为的要小得多。然而，圆盘不可能是无粘性的，例如所谓的垂直剪切不稳定性（VSI）设定了最小粘度。此外，盘风从原行星盘的薄表层带走了角动量，促进了这些层中气体向中心恒星的快速径向传输。我们提出的项目是：(1)建立一个考虑VSI和盘风的原行星盘的现实模型，并再现观测到的恒星吸积率；(2)研究气体的吸积和嵌入这些盘中的行星的迁移。

As of today over 2600 exoplanetary systems that contain over 3500 planets have been discovered. The debiased observations show that the most abundant planets are Super-Earths (planets with 1-20 Earth masses) with orbital periods shorter than 100 days, followed by giant planets at distances of 1-3 astronomical units (AU) from the parent star. The latter outnumber, by at least a factor of ten, the population of hot-Jupiters (at a distance of about 0.1 AU from the star). The mass distribution of giant planets peaks at about 1-3 Jupiter masses; planets with masses larger than that exist but are quite rare. From a theoretical standpoint, these observations are difficult to understand. Planet migration towards the star can easily explain the existence of close-in super-Earths, but it is a problem to understand why only a minority of giant planets reached orbits less than 1 AU in semi major axis. Also, gas accretion onto planetary cores should be very fast. Thus, it is not understood what prevented super-Earths from becoming giant planets and what limited the growth of giant planets to a few Jupiter masses.This proposal is based on the idea that the difficulties in understanding the extrasolar planets' mass and orbital distributions are due to incorrect assumptions on the protoplanetary disk structure. The classic view of a viscous disk, with viscosity generated by strong turbulence driven by the magneto rotational instability, is challenged by modern magneto-hydrodynamic simulations. Disks are probably much less viscous than previously thought. Nevertheless, disks cannot be inviscid, a minimum viscosity is set for example by the so-called vertical shear instability (VSI). In addition, disk winds remove angular momentum from thin surface layers of the protoplanetary disk, promoting the fast radial transport of gas towards the central star in these layers. Our proposed project is (i) to construct a realistic model of protoplanetary disks accounting for both the VSI and disk winds, and reproducing the observed stellar accretion rates and (ii) to study the accretion of gas and the migration of planets embedded in these disks.