Dust Evolution and the Formation of Planetesimals

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.Comment: 43 pages, 6 figures. Chapter in International Space Science Institute (ISSI) Book on "The Disk in Relation to the Formation of Planets and their Proto-atmospheres", published in Space Science Reviews by Springe

Molecular abundances and C/O ratios in chemically evolving planet-forming disk midplanes

“…The physics of grain evolution has been studied extensively (see e.g. Birnstiel et al 2016;Kataoka et al 2014;Okuzumi et al 2012, and references therein).…”

X-ray photoevaporation’s limited success in the formation of planetesimals by the streaming instability

“…If the maximum grain size is limited by radial drift, we assume that 97% of the grains are large; if in the fragmentation dominated regime, this is instead 75%. We use these fractions to compute a weighted average of the radial drift velocity to advect the dust (see Birnstiel et al 2012or Birnstiel et al 2016 for a review). Table 2 summarises the mass accrued in planetesimals throughout the disc for the models summarised in Table 1.…”

Low-velocity collisions of chondrules: How a thin dust cover helps enhance the sticking probability

“…The chondrules collide centrally with relative velocities v varying between 0.05 and 1 m s −1 . Such velocities are typical of relative velocities in the mid-planes of protoplanetary disks (Birnstiel et al 2016). In order to gather some statistics, for each collision three individual impacts have been simulated; these impacts differ from each other by an arbitrary rotation of the chondrules before collision.…”

“…These collisions are of particular importance in protoplanetary disks, where such collisions ultimately lead -by aggregation processes -to the formation of planets and moons (Blum 2010). While many aspects of such collisions have by now been investigated, the role of collisions between mm-to m-sized objects still poses many questions (Birnstiel et al 2016). On the one hand, collision velocities of such objects with each other and with smaller dust particles may occur with higher relative velocities, since these objects have (partly) decoupled from their gas environment.…”