Grain growth in proto-planetary discs: the role of scattering

行星形成盘是系外行星聚集的自然环境。根据核心吸积模型，行星的形成是一个自下而上的过程：最初只有m大小，尘埃颗粒会增长几个数量级，形成大约100公里大的星子。这就是为什么评估在千万年的原行星盘中是否真的发生了谷物生长，对于理解目前观测到的千万年的系外行星是如何诞生的至关重要。此外，对行星形成盘中最大颗粒尺寸的估计对行星形成模型有很强的限制(例如，星子形成的速度有多快？在给定的时间内，行星形成的最小固体量是多少？)。晶粒生长假说已经通过光谱指数进行了检验，光谱指数a（要么是空间分辨的，要么是更常见的圆盘积分的）。在光学薄区和瑞利-金斯近似中，它可以简单地与吸收光谱指数相关。后者又是最大晶粒尺寸的函数。这是过去几年推断的毫米到厘米大颗粒的存在。然而，现在已知的光学厚度的亚结构，如环或螺旋在原行星盘中无处不在，这表明先前的近似不再有效。实际上，在光学厚度的情况下，无论晶粒大小，a都= 2。此外，最近对单一来源的研究表明，在光学薄的情况下，尘埃的自散射也很重要：忽略散射会导致对高反照率颗粒的尘埃大小的高估。在我的项目中，我以自洽的方式研究颗粒生长，以评估粉尘散射在确定最大颗粒尺寸中的作用。面对模型和数据，我的目标是提供可靠的估计，在附近的恒星形成区域的圆盘人口的最大颗粒尺寸。

Planet-forming discs are the natural environment in which exoplanets are assembled. According to the core-accretion model, planet formation is a bottom up process: initially m-sized, dust grains grow several orders of magnitude to form roughly 100 km-large planetesimals. This is why assessing whether grain growth is actually taking place in Myr-old protoplanetary discs is critical to understand how the currently observed Gyr-old exoplanets are born. Furthermore, estimates of the maximum grain size in planet-forming discs posit strong constraints on the planet-formation models (e.g., how rapidly do planetesimals form? Which is the minimum amount of solids in discs at a given time for planet formation to be viable?). The grain-growth hypothesis has been tested relying on the spectral index, a (either spatially resolved or, more commonly, disc-integrated). In the optically thin regime and the Rayleigh-Jeans approximation, it can be simply related to the absorption spectral index. The latter, in turn, is a function of the maximum grain size. This was the presence of mm to cm large grains was inferred in the past years. However, it is now known that optically thick sub-structures such as rings or spirals are ubiquitous in protoplanetary discs suggesting that the previous approximation is no longer valid. Indeed, in the optically thick case a = 2 regardless of the grain size. Moreover, recent works focusing on single sources suggest that also dust self-scattering can be important in the optically thin scenario: overlooking scattering brings about an overestimation of the dust sizes for grains with high albedo. In my project I study grain growth in a self-consistent way to assess the role of dust scattering in the determination of the maximum grain size. Confronting models and data, my aim is that of providing reliable estimates of the maximum grain sizes in populations of discs in the nearby star forming regions.