Eduard Vorobyov - May 4, 2018

The early stages of dust growth in young gravitationally unstable disks


Planets form in gaseous and dusty discs surrounding young stars through 
the process of dust growth from submicron particles to hundreds-km-sized bodies.  
It was thought until recently that planets form in relatively advanced stages 
of disc evolution, in million-year-old discs known as protoplanetary discs. 
However, recent spectacular images of HL Tauri possessing a much younger disc 
show signatures, such as rings and gaps, that are typical for the 
advanced stages of dust growth. These signatures imply that the process of planet 
formation may be coeval with the formation of discs and stars themselves. 

I will report on the early evolution of gravitationally unstable protostellar 
disks, studied using numerical hydrodynamics simulations modified to include
a dust component consisting of two parts: sub-micron-sized dust and grown dust. 
The former is strictly coupled to the gas, while the latter interacts with the 
gas via friction and gravity. The conversion of small to grown dust, dust growth, 
settling, and dust self-gravity are also considered.

I will show that the process of dust growth known for the older protoplanetary 
phase of disk evolution also holds for the early protostellar phase when the 
disk is still surrounded by the parental cloud. The process of small-to-grown 
dust conversion is very fast once the disk is formed. The total mass of grown 
dust in the disk reaches hundreds of Earth masses in 
hundreds-of-thousand-year-old disks.  Dust does not usually grow to radii 
greater than a few cm, with a notable exception of models 
with reduced viscous mass transport, in which case a dead zone forms in the 
inner disk regions where dust can grow to meter-sized boulders. Grown dust 
tend to accumulate in spiral arms near corotation where the azimuthal velocity of
dust grains is closest to the local velocity of the spiral pattern. The 
implications of fast dust growth for planet formation will also be discussed.