Dust Evolution and the Formation of Planetesimals

T Birnstiel,M Fang,A Johansen

doi:10.1007/s11214-016-0256-1

Abstract

The solid content of circumstellar disks is inherited from the interstellar medium: dust particles of at most a micrometer in size. Protoplanetary disks are the environment where these dust grains need to grow at least 13 orders of magnitude in size. Our understanding of this growth process is far from complete, with different physics seemingly posing obstacles to this growth at various stages. Yet, the ubiquity of planets in our galaxy suggests that planet formation is a robust mechanism. This chapter focuses on the earliest stages of planet formation, the growth of small dust grains towards the gravitationally bound "planetesimals", the building blocks of planets. We will introduce some of the key physics involved in the growth processes and discuss how they are expected to shape the global behavior of the solid content of disks. We will consider possible pathways towards the formation of larger bodies and conclude by reviewing some of the recent observational advances in the field.

Highlights

Circumstellar disks consist mainly (99 % by mass) of gas, but the tiny 1 % of condensible material is a crucial ingredient
The field of protoplanetary disks is currently being revolutionized by observational advances in near-infrared imaging (Subaru, NACO, and the upcoming Sphere and GPI instruments) and inmillimeter interferometry with ALMA
It is even more interesting that some transition disks that show gaps/holes in themillimeter interferometric data lack evidence of evolved inner disks in H -band polarized intensity images (Muto et al 2012; Follette et al 2013), which may be due to the decoupling between the spatial distributions of the small and big dust grains inside the holes/gaps caused by dust filtration (Rice et al 2006; Dong et al 2012; Pinilla et al 2012; Zhu et al 2012)

Summary

Introduction

Circumstellar disks consist mainly (99 % by mass) of gas, but the tiny 1 % of condensible material (commonly called solids or dust) is a crucial ingredient. After all it is the material out of which planets and minor bodies are formed, but beyond this it is important for the physical structure and the evolution of the disk: in most parts of the disk, the opacity is dominated by the dust. Dust determines the observational appearance of the disk: on the one hand through its thermal continuum emission and on the other hand by determining the temperature and density structure and the excitation conditions for gas lines. Understanding the evolution of solids is a key element in the puzzle of planet formation

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Conclusion