Dust Coagulation and Settling in Layered Protoplanetary Disks

Abstract

Previous models of dust growth in protoplanetary disks considered either uniformly laminar or turbulent disks. This Letter explores how dust growth occurs in a layered protoplanetary disk in which the magnetorotational instability generates turbulence only in the surface layers of a disk. Two cases are considered: a completely laminar dead zone and a dead zone in which turbulence is ``stirred up'' from the MRI acting above. It is found that dust is depleted from high altitudes in layered disks faster than in those cases of a uniformly laminar or turblent disks. This is a result of the accelerated growth of particles in the turbulent regions and their storage in the lower levels where they escape energetic collisions which would result in disruption. Thus the regions of a protoplanetary disk above a dead zone would become rapidly depleted in small dust grains, whereas the outer regions, where the MRI is active throughout, will maintain a small dust poplulation at all heights due to the disruptive collisions and vertical mixing from turbulence. This structure is similar to that which has been inferred for disks around TW Hydra, GM Auriga, and CoKu Tau/4, which are depleted in dust close to the star, but are optically thick at larger heliocentric distances.Comment: 4 pages, 3 figures, accepted to ApJ Letter

Co‐Accretion of Chondrules and Dust in the Solar Nebula

“…Since all these effects will vary with height, however, it is difficult to predict how these effects will unfold, and which parameters are key. Clearly, additional modeling is needed, where we may even combine these two different modes of turbulence since it is quite natural to expect that different physical processes operate at different heights (Ciesla 2007). However, incremental growth in the dense, particle-dominated midplanes of nonturbulent models then proceeds extremely rapidly Weidenschilling 2000) which is contrary to the evidence from meteorites and asteroids (see Cuzzi et al 2005;, for a discussion).…”

“…Clearly, other compaction processes are needed than can be provided by collisions in turbulence, even between compounds approaching St ¼ 1 in size. One possibility, given the preference for melting of chondrules by Mach 7 shock waves (Desch & Connolly 2002;Hood et al 2005), is that the plausibly more numerous, more prevalent, weaker shocks which are consequently experienced even more routinely by particles, would provide this range of collisional velocities for chondrule-size particles and their fractal aggregates (Ciesla 2006). Another constraint is the rim-matrix distinction.…”

Magnetic fields in protoplanetary disks

“…Advection and/or stirring by magnetically-driven turbulence affects disk chemistry (e.g. Semenov, Wiebe & Henning 2006;Ilgner & Nelson 2006), the aggregation and settling of small particles that are the preliminary stages of planet building (Johansen & Klahr 2005;Turner et al 2006;Fromang & Papaloizou 2006;Ciesla 2007), and magnetic activity at the disk surface may produce a corona (e.g. Fleming & Stone 2003) and affect observational signatures of protoplanetary disks.…”

Formation of stars and planets: the role of magnetic fields

“…Additionally, magneticallydriven mixing strongly influences the chemistry of the disc (e.g. Semenov et al 2006;Ilgner & Nelson 2006) and critically affects the dynamics, aggregation and overall evolution of dust particles (Turner et al 2006;Ciesla 2007) -the assembly blocks of planetesimals according to the 'core accretion' model of planet formation (Pollack et al 1996). Furthermore, magnetic fields can also modify the response of the disc to gravitational perturbations introduced by forming planets, in turn influencing the rate and even the direction of planetary migration through the disc (Terquem 2003;Fromang et al 2005;Muto et al 2008;Johnson et al 2006).…”