On the Importance of Gas Flow through Porous Bodies for the Formation of Planetesimals

Wurm, Gerhard; Paraskov, Georgi; Krauß, Oliver

doi:10.1086/383122

Cited by 30 publications

(17 citation statements)

References 19 publications

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“…In reality, the fate of the fragments will be determined by the local gas flow and the velocity with which they are ejected. For very porous targets, the gas flow through and around the surface of the target might result in these fragments being re-accreted (Wurm et al 2004). If efficient, this re-accretion might be a way to alleviate the destructive influence of erosive collisions.…”

Section: Discussionmentioning

confidence: 99%

Erosion and the limits to planetesimal growth

Krijt

Ormel

Dominik

et al. 2015

A&A

135

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Context. The coagulation of microscopic dust into planetesimals is the first step towards the formation of planets. The composition, size, and shape of the growing aggregates determine the efficiency of this early growth. In particular, it has been proposed that fluffy ice aggregates can grow very efficiently in protoplanetary disks, suffering less from the bouncing and radial drift barriers. Aims. While the collision velocity between icy aggregates of similar size is thought to stay below the fragmentation threshold, they may nonetheless lose mass from collisions with much smaller projectiles. As a result, erosive collisions have the potential to terminate the growth of pre-planetesimal bodies. We investigate the effect of these erosive collisions on the ability of porous ice aggregates to cross the radial drift barrier. Methods. We develop a Monte Carlo code that calculates the evolution of the masses and porosities of growing aggregates, while resolving the entire mass distribution at all times. The aggregate's porosity is treated independently of its mass, and is determined by collisional compaction, gas compaction, and eventually self-gravity compaction. We include erosive collisions and study the effect of the erosion threshold velocity on aggregate growth. Results. For erosion threshold velocities of 20−40 m s −1 , high-velocity collisions with small projectiles prevent the largest aggregates from growing when they start to drift. In these cases, our local simulations result in a steady-state distribution, with most of the dust mass in particles with Stokes numbers close to unity. Only for the highest erosion threshold considered (60 m s −1 ) do porous aggregates manage to cross the radial drift barrier in the inner 10 AU of MMSN-like disks. Conclusions. Erosive collisions are more effective in limiting the growth than fragmentary collisions between similar-size particles. Conceivably, erosion limits the growth before the radial drift barrier, although the robustness of this statement depends on uncertain material properties of icy aggregates. If erosion inhibits planetesimal formation through direct sticking, the sea of ∼10 9 g, highly porous particles appears suitable for triggering streaming instability.

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Section: Discussionmentioning

confidence: 99%

Erosion and the limits to planetesimal growth

Krijt

Ormel

Dominik

et al. 2015

A&A

135

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“…where q boundary can be seen as either the flow velocity inside a porous planetesimal (Wurm et al 2004) or as the flow velocity difference between the dust particles and the gas (Sekiya & Takeda 2005). We use the latter approach to calculate q boundary based on the kinematic theory of gases, which yields q boundary ∆v col = 3 8…”

Section: The Boundary Layermentioning

confidence: 99%

“…Theoretical studies have mostly focused either on grain growth and the formation of planetesimals (e.g., Weidenschilling 1984;Wurm et al 2004;Dullemond & Dominik 2005;Cuzzi et al 2008;Birnstiel et al 2011;Okuzumi et al 2012) or on the growth of a swarm of mutually interacting planetesimals (e.g., Safronov 1972;Wetherill & Stewart 1989;Kokubo & Ida 1998;Chambers 2006;Levison et al 2010;Johansen et al 2014). The interactions of planetesimals and dust in a gas-rich disk have been the focus of less attention, apart from some works to which this study will frequently refer (Rafikov 2004;Ormel & Klahr 2010;Ormel & Kobayashi 2012;Lambrechts & Johansen 2012).…”

Section: Introductionmentioning

confidence: 99%

On the filtering and processing of dust by planetesimals

Guillot

Ida

Ormel

2014

A&A

141

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Context. Circumstellar disks are known to contain a significant mass in dust ranging from micron to centimeter size. Meteorites are evidence that individual grains of those sizes were collected and assembled into planetesimals in the young solar system. Aims. We assess the efficiency of dust collection of a swarm of non-drifting planetesimals with radii ranging from 1 to 10 3 km and beyond. Methods. We calculate the collision probability of dust drifting in the disk due to gas drag by planetesimal accounting for several regimes depending on the size of the planetesimal, dust, and orbital distance: the geometric, Safronov, settling, and three-body regimes. We also include a hydrodynamical regime to account for the fact that small grains tend to be carried by the gas flow around planetesimals. Results. We provide expressions for the collision probability of dust by planetesimals and for the filtering efficiency by a swarm of planetesimals. For standard turbulence conditions (i.e., a turbulence parameter α = 10 −2 ), filtering is found to be inefficient, meaning that when crossing a minimum-mass solar nebula (MMSN) belt of planetesimals extending between 0.1 AU and 35 AU most dust particles are eventually accreted by the central star rather than colliding with planetesimals. However, if the disk is weakly turbulent (α = 10 −4 ) filtering becomes efficient in two regimes: (i) when planetesimals are all smaller than about 10 km in size, in which case collisions mostly take place in the geometric regime; and (ii) when planetary embryos larger than about 1000 km in size dominate the distribution, have a scale height smaller than one tenth of the gas scale height, and dust is of millimeter size or larger in which case most collisions take place in the settling regime. These two regimes have very different properties: we find that the local filtering efficiency x filter,MMSN scales with r −7/4 (where r is the orbital distance) in the geometric regime, but with r −1/4 to r 1/4 in the settling regime. This implies that the filtering of dust by small planetesimals should occur close to the central star and with a short spread in orbital distances. On the other hand, the filtering by embryos in the settling regime is expected to be more gradual and determined by the extent of the disk of embryos. Dust particles much smaller than millimeter size tend only to be captured by the smallest planetesimals because they otherwise move on gas streamlines and their collisions take place in the hydrodynamical regime. Conclusions. Our results hint at an inside-out formation of planetesimals in the infant solar system because small planetesimals in the geometrical limit can filter dust much more efficiently close to the central star. However, even a fully-formed belt of planetesimals such as the MMSN only marginally captures inward-drifting dust and this seems to imply that dust in the protosolar disk has been filtered by planetesimals even smaller than 1 km (not included in this study) or that it has been assembled into planetesi...

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“…Dust grains may however still be able to penetrate to the boulder if the boulder is porous and has gas flow through it (Wurm et al 2004). We shall nevertheless limit ourselves to the Epstein regime in this paper and leave the treatment of the interaction of grains and boulders in the Stokes regime to a future publication.…”

Section: Appendix A: Stability Analysismentioning

confidence: 99%

A coagulation-fragmentation model for the turbulent growth and destruction of preplanetesimals

Johansen¹,

Brauer²,

Dullemond³

et al. 2008

A&A

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To treat the problem of growing protoplanetary disc solids across the meter barrier, we consider a very simplified two-component coagulation-fragmentation model that consists of macroscopic boulders and smaller dust grains, the latter being the result of catastrophic collisions between the boulders. Boulders in turn increase their radii by sweeping up the dust fragments. An analytical solution of the dynamical equations predicts that growth by coagulation-fragmentation can be efficient and allow agglomeration of 10-m-sized objects within the time-scale of the radial drift. These results are supported by computer simulations of the motion of boulders and fragments in 3-D time-dependent magnetorotational turbulence. However allowing the fragments to diffuse freely out of the sedimentary layer of boulders drastically reduces the density of both boulders and fragments in the mid-plane, and thus also the growth of the boulder radius. The reason is that the turbulent diffusion time-scale is so much shorter than the collisional time-scale that dust fragments leak out of the mid-plane layer before they can be swept up by the boulders there. Our conclusion that coagulationfragmentation is not an efficient way to grow across the meter barrier in fully turbulent protoplanetary discs confirms recent results by Brauer, Dullemond, & Henning who solved the coagulation equation in a parameterised turbulence model with collisional fragmentation, cratering, radial drift, and a range of particle sizes. We find that a relatively small population of boulders in a sedimentary mid-plane layer can populate the entire vertical extent of the disc with small grains and that these grains are not first generation dust, but have been through several agglomeration-destruction cycles during the simulations.

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On the Importance of Gas Flow through Porous Bodies for the Formation of Planetesimals

Cited by 30 publications

References 19 publications

Erosion and the limits to planetesimal growth

Erosion and the limits to planetesimal growth

On the filtering and processing of dust by planetesimals

A coagulation-fragmentation model for the turbulent growth and destruction of preplanetesimals

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