Fluffy dust forms icy planetesimals by static compression

Abstract

Context. Several barriers have been proposed in planetesimal formation theory: bouncing, fragmentation, and radial drift problems. Understanding the structure evolution of dust aggregates is a key in planetesimal formation. Dust grains become fluffy by coagulation in protoplanetary disks. However, once they are fluffy, they are not sufficiently compressed by collisional compression to form compact planetesimals. Aims. We aim to reveal the pathway of dust structure evolution from dust grains to compact planetesimals. Methods. Using the compressive strength formula, we analytically investigate how fluffy dust aggregates are compressed by static compression due to ram pressure of the disk gas and self-gravity of the aggregates in protoplanetary disks. Results. We reveal the pathway of the porosity evolution from dust grains via fluffy aggregates to form planetesimals, circumventing the barriers in planetesimal formation. The aggregates are compressed by the disk gas to a density of 10 −3 g/cm 3 in coagulation, which is more compact than is the case with collisional compression. Then, they are compressed more by self-gravity to 10 −1 g/cm 3 when the radius is 10 km. Although the gas compression decelerates the growth, the aggregates grow rapidly enough to avoid the radial drift barrier when the orbital radius is < ∼ 6 AU in a typical disk.Conclusions. We propose a fluffy dust growth scenario from grains to planetesimals. It enables icy planetesimal formation in a wide range beyond the snowline in protoplanetary disks. This result proposes a concrete initial condition of planetesimals for the later stages of the planet formation.

Dust Coagulation with Porosity Evolution

“…This figure is published as a part of Fig. 3 in Kataoka et al (2013a) Interestingly, the mass and filling factor of the resultant aggregates are in relatively good agreement with those of comets, which are believed to be the remnants of planetesimals (A'Hearn 2011). In this way, the overall filling factor evolution has been revealed, with the intermediate stage of extremely fluffy dust aggregates.…”

“…Combining these stages, we finally reveal the overall filling factor evolution in protoplanetary disks, as shown in Fig. 5.2 (Kataoka et al 2013a). The 0.1-msized dust grains become extremely fluffy dust aggregates with a filling factor of 10 5 .…”

“…This figure is published as Fig. 2 in Kataoka et al (2013a) As long as the dust coagulation occurs with similar-sized collisions, therefore, the dust aggregates become more and more porous because of hit-and-stick growth.…”

Grain Size Constraints on Hl Tau With Polarization Signature

“…This infers that the highly porous and To understand what kind of dust aggregates are required to explain both the polarization and the spectral index, here we discuss possible constraints on the dust aggregates from the spectral index assuming that the emission is optically thin. Dust grains coagulate to form porous dust aggregates (e.g., Ossenkopf 1993) The filling factor can be even as small as 10 −4 in disks (Kataoka et al 2013), though how porous the dust aggregates are is still controvercial. However, we can constrain the product of the aggregate radius a and the filling factor f because the absorption opacity is the same if the product af is the same (Kataoka et al 2014).…”

“…To explain the observed index of the absorption opacity, the dust aggregates in HL Tau should have a product of af in the range of a f 5cm 21cm max   in the adopted dust model. This means that, for example, if the filling factor is as small as f=10 −4 (Kataoka et al 2013), the aggregate radius is ∼1 km.…”